From mboxrd@z Thu Jan 1 00:00:00 1970 From: Punit Agrawal Subject: [PATCH v5 3/8] mm, gup: Remove broken VM_BUG_ON_PAGE compound check for hugepages Date: Mon, 19 Jun 2017 18:01:40 +0100 Message-ID: <20170619170145.25577-4-punit.agrawal@arm.com> References: <20170619170145.25577-1-punit.agrawal@arm.com> Return-path: In-Reply-To: <20170619170145.25577-1-punit.agrawal@arm.com> Sender: owner-linux-mm@kvack.org To: akpm@linux-foundation.org Cc: Will Deacon , linux-mm@kvack.org, linux-kernel@vger.kernel.org, linux-arm-kernel@lists.infradead.org, catalin.marinas@arm.com, n-horiguchi@ah.jp.nec.com, kirill.shutemov@linux.intel.com, mike.kravetz@oracle.com, steve.capper@arm.com, mark.rutland@arm.com, linux-arch@vger.kernel.org, aneesh.kumar@linux.vnet.ibm.com, Punit Agrawal List-Id: linux-arch.vger.kernel.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. Signed-off-by: Will Deacon Acked-by: Steve Capper Signed-off-by: Punit Agrawal Acked-by: "Kirill A. Shutemov" Cc: Aneesh Kumar K.V --- mm/gup.c | 3 --- 1 file changed, 3 deletions(-) diff --git a/mm/gup.c b/mm/gup.c index b3c7214d710d..e74e0b5a0c7c 100644 --- a/mm/gup.c +++ b/mm/gup.c @@ -1357,7 +1357,6 @@ 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++; @@ -1396,7 +1395,6 @@ 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++; @@ -1434,7 +1432,6 @@ 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 -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@kvack.org. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: email@kvack.org From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from foss.arm.com ([217.140.101.70]:54468 "EHLO foss.arm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752520AbdFSRCp (ORCPT ); Mon, 19 Jun 2017 13:02:45 -0400 From: Punit Agrawal Subject: [PATCH v5 3/8] mm, gup: Remove broken VM_BUG_ON_PAGE compound check for hugepages Date: Mon, 19 Jun 2017 18:01:40 +0100 Message-ID: <20170619170145.25577-4-punit.agrawal@arm.com> In-Reply-To: <20170619170145.25577-1-punit.agrawal@arm.com> References: <20170619170145.25577-1-punit.agrawal@arm.com> Sender: linux-arch-owner@vger.kernel.org List-ID: To: akpm@linux-foundation.org Cc: Will Deacon , linux-mm@kvack.org, linux-kernel@vger.kernel.org, linux-arm-kernel@lists.infradead.org, catalin.marinas@arm.com, n-horiguchi@ah.jp.nec.com, kirill.shutemov@linux.intel.com, mike.kravetz@oracle.com, steve.capper@arm.com, mark.rutland@arm.com, linux-arch@vger.kernel.org, aneesh.kumar@linux.vnet.ibm.com, Punit Agrawal Message-ID: <20170619170140.Vqx8vBtEWwpWWqFK7zJ4vDx4xosCJobE2mGmkfSNtDc@z> 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. Signed-off-by: Will Deacon Acked-by: Steve Capper Signed-off-by: Punit Agrawal Acked-by: "Kirill A. Shutemov" Cc: Aneesh Kumar K.V --- mm/gup.c | 3 --- 1 file changed, 3 deletions(-) diff --git a/mm/gup.c b/mm/gup.c index b3c7214d710d..e74e0b5a0c7c 100644 --- a/mm/gup.c +++ b/mm/gup.c @@ -1357,7 +1357,6 @@ 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++; @@ -1396,7 +1395,6 @@ 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++; @@ -1434,7 +1432,6 @@ 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