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mm, gup: remove broken VM_BUG_ON_PAGE compound check for hugepages

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When operating on hugepages with DEBUG_VM enabled, the GUP code checks
the compound head for each tail page prior to calling
page_cache_add_speculative.  This is broken, because on the fast-GUP
path (where we don't hold any page table locks) we can be racing with a
concurrent invocation of split_huge_page_to_list.

split_huge_page_to_list deals with this race by using page_ref_freeze to
freeze the page and force concurrent GUPs to fail whilst the component
pages are modified.  This modification includes clearing the
compound_head field for the tail pages, so checking this prior to a
successful call to page_cache_add_speculative can lead to false
positives: In fact, page_cache_add_speculative *already* has this check
once the page refcount has been successfully updated, so we can simply
remove the broken calls to VM_BUG_ON_PAGE.

Link: http://lkml.kernel.org/r/[email protected]
Signed-off-by: Will Deacon <[email protected]>
Signed-off-by: Punit Agrawal <[email protected]>
Acked-by: Steve Capper <[email protected]>
Acked-by: Kirill A. Shutemov <[email protected]>
Cc: Aneesh Kumar K.V <[email protected]>
Cc: Catalin Marinas <[email protected]>
Cc: Naoya Horiguchi <[email protected]>
Cc: Mark Rutland <[email protected]>
Cc: Hillf Danton <[email protected]>
Cc: Michal Hocko <[email protected]>
Cc: Mike Kravetz <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>
This commit is contained in:
Will Deacon 2017-07-06 15:39:36 -07:00 • committed by Linus Torvalds
commit a3e328556d
1 changed files with 0 additions and 3 deletions
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3
mm/gup.c
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@ -1426,7 +1426,6 @@ static int gup_huge_pmd(pmd_t orig, pmd_t *pmdp, unsigned long addr,
head = pmd_page(orig);
page = head + ((addr & ~PMD_MASK) >> PAGE_SHIFT);
do {
VM_BUG_ON_PAGE(compound_head(page) != head, page);
pages[*nr] = page;
(*nr)++;
page++;
@ -1465,7 +1464,6 @@ static int gup_huge_pud(pud_t orig, pud_t *pudp, unsigned long addr,
head = pud_page(orig);
page = head + ((addr & ~PUD_MASK) >> PAGE_SHIFT);
do {
VM_BUG_ON_PAGE(compound_head(page) != head, page);
pages[*nr] = page;
(*nr)++;
page++;
@ -1503,7 +1501,6 @@ static int gup_huge_pgd(pgd_t orig, pgd_t *pgdp, unsigned long addr,
head = pgd_page(orig);
page = head + ((addr & ~PGDIR_MASK) >> PAGE_SHIFT);
do {
VM_BUG_ON_PAGE(compound_head(page) != head, page);
pages[*nr] = page;
(*nr)++;
page++;