4.9-stable review patch. If anyone has any objections, please let me know.
------------------ 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/[email protected] Fixes: f3f0e1d2150b2 ("khugepaged: add support of collapse for tmpfs/shmem pages") Signed-off-by: Hugh Dickins <[email protected]> Cc: Kirill A. Shutemov <[email protected]> Cc: Jerome Glisse <[email protected]> Cc: Konstantin Khlebnikov <[email protected]> Cc: Matthew Wilcox <[email protected]> Cc: <[email protected]> [4.8+] Signed-off-by: Andrew Morton <[email protected]> Signed-off-by: Linus Torvalds <[email protected]> Signed-off-by: Sasha Levin <[email protected]> --- mm/khugepaged.c | 10 +++++++++- 1 file changed, 9 insertions(+), 1 deletion(-) diff --git a/mm/khugepaged.c b/mm/khugepaged.c index b87bd43993bd..e0cfc3a54b6a 100644 --- a/mm/khugepaged.c +++ b/mm/khugepaged.c @@ -1400,7 +1400,15 @@ 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; -- 2.17.1
