From: Will Deacon <will.deacon@xxxxxxx> 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@xxxxxxx Signed-off-by: Will Deacon <will.deacon@xxxxxxx> Signed-off-by: Punit Agrawal <punit.agrawal@xxxxxxx> Acked-by: Steve Capper <steve.capper@xxxxxxx> Acked-by: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx> Cc: Aneesh Kumar K.V <aneesh.kumar@xxxxxxxxxxxxxxxxxx> Cc: Catalin Marinas <catalin.marinas@xxxxxxx> Cc: Naoya Horiguchi <n-horiguchi@xxxxxxxxxxxxx> Cc: Mark Rutland <mark.rutland@xxxxxxx> Cc: Hillf Danton <hillf.zj@xxxxxxxxxxxxxxx> Cc: Michal Hocko <mhocko@xxxxxxxx> Cc: Mike Kravetz <mike.kravetz@xxxxxxxxxx> Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> Signed-off-by: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx> Signed-off-by: Srivatsa S. Bhat (VMware) <srivatsa@xxxxxxxxxxxxx> Signed-off-by: Ajay Kaher <akaher@xxxxxxxxxx> Signed-off-by: Vlastimil Babka <vbabka@xxxxxxx> Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx> --- mm/gup.c | 3 --- 1 file changed, 3 deletions(-) --- a/mm/gup.c +++ b/mm/gup.c @@ -1134,7 +1134,6 @@ static int gup_huge_pmd(pmd_t orig, pmd_ page = head + ((addr & ~PMD_MASK) >> PAGE_SHIFT); tail = page; do { - VM_BUG_ON_PAGE(compound_head(page) != head, page); pages[*nr] = page; (*nr)++; page++; @@ -1181,7 +1180,6 @@ static int gup_huge_pud(pud_t orig, pud_ page = head + ((addr & ~PUD_MASK) >> PAGE_SHIFT); tail = page; do { - VM_BUG_ON_PAGE(compound_head(page) != head, page); pages[*nr] = page; (*nr)++; page++; @@ -1224,7 +1222,6 @@ static int gup_huge_pgd(pgd_t orig, pgd_ page = head + ((addr & ~PGDIR_MASK) >> PAGE_SHIFT); tail = page; do { - VM_BUG_ON_PAGE(compound_head(page) != head, page); pages[*nr] = page; (*nr)++; page++;
