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);}- }
+}
[...]