A hugetlb page will have a mapcount of 1 if mapped by multiple processes via a shared PMD. This is because only the first process increases the map count, and subsequent processes just add the shared PMD page to their page table.
page_mapcount is being used to decide if a hugetlb page is shared or private in /proc/PID/smaps. Pages referenced via a shared PMD were incorrectly being counted as private.
To fix, check for a shared PMD if mapcount is 1. If a shared PMD is found count the hugetlb page as shared. A new helper to check for a shared PMD is added.
Fixes: 25ee01a2fca0 ("mm: hugetlb: proc: add hugetlb-related fields to /proc/PID/smaps") Cc: stable@vger.kernel.org Signed-off-by: Mike Kravetz mike.kravetz@oracle.com --- fs/proc/task_mmu.c | 10 ++++++++-- include/linux/hugetlb.h | 12 ++++++++++++ 2 files changed, 20 insertions(+), 2 deletions(-)
diff --git a/fs/proc/task_mmu.c b/fs/proc/task_mmu.c index e35a0398db63..cb9539879402 100644 --- a/fs/proc/task_mmu.c +++ b/fs/proc/task_mmu.c @@ -749,8 +749,14 @@ static int smaps_hugetlb_range(pte_t *pte, unsigned long hmask,
if (mapcount >= 2) mss->shared_hugetlb += huge_page_size(hstate_vma(vma)); - else - mss->private_hugetlb += huge_page_size(hstate_vma(vma)); + else { + if (hugetlb_pmd_shared(pte)) + mss->shared_hugetlb += + huge_page_size(hstate_vma(vma)); + else + mss->private_hugetlb += + huge_page_size(hstate_vma(vma)); + } } return 0; } diff --git a/include/linux/hugetlb.h b/include/linux/hugetlb.h index e3aa336df900..8e65920e4363 100644 --- a/include/linux/hugetlb.h +++ b/include/linux/hugetlb.h @@ -1225,6 +1225,18 @@ static inline __init void hugetlb_cma_reserve(int order) } #endif
+#ifdef CONFIG_ARCH_WANT_HUGE_PMD_SHARE +static inline bool hugetlb_pmd_shared(pte_t *pte) +{ + return page_count(virt_to_page(pte)) > 1; +} +#else +static inline bool hugetlb_pmd_shared(pte_t *pte) +{ + return false; +} +#endif + bool want_pmd_share(struct vm_area_struct *vma, unsigned long addr);
#ifndef __HAVE_ARCH_FLUSH_HUGETLB_TLB_RANGE
On 26.01.23 23:27, Mike Kravetz wrote:
A hugetlb page will have a mapcount of 1 if mapped by multiple processes via a shared PMD. This is because only the first process increases the map count, and subsequent processes just add the shared PMD page to their page table.
page_mapcount is being used to decide if a hugetlb page is shared or private in /proc/PID/smaps. Pages referenced via a shared PMD were incorrectly being counted as private.
To fix, check for a shared PMD if mapcount is 1. If a shared PMD is found count the hugetlb page as shared. A new helper to check for a shared PMD is added.
Fixes: 25ee01a2fca0 ("mm: hugetlb: proc: add hugetlb-related fields to /proc/PID/smaps") Cc: stable@vger.kernel.org Signed-off-by: Mike Kravetz mike.kravetz@oracle.com
fs/proc/task_mmu.c | 10 ++++++++-- include/linux/hugetlb.h | 12 ++++++++++++ 2 files changed, 20 insertions(+), 2 deletions(-)
diff --git a/fs/proc/task_mmu.c b/fs/proc/task_mmu.c index e35a0398db63..cb9539879402 100644 --- a/fs/proc/task_mmu.c +++ b/fs/proc/task_mmu.c @@ -749,8 +749,14 @@ static int smaps_hugetlb_range(pte_t *pte, unsigned long hmask, if (mapcount >= 2) mss->shared_hugetlb += huge_page_size(hstate_vma(vma));
elsemss->private_hugetlb += huge_page_size(hstate_vma(vma));
else {
Better:
if (mapcount >= 2 || hugetlb_pmd_shared(pte)) mss->shared_hugetlb += huge_page_size(hstate_vma(vma)); else mss->private_hugetlb += huge_page_size(hstate_vma(vma));
On Fri, 27 Jan 2023 17:23:39 +0100 David Hildenbrand david@redhat.com wrote:
On 26.01.23 23:27, Mike Kravetz wrote:
A hugetlb page will have a mapcount of 1 if mapped by multiple processes via a shared PMD. This is because only the first process increases the map count, and subsequent processes just add the shared PMD page to their page table.
page_mapcount is being used to decide if a hugetlb page is shared or private in /proc/PID/smaps. Pages referenced via a shared PMD were incorrectly being counted as private.
To fix, check for a shared PMD if mapcount is 1. If a shared PMD is found count the hugetlb page as shared. A new helper to check for a shared PMD is added.
...
--- a/fs/proc/task_mmu.c +++ b/fs/proc/task_mmu.c @@ -749,8 +749,14 @@ static int smaps_hugetlb_range(pte_t *pte, unsigned long hmask, if (mapcount >= 2) mss->shared_hugetlb += huge_page_size(hstate_vma(vma));
elsemss->private_hugetlb += huge_page_size(hstate_vma(vma));
else {Better:
if (mapcount >= 2 || hugetlb_pmd_shared(pte)) mss->shared_hugetlb += huge_page_size(hstate_vma(vma)); else mss->private_hugetlb += huge_page_size(hstate_vma(vma));
Yup. And that local doesn't add any value?
--- a/fs/proc/task_mmu.c~mm-hugetlb-proc-check-for-hugetlb-shared-pmd-in-proc-pid-smaps-fix +++ a/fs/proc/task_mmu.c @@ -745,18 +745,10 @@ static int smaps_hugetlb_range(pte_t *pt page = pfn_swap_entry_to_page(swpent); } if (page) { - int mapcount = page_mapcount(page); - - if (mapcount >= 2) + if (page_mapcount(page) >= 2 || hugetlb_pmd_shared(pte)) mss->shared_hugetlb += huge_page_size(hstate_vma(vma)); - else { - if (hugetlb_pmd_shared(pte)) - mss->shared_hugetlb += - huge_page_size(hstate_vma(vma)); - else - mss->private_hugetlb += - huge_page_size(hstate_vma(vma)); - } + else + mss->private_hugetlb += huge_page_size(hstate_vma(vma)); } return 0; } _
On 01/27/23 15:04, Andrew Morton wrote:
On Fri, 27 Jan 2023 17:23:39 +0100 David Hildenbrand david@redhat.com wrote:
On 26.01.23 23:27, Mike Kravetz wrote:
A hugetlb page will have a mapcount of 1 if mapped by multiple processes via a shared PMD. This is because only the first process increases the map count, and subsequent processes just add the shared PMD page to their page table.
page_mapcount is being used to decide if a hugetlb page is shared or private in /proc/PID/smaps. Pages referenced via a shared PMD were incorrectly being counted as private.
To fix, check for a shared PMD if mapcount is 1. If a shared PMD is found count the hugetlb page as shared. A new helper to check for a shared PMD is added.
...
--- a/fs/proc/task_mmu.c +++ b/fs/proc/task_mmu.c @@ -749,8 +749,14 @@ static int smaps_hugetlb_range(pte_t *pte, unsigned long hmask, if (mapcount >= 2) mss->shared_hugetlb += huge_page_size(hstate_vma(vma));
elsemss->private_hugetlb += huge_page_size(hstate_vma(vma));
else {Better:
if (mapcount >= 2 || hugetlb_pmd_shared(pte)) mss->shared_hugetlb += huge_page_size(hstate_vma(vma)); else mss->private_hugetlb += huge_page_size(hstate_vma(vma));
Yup. And that local doesn't add any value?
--- a/fs/proc/task_mmu.c~mm-hugetlb-proc-check-for-hugetlb-shared-pmd-in-proc-pid-smaps-fix +++ a/fs/proc/task_mmu.c @@ -745,18 +745,10 @@ static int smaps_hugetlb_range(pte_t *pt page = pfn_swap_entry_to_page(swpent); } if (page) {
int mapcount = page_mapcount(page);if (mapcount >= 2)
if (page_mapcount(page) >= 2 || hugetlb_pmd_shared(pte)) mss->shared_hugetlb += huge_page_size(hstate_vma(vma));
else {if (hugetlb_pmd_shared(pte))mss->shared_hugetlb +=huge_page_size(hstate_vma(vma));elsemss->private_hugetlb +=huge_page_size(hstate_vma(vma));}
elsemss->private_hugetlb += huge_page_size(hstate_vma(vma));}
Thank you both! That looks much better.
On Fri 27-01-23 17:12:05, Mike Kravetz wrote:
On 01/27/23 15:04, Andrew Morton wrote:
On Fri, 27 Jan 2023 17:23:39 +0100 David Hildenbrand david@redhat.com wrote:
On 26.01.23 23:27, Mike Kravetz wrote:
A hugetlb page will have a mapcount of 1 if mapped by multiple processes via a shared PMD. This is because only the first process increases the map count, and subsequent processes just add the shared PMD page to their page table.
page_mapcount is being used to decide if a hugetlb page is shared or private in /proc/PID/smaps. Pages referenced via a shared PMD were incorrectly being counted as private.
To fix, check for a shared PMD if mapcount is 1. If a shared PMD is found count the hugetlb page as shared. A new helper to check for a shared PMD is added.
...
--- a/fs/proc/task_mmu.c +++ b/fs/proc/task_mmu.c @@ -749,8 +749,14 @@ static int smaps_hugetlb_range(pte_t *pte, unsigned long hmask, if (mapcount >= 2) mss->shared_hugetlb += huge_page_size(hstate_vma(vma));
elsemss->private_hugetlb += huge_page_size(hstate_vma(vma));
else {Better:
if (mapcount >= 2 || hugetlb_pmd_shared(pte)) mss->shared_hugetlb += huge_page_size(hstate_vma(vma)); else mss->private_hugetlb += huge_page_size(hstate_vma(vma));
Yup. And that local doesn't add any value?
--- a/fs/proc/task_mmu.c~mm-hugetlb-proc-check-for-hugetlb-shared-pmd-in-proc-pid-smaps-fix +++ a/fs/proc/task_mmu.c @@ -745,18 +745,10 @@ static int smaps_hugetlb_range(pte_t *pt page = pfn_swap_entry_to_page(swpent); } if (page) {
int mapcount = page_mapcount(page);if (mapcount >= 2)
if (page_mapcount(page) >= 2 || hugetlb_pmd_shared(pte)) mss->shared_hugetlb += huge_page_size(hstate_vma(vma));
else {if (hugetlb_pmd_shared(pte))mss->shared_hugetlb +=huge_page_size(hstate_vma(vma));elsemss->private_hugetlb +=huge_page_size(hstate_vma(vma));}
elsemss->private_hugetlb += huge_page_size(hstate_vma(vma));}
Thank you both! That looks much better.
Yes, this looks simple enough. My only concern would be that this special casing might be required on other places which is hard to evaluate. I thought PSS reported by smaps would be broken as well but it seems pss is not really accounted for hugetlb mappings at all.
Have you tried to look into {in,de}creasing the map count of the page when the the pte is {un}shared for it?
On 01/30/23 13:36, Michal Hocko wrote:
On Fri 27-01-23 17:12:05, Mike Kravetz wrote:
On 01/27/23 15:04, Andrew Morton wrote:
On Fri, 27 Jan 2023 17:23:39 +0100 David Hildenbrand david@redhat.com wrote:
On 26.01.23 23:27, Mike Kravetz wrote:
Yes, this looks simple enough. My only concern would be that this special casing might be required on other places which is hard to evaluate. I thought PSS reported by smaps would be broken as well but it seems pss is not really accounted for hugetlb mappings at all.
Have you tried to look into {in,de}creasing the map count of the page when the the pte is {un}shared for it?
A quick thought is that it would not be too difficult. It would need to include the following: - At PMD share time in huge_pmd_share(), Go through all entries in the PMD, and increment map and ref count for all referenced pages. huge_pmd_share is just adding another sharing process. - At PMD unshare time in huge_pmd_unshare(), Go through all entries in the PMD, and decrement map and ref count for all referenced pages. huge_pmd_unshare is just removing one sharing process. - At page fault time, check if we are adding a new entry to a shared PMD. If yes, add 'num_of_sharing__processes - 1' to the ref and map count.
In each of the above operations, we are holding the PTL lock (which is really the split/PMD lock) so synchronization should not be an issue.
Although I mention processes sharing the PMD above, it is really mappings/vmas sharing the PMD. You could have two mappings of the same object in the same process sharing PMDs.
I'll code this up and see how it looks.
However, unless you have an objection I would prefer the simple patches move forward, especially for stable backports.
On Mon 30-01-23 14:08:47, Mike Kravetz wrote:
On 01/30/23 13:36, Michal Hocko wrote:
On Fri 27-01-23 17:12:05, Mike Kravetz wrote:
On 01/27/23 15:04, Andrew Morton wrote:
On Fri, 27 Jan 2023 17:23:39 +0100 David Hildenbrand david@redhat.com wrote:
On 26.01.23 23:27, Mike Kravetz wrote:
Yes, this looks simple enough. My only concern would be that this special casing might be required on other places which is hard to evaluate. I thought PSS reported by smaps would be broken as well but it seems pss is not really accounted for hugetlb mappings at all.
Have you tried to look into {in,de}creasing the map count of the page when the the pte is {un}shared for it?
A quick thought is that it would not be too difficult. It would need to include the following:
- At PMD share time in huge_pmd_share(), Go through all entries in the PMD, and increment map and ref count for all referenced pages. huge_pmd_share is just adding another sharing process.
- At PMD unshare time in huge_pmd_unshare(), Go through all entries in the PMD, and decrement map and ref count for all referenced pages. huge_pmd_unshare is just removing one sharing process.
- At page fault time, check if we are adding a new entry to a shared PMD. If yes, add 'num_of_sharing__processes - 1' to the ref and map count.
In each of the above operations, we are holding the PTL lock (which is really the split/PMD lock) so synchronization should not be an issue.
Although I mention processes sharing the PMD above, it is really mappings/vmas sharing the PMD. You could have two mappings of the same object in the same process sharing PMDs.
I'll code this up and see how it looks.
Thanks!
However, unless you have an objection I would prefer the simple patches move forward, especially for stable backports.
Yes, the current patch is much simpler and more suitable for stable backports. If the explicit map count modifications are not all that terrible then this would sound like a more appropriate long term plan though.
Thanks!
On 02/01/23 08:47, Michal Hocko wrote:
On Mon 30-01-23 14:08:47, Mike Kravetz wrote:
On 01/30/23 13:36, Michal Hocko wrote:
On Fri 27-01-23 17:12:05, Mike Kravetz wrote:
On 01/27/23 15:04, Andrew Morton wrote:
On Fri, 27 Jan 2023 17:23:39 +0100 David Hildenbrand david@redhat.com wrote:
On 26.01.23 23:27, Mike Kravetz wrote:
Yes, this looks simple enough. My only concern would be that this special casing might be required on other places which is hard to evaluate. I thought PSS reported by smaps would be broken as well but it seems pss is not really accounted for hugetlb mappings at all.
Have you tried to look into {in,de}creasing the map count of the page when the the pte is {un}shared for it?
A quick thought is that it would not be too difficult. It would need to include the following:
- At PMD share time in huge_pmd_share(), Go through all entries in the PMD, and increment map and ref count for all referenced pages. huge_pmd_share is just adding another sharing process.
- At PMD unshare time in huge_pmd_unshare(), Go through all entries in the PMD, and decrement map and ref count for all referenced pages. huge_pmd_unshare is just removing one sharing process.
- At page fault time, check if we are adding a new entry to a shared PMD. If yes, add 'num_of_sharing__processes - 1' to the ref and map count.
In each of the above operations, we are holding the PTL lock (which is really the split/PMD lock) so synchronization should not be an issue.
Although I mention processes sharing the PMD above, it is really mappings/vmas sharing the PMD. You could have two mappings of the same object in the same process sharing PMDs.
I'll code this up and see how it looks.
Thanks!
However, unless you have an objection I would prefer the simple patches move forward, especially for stable backports.
Yes, the current patch is much simpler and more suitable for stable backports. If the explicit map count modifications are not all that terrible then this would sound like a more appropriate long term plan though.
The approach mentioned above seems to be simple enough. Patch is below.
I 'tested' with the same method and tests used to measure fault scalabilty when developing vma based locking [1]. I figured this would be a good stress of the share, unshare and fault paths. With the patch, we are doing more with the page table lock held, so I expected to see a little difference in scalability, but not as much as actually measured:
next-20230131 test instances unmodified patched -------------------------------------------------------------------------- Combined faults 24 61888.4 58314.8 Combined forks 24 157.3 130.1
These tests could seem a bit like a micro-benchmark targeting these code paths. However, I put them together based on the description of a customer workload that prompted the vma based locking work. And, performance of these tests seems to reflect performance of their workloads.
This extra overhead is the cost needed to make shared PMD map counts be accurate and in line with what is normal and expected. I think it is worth the cost. Other opinions? Of course, the patch below may have issues so please take a look.
[1] https://lore.kernel.org/linux-mm/20220914221810.95771-1-mike.kravetz@oracle....
From bff5a717521f96b0e5075ac4b5a1ef84a3589b7e Mon Sep 17 00:00:00 2001 From: Mike Kravetz mike.kravetz@oracle.com Date: Mon, 30 Jan 2023 20:14:14 -0800 Subject: [PATCH] hugetlb: Adjust hugetlbp page ref/map counts for PMD sharing
When hugetlb PMDS are shared, the sharing code simply adds the shared PMD to another processes page table. It will not update the ref/map counts of pages referenced by the shared PMD. As a result, the ref/map count will only reflect when the page was added to the shared PMD. Even though the shared PMD may be in MANY process page tables, ref/map counts on the pages will only appear to be that of a single process.
Update ref/map counts to take PMD sharing into account. This is done in three distinct places: 1) At PMD share time in huge_pmd_share(), Go through all entries in the PMD, and increment map and ref count for all referenced pages. huge_pmd_share is just adding another use and mapping of each page. 2) At PMD unshare time in huge_pmd_unshare(), Go through all entries in the PMD, and decrement map and ref count for all referenced pages. huge_pmd_unshare is just removing one use and mapping of each page. 3) When faulting in a new hugetlb page, Check if we are adding a new entry to a shared PMD. If yes, add 'num_of_sharing__processes - 1' to the ref and map count.
Signed-off-by: Mike Kravetz mike.kravetz@oracle.com --- mm/hugetlb.c | 56 ++++++++++++++++++++++++++++++++++++++++++++++++---- 1 file changed, 52 insertions(+), 4 deletions(-)
diff --git a/mm/hugetlb.c b/mm/hugetlb.c index 3a01a9dbf445..c7b1c6307a82 100644 --- a/mm/hugetlb.c +++ b/mm/hugetlb.c @@ -96,6 +96,7 @@ static void hugetlb_vma_lock_alloc(struct vm_area_struct *vma); static void __hugetlb_vma_unlock_write_free(struct vm_area_struct *vma); static void hugetlb_unshare_pmds(struct vm_area_struct *vma, unsigned long start, unsigned long end); +static void adjust_page_counts_for_shared_pmd(pte_t *ptep, struct folio *folio);
static inline bool subpool_is_free(struct hugepage_subpool *spool) { @@ -5905,10 +5906,12 @@ static vm_fault_t hugetlb_no_page(struct mm_struct *mm, if (!pte_same(huge_ptep_get(ptep), old_pte)) goto backout;
- if (anon_rmap) + if (anon_rmap) { hugepage_add_new_anon_rmap(folio, vma, haddr); - else + } else { page_dup_file_rmap(&folio->page, true); + adjust_page_counts_for_shared_pmd(ptep, folio); + } new_pte = make_huge_pte(vma, &folio->page, ((vma->vm_flags & VM_WRITE) && (vma->vm_flags & VM_SHARED))); /* @@ -7036,6 +7039,43 @@ void adjust_range_if_pmd_sharing_possible(struct vm_area_struct *vma, *end = ALIGN(*end, PUD_SIZE); }
+static void adjust_page_counts_for_shared_pmd(pte_t *ptep, struct folio *folio) +{ + int shared_count = page_count(virt_to_page(ptep)); + + if (shared_count < 2) + return; + + folio_ref_add(folio, shared_count - 1); + atomic_add(shared_count - 1, &folio->_entire_mapcount); +} + +static void adjust_shared_pmd_page_counts(pmd_t *pmd_start, int delta) +{ + struct folio *folio; + struct page *page; + pte_t *ptep, pte; + int i; + + for (i= 0; i < PTRS_PER_PMD; i++) { + ptep = (pte_t *)(pmd_start + i); + + pte = huge_ptep_get(ptep); + if (huge_pte_none(pte) || !pte_present(pte)) + continue; + + page = pte_page(pte); + folio = (struct folio *)page; + if (delta > 0) { + folio_get(folio); + atomic_inc(&folio->_entire_mapcount); + } else { + folio_put(folio); + atomic_dec(&folio->_entire_mapcount); + } + } +} + /* * Search for a shareable pmd page for hugetlb. In any case calls pmd_alloc() * and returns the corresponding pte. While this is not necessary for the @@ -7078,9 +7118,11 @@ pte_t *huge_pmd_share(struct mm_struct *mm, struct vm_area_struct *vma,
ptl = huge_pte_lock(hstate_vma(vma), mm, spte); if (pud_none(*pud)) { - pud_populate(mm, pud, - (pmd_t *)((unsigned long)spte & PAGE_MASK)); + pmd_t *pmdp = (pmd_t *)((unsigned long)spte & PAGE_MASK); + + pud_populate(mm, pud, pmdp); mm_inc_nr_pmds(mm); + adjust_shared_pmd_page_counts(pmdp, 1); } else { put_page(virt_to_page(spte)); } @@ -7118,12 +7160,18 @@ int huge_pmd_unshare(struct mm_struct *mm, struct vm_area_struct *vma,
pud_clear(pud); put_page(virt_to_page(ptep)); + adjust_shared_pmd_page_counts( + (pmd_t *)((unsigned long)ptep & PAGE_MASK), -1); mm_dec_nr_pmds(mm); return 1; }
#else /* !CONFIG_ARCH_WANT_HUGE_PMD_SHARE */
+static void adjust_page_counts_for_shared_pmd(pte_t *ptep, struct folio *folio) +{ +} + pte_t *huge_pmd_share(struct mm_struct *mm, struct vm_area_struct *vma, unsigned long addr, pud_t *pud) {
On Wed 01-02-23 13:05:35, Mike Kravetz wrote:
On 02/01/23 08:47, Michal Hocko wrote:
On Mon 30-01-23 14:08:47, Mike Kravetz wrote:
On 01/30/23 13:36, Michal Hocko wrote:
On Fri 27-01-23 17:12:05, Mike Kravetz wrote:
On 01/27/23 15:04, Andrew Morton wrote:
On Fri, 27 Jan 2023 17:23:39 +0100 David Hildenbrand david@redhat.com wrote: > On 26.01.23 23:27, Mike Kravetz wrote:
Yes, this looks simple enough. My only concern would be that this special casing might be required on other places which is hard to evaluate. I thought PSS reported by smaps would be broken as well but it seems pss is not really accounted for hugetlb mappings at all.
Have you tried to look into {in,de}creasing the map count of the page when the the pte is {un}shared for it?
A quick thought is that it would not be too difficult. It would need to include the following:
- At PMD share time in huge_pmd_share(), Go through all entries in the PMD, and increment map and ref count for all referenced pages. huge_pmd_share is just adding another sharing process.
- At PMD unshare time in huge_pmd_unshare(), Go through all entries in the PMD, and decrement map and ref count for all referenced pages. huge_pmd_unshare is just removing one sharing process.
- At page fault time, check if we are adding a new entry to a shared PMD. If yes, add 'num_of_sharing__processes - 1' to the ref and map count.
In each of the above operations, we are holding the PTL lock (which is really the split/PMD lock) so synchronization should not be an issue.
Although I mention processes sharing the PMD above, it is really mappings/vmas sharing the PMD. You could have two mappings of the same object in the same process sharing PMDs.
I'll code this up and see how it looks.
Thanks!
However, unless you have an objection I would prefer the simple patches move forward, especially for stable backports.
Yes, the current patch is much simpler and more suitable for stable backports. If the explicit map count modifications are not all that terrible then this would sound like a more appropriate long term plan though.
The approach mentioned above seems to be simple enough. Patch is below.
I 'tested' with the same method and tests used to measure fault scalabilty when developing vma based locking [1]. I figured this would be a good stress of the share, unshare and fault paths. With the patch, we are doing more with the page table lock held, so I expected to see a little difference in scalability, but not as much as actually measured:
next-20230131test instances unmodified patched
Combined faults 24 61888.4 58314.8 Combined forks 24 157.3 130.1
So faults are 6 % slower while forks are hit by 18%. This is quite a lot and more than I expected. pmd sharing shouldn't really be a common operation AFAICS. It should only happen with the first mapping in the pud (so once every 1GB/2MB faults for fully populated mappings).
It would be good to know whether this is purely lock contention based or the additional work in each #pf and unmapping makes a big impact as well.
These tests could seem a bit like a micro-benchmark targeting these code paths. However, I put them together based on the description of a customer workload that prompted the vma based locking work. And, performance of these tests seems to reflect performance of their workloads.
This extra overhead is the cost needed to make shared PMD map counts be accurate and in line with what is normal and expected. I think it is worth the cost. Other opinions? Of course, the patch below may have issues so please take a look.
If 18% slowdown really reflects a real workload then this might just be too expensive I am afraid.
[1] https://lore.kernel.org/linux-mm/20220914221810.95771-1-mike.kravetz@oracle....
From bff5a717521f96b0e5075ac4b5a1ef84a3589b7e Mon Sep 17 00:00:00 2001
From: Mike Kravetz mike.kravetz@oracle.com Date: Mon, 30 Jan 2023 20:14:14 -0800 Subject: [PATCH] hugetlb: Adjust hugetlbp page ref/map counts for PMD sharing
When hugetlb PMDS are shared, the sharing code simply adds the shared PMD to another processes page table. It will not update the ref/map counts of pages referenced by the shared PMD. As a result, the ref/map count will only reflect when the page was added to the shared PMD. Even though the shared PMD may be in MANY process page tables, ref/map counts on the pages will only appear to be that of a single process.
Update ref/map counts to take PMD sharing into account. This is done in three distinct places:
- At PMD share time in huge_pmd_share(), Go through all entries in the PMD, and increment map and ref count for all referenced pages. huge_pmd_share is just adding another use and mapping of each page.
- At PMD unshare time in huge_pmd_unshare(), Go through all entries in the PMD, and decrement map and ref count for all referenced pages. huge_pmd_unshare is just removing one use and mapping of each page.
- When faulting in a new hugetlb page, Check if we are adding a new entry to a shared PMD. If yes, add 'num_of_sharing__processes - 1' to the ref and map count.
Honestly, I didn't really have much time to think about this very deeply so I might be missing something here. The patch seems correct to me. adjust_shared_pmd_page_counts's delta parameter is confusing because it implies a delta adjustments while it justs want to be "bool increase" instead.
Thanks for looking into this Mike! [...]
+static void adjust_shared_pmd_page_counts(pmd_t *pmd_start, int delta) +{
- struct folio *folio;
- struct page *page;
- pte_t *ptep, pte;
- int i;
- for (i= 0; i < PTRS_PER_PMD; i++) {
ptep = (pte_t *)(pmd_start + i);pte = huge_ptep_get(ptep);if (huge_pte_none(pte) || !pte_present(pte))continue;page = pte_page(pte);folio = (struct folio *)page;if (delta > 0) {folio_get(folio);atomic_inc(&folio->_entire_mapcount);} else {folio_put(folio);atomic_dec(&folio->_entire_mapcount);}- }
+}
[...]
On 02/03/23 14:40, Michal Hocko wrote:
On Wed 01-02-23 13:05:35, Mike Kravetz wrote:
On 02/01/23 08:47, Michal Hocko wrote:
process.
- At page fault time, check if we are adding a new entry to a shared PMD. If yes, add 'num_of_sharing__processes - 1' to the ref and map count.
In each of the above operations, we are holding the PTL lock (which is really the split/PMD lock) so synchronization should not be an issue.
Although I mention processes sharing the PMD above, it is really mappings/vmas sharing the PMD. You could have two mappings of the same object in the same process sharing PMDs.
I'll code this up and see how it looks.
Thanks!
However, unless you have an objection I would prefer the simple patches move forward, especially for stable backports.
Yes, the current patch is much simpler and more suitable for stable backports. If the explicit map count modifications are not all that terrible then this would sound like a more appropriate long term plan though.
The approach mentioned above seems to be simple enough. Patch is below.
I 'tested' with the same method and tests used to measure fault scalabilty when developing vma based locking [1]. I figured this would be a good stress of the share, unshare and fault paths. With the patch, we are doing more with the page table lock held, so I expected to see a little difference in scalability, but not as much as actually measured:
next-20230131test instances unmodified patched
Combined faults 24 61888.4 58314.8 Combined forks 24 157.3 130.1
So faults are 6 % slower while forks are hit by 18%. This is quite a lot and more than I expected. pmd sharing shouldn't really be a common operation AFAICS. It should only happen with the first mapping in the pud (so once every 1GB/2MB faults for fully populated mappings).
Just want to be perfectly clear on what in being measured in these tests: - faults program does the following in a loop . mmap 250GB hugetlb file PUD_SIZE aligned . read fault each hugetlb page (so all on shared PMDs) . unmap file measurement is how many times this map/read/unmap loop is completed - fork program does the following . mmap 250GB hugetlb file PUD_SIZE aligned . read fault 3 pages on different PUDs . fork . child write faults 3 pages on different PUDs . child exits measurement is how many children can be created sequentially For the results above, 24 instances of the fault program are being run in parallel with 24 instances of the fork program.
It would be good to know whether this is purely lock contention based or the additional work in each #pf and unmapping makes a big impact as well.
I did not do a deep dive into the exact cause of the slowdown. Do note how much we are executing the PMD sharing paths in these tests. I did not really plan it that way, but was trying to simulate what might be happening in a customer environment.
These tests could seem a bit like a micro-benchmark targeting these code paths. However, I put them together based on the description of a customer workload that prompted the vma based locking work. And, performance of these tests seems to reflect performance of their workloads.
This extra overhead is the cost needed to make shared PMD map counts be accurate and in line with what is normal and expected. I think it is worth the cost. Other opinions? Of course, the patch below may have issues so please take a look.
If 18% slowdown really reflects a real workload then this might just be too expensive I am afraid.
On second thought, I tend to agree. The fixes already done cover all known exposures from inaccurate counts due to PMD sharing. If we want to move forward with the approach here, I would like to: - Do more analysis in order to explain exactly why this is happening. - Try to run the proposed patch is a more accurate customer environment simulation to determine if slowdown is actually visible. I do not have access to such an environment, so will require cooperation from external vendor/customer.
Unless someone thinks we should move forward, I will not push the code for this approach now. It will also be interesting to see if this is impacted at all by the outcome of discussions to perhaps redesign mapcount.
On Fri 03-02-23 12:16:04, Mike Kravetz wrote: [...]
Unless someone thinks we should move forward, I will not push the code for this approach now. It will also be interesting to see if this is impacted at all by the outcome of discussions to perhaps redesign mapcount.
Yes, I do agree. We might want to extend page_mapcount documentation a bit though. The comment is explicit about the order-0 pages but a note about hugetlb and pmd sharing wouldn't hurt. WDYT?
On 02/13/23 19:01, Michal Hocko wrote:
On Fri 03-02-23 12:16:04, Mike Kravetz wrote: [...]
Unless someone thinks we should move forward, I will not push the code for this approach now. It will also be interesting to see if this is impacted at all by the outcome of discussions to perhaps redesign mapcount.
Yes, I do agree. We might want to extend page_mapcount documentation a bit though. The comment is explicit about the order-0 pages but a note about hugetlb and pmd sharing wouldn't hurt. WDYT?
Looks like that comment about 'Mapcount of 0-order page' has been removed in the latest version of page_mapcount(). It would not surprise me if the calls to page_mapcount after which we check for shared PMDs will soon be replaced with calls to folio_mapcount().
Perhaps Matthew has an opinion as to where map counts for hugetlb shared PMDs might be mentioned.
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