From mboxrd@z Thu Jan 1 00:00:00 1970 From: punit.agrawal@arm.com (Punit Agrawal) Date: Mon, 22 May 2017 14:35:59 +0100 Subject: [PATCH v3 1/6] mm, gup: Remove broken VM_BUG_ON_PAGE compound check for hugepages In-Reply-To: <20170522133604.11392-1-punit.agrawal@arm.com> References: <20170522133604.11392-1-punit.agrawal@arm.com> Message-ID: <20170522133604.11392-2-punit.agrawal@arm.com> To: linux-arm-kernel@lists.infradead.org List-Id: linux-arm-kernel.lists.infradead.org From: Will Deacon 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. Cc: "Kirill A. Shutemov" Cc: Aneesh Kumar K.V Signed-off-by: Will Deacon Acked-by: Steve Capper Signed-off-by: Punit Agrawal --- mm/gup.c | 3 --- 1 file changed, 3 deletions(-) diff --git a/mm/gup.c b/mm/gup.c index d9e6fddcc51f..ccf8cb38234f 100644 --- a/mm/gup.c +++ b/mm/gup.c @@ -1361,7 +1361,6 @@ 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++; @@ -1400,7 +1399,6 @@ 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++; @@ -1438,7 +1436,6 @@ 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++; -- 2.11.0