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From:   Vlastimil Babka <vbabka@suse.cz>
To:     stable@vger.kernel.org
Cc:     linux-mm@kvack.org, linux-kernel@vger.kernel.org,
        Ajay Kaher <akaher@vmware.com>,
        Will Deacon <will.deacon@arm.com>,
        Punit Agrawal <punit.agrawal@arm.com>,
        Steve Capper <steve.capper@arm.com>,
        "Kirill A . Shutemov" <kirill.shutemov@linux.intel.com>,
        "Aneesh Kumar K . V" <aneesh.kumar@linux.vnet.ibm.com>,
        Catalin Marinas <catalin.marinas@arm.com>,
        Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>,
        Mark Rutland <mark.rutland@arm.com>,
        Hillf Danton <hillf.zj@alibaba-inc.com>,
        Michal Hocko <mhocko@suse.com>,
        Mike Kravetz <mike.kravetz@oracle.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Vlastimil Babka <vbabka@suse.cz>
Subject: [PATCH STABLE 4.4 1/8] mm, gup: remove broken VM_BUG_ON_PAGE compound check for hugepages
Date:   Fri,  8 Nov 2019 10:38:07 +0100
Message-Id: <20191108093814.16032-2-vbabka@suse.cz>
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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: Vlastimil Babka <vbabka@suse.cz>
---
 mm/gup.c | 3 ---
 1 file changed, 3 deletions(-)

diff --git a/mm/gup.c b/mm/gup.c
index 2cd3b31e3666..6f9088cb8ebe 100644
--- a/mm/gup.c
+++ b/mm/gup.c
@@ -1134,7 +1134,6 @@ static int gup_huge_pmd(pmd_t orig, pmd_t *pmdp, unsigned long addr,
 	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_t *pudp, unsigned long addr,
 	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_t *pgdp, unsigned long addr,
 	page = head + ((addr & ~PGDIR_MASK) >> PAGE_SHIFT);
 	tail = page;
 	do {
-		VM_BUG_ON_PAGE(compound_head(page) != head, page);
 		pages[*nr] = page;
 		(*nr)++;
 		page++;
-- 
2.23.0