From 8b37c40503eadc6fdbc4619455dc831d041eea15 Mon Sep 17 00:00:00 2001 From: Hugh Dickins Date: Fri, 30 Nov 2018 14:10:47 -0800 Subject: [PATCH] mm/khugepaged: collapse_shmem() do not crash on Compound commit 06a5e1268a5fb9c2b346a3da6b97e85f2eba0f07 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: f3f0e1d2150b2 ("khugepaged: add support of collapse for tmpfs/shmem pages") Signed-off-by: Hugh Dickins Cc: Kirill A. Shutemov Cc: Jerome Glisse Cc: Konstantin Khlebnikov Cc: Matthew Wilcox Cc: [4.8+] Signed-off-by: Andrew Morton Signed-off-by: Linus Torvalds Signed-off-by: Sasha Levin --- mm/khugepaged.c | 10 +++++++++- 1 file changed, 9 insertions(+), 1 deletion(-) diff --git a/mm/khugepaged.c b/mm/khugepaged.c index e2b13c0..fde5820 100644 --- a/mm/khugepaged.c +++ b/mm/khugepaged.c @@ -1398,7 +1398,15 @@ static void collapse_shmem(struct mm_struct *mm, */ VM_BUG_ON_PAGE(!PageLocked(page), page); VM_BUG_ON_PAGE(!PageUptodate(page), page); - VM_BUG_ON_PAGE(PageTransCompound(page), page); + + /* + * If file was truncated then extended, or hole-punched, before + * we locked the first page, then a THP might be there already. + */ + if (PageTransCompound(page)) { + result = SCAN_PAGE_COMPOUND; + goto out_unlock; + } if (page_mapping(page) != mapping) { result = SCAN_TRUNCATED; -- 2.7.4