mm/khugepaged: collapse_shmem() do not crash on Compound
commit06a5e1268a
upstream. collapse_shmem()'s VM_BUG_ON_PAGE(PageTransCompound) was unsafe: before it holds page lock of the first page, racing truncation then extension might conceivably have inserted a hugepage there already. Fail with the SCAN_PAGE_COMPOUND result, instead of crashing (CONFIG_DEBUG_VM=y) or otherwise mishandling the unexpected hugepage - though later we might code up a more constructive way of handling it, with SCAN_SUCCESS. Link: http://lkml.kernel.org/r/alpine.LSU.2.11.1811261529310.2275@eggly.anvils Fixes:f3f0e1d215
("khugepaged: add support of collapse for tmpfs/shmem pages") Signed-off-by: Hugh Dickins <hughd@google.com> Cc: Kirill A. Shutemov <kirill.shutemov@linux.intel.com> Cc: Jerome Glisse <jglisse@redhat.com> Cc: Konstantin Khlebnikov <khlebnikov@yandex-team.ru> Cc: Matthew Wilcox <willy@infradead.org> Cc: <stable@vger.kernel.org> [4.8+] Signed-off-by: Andrew Morton <akpm@linux-foundation.org> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org> Signed-off-by: Sasha Levin <sashal@kernel.org>
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@ -1400,7 +1400,15 @@ static void collapse_shmem(struct mm_struct *mm,
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*/
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VM_BUG_ON_PAGE(!PageLocked(page), page);
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VM_BUG_ON_PAGE(!PageUptodate(page), page);
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VM_BUG_ON_PAGE(PageTransCompound(page), page);
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/*
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* If file was truncated then extended, or hole-punched, before
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* we locked the first page, then a THP might be there already.
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*/
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if (PageTransCompound(page)) {
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result = SCAN_PAGE_COMPOUND;
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goto out_unlock;
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}
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if (page_mapping(page) != mapping) {
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result = SCAN_TRUNCATED;
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