From: Will Deacon will.deacon@arm.com
commit a3e328556d41bb61c55f9dfcc62d6a826ea97b85 upstream.
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/20170522133604.11392-2-punit.agrawal@arm.com Signed-off-by: Will Deacon will.deacon@arm.com Signed-off-by: Punit Agrawal punit.agrawal@arm.com Acked-by: Steve Capper steve.capper@arm.com Acked-by: Kirill A. Shutemov kirill.shutemov@linux.intel.com Cc: Aneesh Kumar K.V aneesh.kumar@linux.vnet.ibm.com Cc: Catalin Marinas catalin.marinas@arm.com Cc: Naoya Horiguchi n-horiguchi@ah.jp.nec.com Cc: Mark Rutland mark.rutland@arm.com Cc: Hillf Danton hillf.zj@alibaba-inc.com Cc: Michal Hocko mhocko@suse.com Cc: Mike Kravetz mike.kravetz@oracle.com Signed-off-by: Andrew Morton akpm@linux-foundation.org Signed-off-by: Linus Torvalds torvalds@linux-foundation.org Signed-off-by: Ben Hutchings ben.hutchings@codethink.co.uk Signed-off-by: Greg Kroah-Hartman gregkh@linuxfoundation.org --- mm/gup.c | 3 --- 1 file changed, 3 deletions(-)
--- a/mm/gup.c +++ b/mm/gup.c @@ -1316,7 +1316,6 @@ static int gup_huge_pmd(pmd_t orig, pmd_ 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++; @@ -1351,7 +1350,6 @@ static int gup_huge_pud(pud_t orig, pud_ 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++; @@ -1387,7 +1385,6 @@ static int gup_huge_pgd(pgd_t orig, pgd_ 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++;