From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-pf0-f198.google.com (mail-pf0-f198.google.com [209.85.192.198]) by kanga.kvack.org (Postfix) with ESMTP id 5C60D831F4 for ; Mon, 22 May 2017 09:36:48 -0400 (EDT) Received: by mail-pf0-f198.google.com with SMTP id y65so128876683pff.13 for ; Mon, 22 May 2017 06:36:48 -0700 (PDT) Received: from foss.arm.com (foss.arm.com. [217.140.101.70]) by mx.google.com with ESMTP id o82si17383529pfi.82.2017.05.22.06.36.45 for ; Mon, 22 May 2017 06:36:45 -0700 (PDT) From: Punit Agrawal Subject: [PATCH v3 1/6] mm, gup: Remove broken VM_BUG_ON_PAGE compound check for hugepages Date: Mon, 22 May 2017 14:35:59 +0100 Message-Id: <20170522133604.11392-2-punit.agrawal@arm.com> In-Reply-To: <20170522133604.11392-1-punit.agrawal@arm.com> References: <20170522133604.11392-1-punit.agrawal@arm.com> Sender: owner-linux-mm@kvack.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, hillf.zj@alibaba-inc.com, linux-arch@vger.kernel.org, aneesh.kumar@linux.vnet.ibm.com, Punit Agrawal 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 -- 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