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mm,thp: recheck each page before collapsing file THP

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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
happens.

syzbot reported bug on previous version of this patch.

Link: http://lkml.kernel.org/r/20191106060930.2571389-2-songliubraving@fb.com
Fixes: 99cb0dbd47 ("mm,thp: add read-only THP support for (non-shmem) FS")
Signed-off-by: Song Liu <songliubraving@fb.com>
Reported-by: syzbot+efb9e48b9fbdc49bb34a@syzkaller.appspotmail.com
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Cc: Hugh Dickins <hughd@google.com>
Cc: William Kucharski <william.kucharski@oracle.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
This commit is contained in:
Song Liu 2019-11-15 17:34:53 -08:00 committed by Linus Torvalds
parent aea4df4c53
commit 4655e5e5f3
1 changed files with 16 additions and 12 deletions
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28
mm/khugepaged.c
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@ -1602,17 +1602,6 @@ static void collapse_file(struct mm_struct *mm,
result = SCAN_FAIL;
goto xa_unlocked;
}
} else if (!PageUptodate(page)) {
xas_unlock_irq(&xas);
wait_on_page_locked(page);
if (!trylock_page(page)) {
result = SCAN_PAGE_LOCK;
goto xa_unlocked;
}
get_page(page);
} else if (PageDirty(page)) {
result = SCAN_FAIL;
goto xa_locked;
} else if (trylock_page(page)) {
get_page(page);
xas_unlock_irq(&xas);
@ -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)) {
result = SCAN_DEL_PAGE_LRU;
goto out_unlock;