[PATCH 3.10 002/268] Btrfs: fix memory leak in reading btree blocks

Willy Tarreau
2017-06-19T19:41:55Z

From: Liu Bo <bo.li.liu@oracle.com>

commit 2571e739677f1e4c0c63f5ed49adcc0857923625 upstream.

So we can read a btree block via readahead or intentional read,
and we can end up with a memory leak when something happens as
follows,
1) readahead starts to read block A but does not wait for read
completion,
2) btree_readpage_end_io_hook finds that block A is corrupted,
and it needs to clear all block A's pages' uptodate bit.
3) meanwhile an intentional read kicks in and checks block A's
pages' uptodate to decide which page needs to be read.
4) when some pages have the uptodate bit during 3)'s check so
3) doesn't count them for eb->io_pages, but they are later
cleared by 2) so we has to readpage on the page, we get
the wrong eb->io_pages which results in a memory leak of
this block.

This fixes the problem by firstly getting all pages's locking and
then checking pages' uptodate bit.

t1(readahead) t2(readahead endio) t3(the following read)
read_extent_buffer_pages end_bio_extent_readpage
for pg in eb: for page 0,1,2 in eb:
if pg is uptodate: btree_readpage_end_io_hook(pg)
num_reads++ if uptodate:
eb->io_pages = num_reads SetPageUptodate(pg) _______________
for pg in eb: for page 3 in eb: read_extent_buffer_pages
if pg is NOT uptodate: btree_readpage_end_io_hook(pg) for pg in eb:
__extent_read_full_page(pg) sanity check reports something wrong if pg is uptodate:
clear_extent_buffer_uptodate(eb) num_reads++
for pg in eb: eb->io_pages = num_reads
ClearPageUptodate(page) _______________
for pg in eb:
if pg is NOT uptodate:
__extent_read_full_page(pg)

So t3's eb->io_pages is not consistent with the number of pages it's reading,
and during endio(), atomic_dec_and_test(&eb->io_pages) will get a negative
number so that we're not able to free the eb.

Signed-off-by: Liu Bo <bo.li.liu@oracle.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>
Signed-off-by: Willy Tarreau <w@1wt.eu>
---
fs/btrfs/extent_io.c | 9 +++++++++
1 file changed, 9 insertions(+)

diff --git a/fs/btrfs/extent_io.c b/fs/btrfs/extent_io.c
index be7e31a..7f0d9be 100644
--- a/fs/btrfs/extent_io.c
+++ b/fs/btrfs/extent_io.c
@@ -4661,11 +4661,20 @@ int read_extent_buffer_pages(struct extent_io_tree *tree,
lock_page(page);
}
locked_pages++;
+ }
+ /*
+ * We need to firstly lock all pages to make sure that
+ * the uptodate bit of our pages won't be affected by
+ * clear_extent_buffer_uptodate().
+ */
+ for (i = start_i; i < num_pages; i++) {
+ page = eb->pages[i];
if (!PageUptodate(page)) {
num_reads++;
all_uptodate = 0;
}
}
+
if (all_uptodate) {
if (start_i == 0)
set_bit(EXTENT_BUFFER_UPTODATE, &eb->bflags);
--
2.8.0.rc2.1.gbe9624a