While testing 5.2 we ran into the following panic
[52238.017028] BUG: kernel NULL pointer dereference, address: 0000000000000001
[52238.105608] RIP: 0010:drop_buffers+0x3d/0x150
[52238.304051] Call Trace:
[52238.308958] try_to_free_buffers+0x15b/0x1b0
[52238.317503] shrink_page_list+0x1164/0x1780
[52238.325877] shrink_inactive_list+0x18f/0x3b0
[52238.334596] shrink_node_memcg+0x23e/0x7d0
[52238.342790] ? do_shrink_slab+0x4f/0x290
[52238.350648] shrink_node+0xce/0x4a0
[52238.357628] balance_pgdat+0x2c7/0x510
[52238.365135] kswapd+0x216/0x3e0
[52238.371425] ? wait_woken+0x80/0x80
[52238.378412] ? balance_pgdat+0x510/0x510
[52238.386265] kthread+0x111/0x130
[52238.392727] ? kthread_create_on_node+0x60/0x60
[52238.401782] ret_from_fork+0x1f/0x30
The page we were trying to drop had a page->private, but had no
page->mapping and so called drop_buffers, assuming that we had a
buffer_head on the page, and then panic'ed trying to deref 1, which is
our page->private for data pages.
This is happening because we're truncating the free space cache while
we're trying to load the free space cache. This isn't supposed to
happen, and I'll fix that in a followup patch. However we still
shouldn't allow those sort of mistakes to result in messing with pages
that do not belong to us. So add the page->mapping check to verify that
we still own this page after dropping and re-acquiring the page lock.
Signed-off-by: Josef Bacik <jo...@toxicpanda.com>
---
fs/btrfs/free-space-cache.c | 6 ++++++
1 file changed, 6 insertions(+)
diff --git a/fs/btrfs/free-space-cache.c b/fs/btrfs/free-space-cache.c
index d54dcd0ab230..d86ada9c3c54 100644
--- a/fs/btrfs/free-space-cache.c
+++ b/fs/btrfs/free-space-cache.c
@@ -385,6 +385,12 @@ static int io_ctl_prepare_pages(struct btrfs_io_ctl
*io_ctl, struct inode *inode
if (uptodate && !PageUptodate(page)) {
btrfs_readpage(NULL, page);
lock_page(page);
+ if (page->mapping != inode->i_mapping) {
+ btrfs_err(BTRFS_I(inode)->root->fs_info,
+ "free space cache page truncated");
+ io_ctl_drop_pages(io_ctl);
+ return -EIO;
+ }
if (!PageUptodate(page)) {
btrfs_err(BTRFS_I(inode)->root->fs_info,
"error reading free space cache");
--
2.21.0
