On 11/4/19 10:52 AM, Jerome Glisse wrote:
On Sun, Nov 03, 2019 at 01:18:07PM -0800, John Hubbard wrote:
Add tracking of pages that were pinned via FOLL_PIN.
As mentioned in the FOLL_PIN documentation, callers who effectively set FOLL_PIN are required to ultimately free such pages via put_user_page(). The effect is similar to FOLL_GET, and may be thought of as "FOLL_GET for DIO and/or RDMA use".
Pages that have been pinned via FOLL_PIN are identifiable via a new function call:
bool page_dma_pinned(struct page *page);
What to do in response to encountering such a page, is left to later patchsets. There is discussion about this in [1].
This also changes a BUG_ON(), to a WARN_ON(), in follow_page_mask().
This also has a couple of trivial, non-functional change fixes to try_get_compound_head(). That function got moved to the top of the file.
Maybe split that as a separate trivial patch.
Will do.
This includes the following fix from Ira Weiny:
DAX requires detection of a page crossing to a ref count of 1. Fix this for GUP pages by introducing put_devmap_managed_user_page() which accounts for GUP_PIN_COUNTING_BIAS now used by GUP.
Please do the put_devmap_managed_page() changes in a separate patch, it would be a lot easier to follow, also on that front see comments below.
Oh! OK. It makes sense when you say it out loud. :)
...
+static inline bool put_devmap_managed_page(struct page *page) +{
- bool is_devmap = page_is_devmap_managed(page);
- if (is_devmap) {
int count = page_ref_dec_return(page);__put_devmap_managed_page(page, count);- }
- return is_devmap;
+}
I think the __put_devmap_managed_page() should be rename to free_devmap_managed_page() and that the count != 1 case move to this inline function ie:
static inline bool put_devmap_managed_page(struct page *page) { bool is_devmap = page_is_devmap_managed(page);
if (is_devmap) { int count = page_ref_dec_return(page);
/* * If refcount is 1 then page is freed and refcount is stable as nobody * holds a reference on the page. */ if (count == 1) free_devmap_managed_page(page, count); else if (!count) __put_page(page);}
return is_devmap; }
Thanks, that does look cleaner and easier to read.
#else /* CONFIG_DEV_PAGEMAP_OPS */ static inline bool put_devmap_managed_page(struct page *page) { @@ -1038,6 +1051,8 @@ static inline __must_check bool try_get_page(struct page *page) return true; } +__must_check bool user_page_ref_inc(struct page *page);
What about having it as an inline here as it is pretty small.
You mean move it to a static inline function in mm.h? It's worse than it looks, though: *everything* that it calls is also a static function, local to gup.c. So I'd have to expose both try_get_compound_head() and __update_proc_vmstat(). And that also means calling mod_node_page_state() from mm.h, and it goes south right about there. :)
...
+/**
- page_dma_pinned() - report if a page is pinned by a call to pin_user_pages*()
- or pin_longterm_pages*()
- @page: pointer to page to be queried.
- @Return: True, if it is likely that the page has been "dma-pinned".
False, if the page is definitely not dma-pinned.- */
Maybe add a small comment about wrap around :)
I don't *think* the count can wrap around, due to the checks in user_page_ref_inc().
But it's true that the documentation is a little light here...What did you have in mind?
[...]
@@ -1930,12 +2028,20 @@ static int __gup_device_huge(unsigned long pfn, unsigned long addr, pgmap = get_dev_pagemap(pfn, pgmap); if (unlikely(!pgmap)) {
undo_dev_pagemap(nr, nr_start, pages);
undo_dev_pagemap(nr, nr_start, flags, pages); return 0;
get_page(page);
if (flags & FOLL_PIN) {if (unlikely(!user_page_ref_inc(page))) {undo_dev_pagemap(nr, nr_start, flags, pages);return 0;}Maybe add a comment about a case that should never happens ie user_page_ref_inc() fails after the second iteration of the loop as it would be broken and a bug to call undo_dev_pagemap() after the first iteration of that loop.
Also i believe that this should never happens as if first iteration succeed than __page_cache_add_speculative() will succeed for all the iterations.
Note that the pgmap case above follows that too ie the call to get_dev_pagemap() can only fail on first iteration of the loop, well i assume you can never have a huge device page that span different pgmap ie different devices (which is a reasonable assumption). So maybe this code needs fixing ie :
pgmap = get_dev_pagemap(pfn, pgmap); if (unlikely(!pgmap)) return 0;
OK, yes that does make sense. And I think a comment is adequate, no need to check for bugs during every tail page iteration. So how about this, as a preliminary patch:
diff --git a/mm/gup.c b/mm/gup.c index 8f236a335ae9..a4a81e125832 100644 --- a/mm/gup.c +++ b/mm/gup.c @@ -1892,17 +1892,18 @@ static int gup_pte_range(pmd_t pmd, unsigned long addr, unsigned long end, static int __gup_device_huge(unsigned long pfn, unsigned long addr, unsigned long end, struct page **pages, int *nr) { - int nr_start = *nr; - struct dev_pagemap *pgmap = NULL; + /* + * Huge pages should never cross dev_pagemap boundaries. Therefore, use + * this same pgmap for the entire huge page. + */ + struct dev_pagemap *pgmap = get_dev_pagemap(pfn, NULL); + + if (unlikely(!pgmap)) + return 0;
do { struct page *page = pfn_to_page(pfn);
- pgmap = get_dev_pagemap(pfn, pgmap); - if (unlikely(!pgmap)) { - undo_dev_pagemap(nr, nr_start, pages); - return 0; - } SetPageReferenced(page); pages[*nr] = page; get_page(page);
} elseget_page(page);- (*nr)++; pfn++; } while (addr += PAGE_SIZE, addr != end);
[...]
@@ -2409,7 +2540,7 @@ static int internal_get_user_pages_fast(unsigned long start, int nr_pages, unsigned long addr, len, end; int nr = 0, ret = 0;
- if (WARN_ON_ONCE(gup_flags & ~(FOLL_WRITE | FOLL_LONGTERM)))
- if (WARN_ON_ONCE(gup_flags & ~(FOLL_WRITE | FOLL_LONGTERM | FOLL_PIN)))
Maybe add a comments to explain, something like:
/*
- The only flags allowed here are: FOLL_WRITE, FOLL_LONGTERM, FOLL_PIN
- Note that get_user_pages_fast() imply FOLL_GET flag by default but
- callers can over-ride this default to pin case by setting FOLL_PIN.
*/
Good idea. Here's the draft now:
/* * The only flags allowed here are: FOLL_WRITE, FOLL_LONGTERM, FOLL_PIN. * * Note that get_user_pages_fast() implies FOLL_GET flag by default, but * callers can override this default by setting FOLL_PIN instead of * FOLL_GET. */ if (WARN_ON_ONCE(gup_flags & ~(FOLL_WRITE | FOLL_LONGTERM | FOLL_PIN))) return -EINVAL;
return -EINVAL;start = untagged_addr(start) & PAGE_MASK; diff --git a/mm/huge_memory.c b/mm/huge_memory.c index 13cc93785006..66bf4c8b88f1 100644 --- a/mm/huge_memory.c +++ b/mm/huge_memory.c
[...]
@@ -968,7 +973,12 @@ struct page *follow_devmap_pmd(struct vm_area_struct *vma, unsigned long addr, if (!*pgmap) return ERR_PTR(-EFAULT); page = pfn_to_page(pfn);
- get_page(page);
- if (flags & FOLL_GET)
get_page(page);- else if (flags & FOLL_PIN)
if (unlikely(!user_page_ref_inc(page)))page = ERR_PTR(-ENOMEM);While i agree that user_page_ref_inc() (ie page_cache_add_speculative()) should never fails here as we are holding the pmd lock and thus no one can unmap the pmd and free the page it points to. I believe you should return -EFAULT like for the pgmap and not -ENOMEM as the pgmap should not fail either for the same reason. Thus it would be better to have consistent error. Maybe also add a comments explaining that it should not fail here.
OK. I'll take a pass through and fix up the remaining points about these sorts of cases below, as well, in v3. Those all make sense.
return page; }
[...]
@@ -1100,7 +1115,7 @@ struct page *follow_devmap_pud(struct vm_area_struct *vma, unsigned long addr, * device mapped pages can only be returned if the * caller will manage the page reference count. */
- if (!(flags & FOLL_GET))
- if (!(flags & (FOLL_GET | FOLL_PIN))) return ERR_PTR(-EEXIST);
Maybe add a comment that FOLL_GET or FOLL_PIN must be set.
pfn += (addr & ~PUD_MASK) >> PAGE_SHIFT; @@ -1108,7 +1123,12 @@ struct page *follow_devmap_pud(struct vm_area_struct *vma, unsigned long addr, if (!*pgmap) return ERR_PTR(-EFAULT); page = pfn_to_page(pfn);
- get_page(page);
- if (flags & FOLL_GET)
get_page(page);- else if (flags & FOLL_PIN)
if (unlikely(!user_page_ref_inc(page)))page = ERR_PTR(-ENOMEM);Same as for follow_devmap_pmd() see above.
return page; } @@ -1522,8 +1542,12 @@ struct page *follow_trans_huge_pmd(struct vm_area_struct *vma, skip_mlock: page += (addr & ~HPAGE_PMD_MASK) >> PAGE_SHIFT; VM_BUG_ON_PAGE(!PageCompound(page) && !is_zone_device_page(page), page);
- if (flags & FOLL_GET) get_page(page);
- else if (flags & FOLL_PIN)
if (unlikely(!user_page_ref_inc(page)))page = NULL;This should not fail either as we are holding the pmd lock maybe add a comment. Dunno if we want a WARN() or something to catch this degenerate case, or dump the page.
out: return page; diff --git a/mm/hugetlb.c b/mm/hugetlb.c index b45a95363a84..da335b1cd798 100644 --- a/mm/hugetlb.c +++ b/mm/hugetlb.c @@ -4462,7 +4462,17 @@ long follow_hugetlb_page(struct mm_struct *mm, struct vm_area_struct *vma, same_page: if (pages) { pages[i] = mem_map_offset(page, pfn_offset);
get_page(pages[i]);
if (flags & FOLL_GET)get_page(pages[i]);else if (flags & FOLL_PIN)if (unlikely(!user_page_ref_inc(pages[i]))) {spin_unlock(ptl);remainder = 0;err = -ENOMEM;WARN_ON_ONCE(1);break;}user_page_ref_inc() should not fail here either because we hold the ptl, so the WAR_ON_ONCE() is right but maybe add a comment.
if (vmas)
[...]
@@ -5034,8 +5050,14 @@ follow_huge_pmd(struct mm_struct *mm, unsigned long address, pte = huge_ptep_get((pte_t *)pmd); if (pte_present(pte)) { page = pmd_page(*pmd) + ((address & ~PMD_MASK) >> PAGE_SHIFT);
- if (flags & FOLL_GET) get_page(page);
else if (flags & FOLL_PIN)if (unlikely(!user_page_ref_inc(page))) {page = NULL;goto out;}This should not fail either (again holding pmd lock), dunno if we want a warn or something to catch this degenerate case.
} else { if (is_hugetlb_entry_migration(pte)) { spin_unlock(ptl);
[...]
Those are all good points, working on them now.
thanks,