Commit 4655e5e5 authored by Song Liu's avatar Song Liu Committed by Linus Torvalds
Browse files

mm,thp: recheck each page before collapsing file THP

In collapse_file(), for !is_shmem case, current check cannot guarantee
the locked page is up-to-date.  Specifically, xas_unlock_irq() should
not be called before lock_page() and get_page(); and it is necessary to
recheck PageUptodate() after locking the page.

With this bug and CONFIG_READ_ONLY_THP_FOR_FS=y, madvise(HUGE)'ed .text
may contain corrupted data.  This is because khugepaged mistakenly
collapses some not up-to-date sub pages into a huge page, and assumes
the huge page is up-to-date.  This will NOT corrupt data in the disk,
because the page is read-only and never written back.  Fix this by
properly checking PageUptodate() after locking the page.  This check
replaces "VM_BUG_ON_PAGE(!PageUptodate(page), page);".

Also, move PageDirty() check after locking the page.  Current khugepaged
should not try to collapse dirty file THP, because it is limited to
read-only .text.  The only case we hit a dirty page here is when the
page hasn't been written since write.  Bail out and retry when this

syzbot reported bug on previous version of this patch.

Fixes: 99cb0dbd

 ("mm,thp: add read-only THP support for (non-shmem) FS")
Signed-off-by: default avatarSong Liu <>
Reported-by: default avatar <>
Cc: Johannes Weiner <>
Cc: Kirill A. Shutemov <>
Cc: Hugh Dickins <>
Cc: William Kucharski <>
Signed-off-by: default avatarAndrew Morton <>
Signed-off-by: default avatarLinus Torvalds <>
parent aea4df4c
......@@ -1602,17 +1602,6 @@ static void collapse_file(struct mm_struct *mm,
result = SCAN_FAIL;
goto xa_unlocked;
} else if (!PageUptodate(page)) {
if (!trylock_page(page)) {
result = SCAN_PAGE_LOCK;
goto xa_unlocked;
} else if (PageDirty(page)) {
result = SCAN_FAIL;
goto xa_locked;
} else if (trylock_page(page)) {
......@@ -1627,7 +1616,12 @@ static void collapse_file(struct mm_struct *mm,
* without racing with truncate.
VM_BUG_ON_PAGE(!PageLocked(page), page);
VM_BUG_ON_PAGE(!PageUptodate(page), page);
/* make sure the page is up to date */
if (unlikely(!PageUptodate(page))) {
result = SCAN_FAIL;
goto out_unlock;
* If file was truncated then extended, or hole-punched, before
......@@ -1643,6 +1637,16 @@ static void collapse_file(struct mm_struct *mm,
goto out_unlock;
if (!is_shmem && PageDirty(page)) {
* khugepaged only works on read-only fd, so this
* page is dirty because it hasn't been flushed
* since first write.
result = SCAN_FAIL;
goto out_unlock;
if (isolate_lru_page(page)) {
goto out_unlock;
Supports Markdown
0% or .
You are about to add 0 people to the discussion. Proceed with caution.
Finish editing this message first!
Please register or to comment