| Message ID | 1470252781-9455-1-git-send-email-bo.li.liu@oracle.com (mailing list archive) |
|---|---|
| State | Accepted |
| Headers | show |
Hi David, On Wed, Aug 03, 2016 at 12:33:01PM -0700, Liu Bo wrote: > 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. Thanks, -liubo > > Signed-off-by: Liu Bo <bo.li.liu@oracle.com> > --- > 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 bd29b9b..a77050e 100644 > --- a/fs/btrfs/extent_io.c > +++ b/fs/btrfs/extent_io.c > @@ -5215,11 +5215,20 @@ 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); > -- > 2.5.5 > > -- > To unsubscribe from this list: send the line "unsubscribe linux-btrfs" in > the body of a message to majordomo@vger.kernel.org > More majordomo info at http://vger.kernel.org/majordomo-info.html -- To unsubscribe from this list: send the line "unsubscribe linux-btrfs" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
On Wed, Aug 24, 2016 at 04:15:50PM -0700, Liu Bo wrote: > Hi David, > > On Wed, Aug 03, 2016 at 12:33:01PM -0700, Liu Bo wrote: > > 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. Thanks for the details, that helped me. I'll add the schematic to the commit log. Reviewed-by: David Sterba <dsterba@suse.com> -- To unsubscribe from this list: send the line "unsubscribe linux-btrfs" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
diff --git a/fs/btrfs/extent_io.c b/fs/btrfs/extent_io.c index bd29b9b..a77050e 100644 --- a/fs/btrfs/extent_io.c +++ b/fs/btrfs/extent_io.c @@ -5215,11 +5215,20 @@ 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);
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. Signed-off-by: Liu Bo <bo.li.liu@oracle.com> --- fs/btrfs/extent_io.c | 9 +++++++++ 1 file changed, 9 insertions(+)