3.16.82-rc1 review patch. If anyone has any objections, please let me know.
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From: Josef Bacik josef@toxicpanda.com
commit 3797136b626ad4b6582223660c041efdea8f26b2 upstream.
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.
This page being unlocked as: btrfs_readpage extent_read_full_page __extent_read_full_page __do_readpage if (!nr) unlock_page <-- nr can be 0 only if submit_extent_page returns an error
Reviewed-by: Filipe Manana fdmanana@suse.com Reviewed-by: Nikolay Borisov nborisov@suse.com Signed-off-by: Josef Bacik josef@toxicpanda.com [ add callchain ] Signed-off-by: David Sterba dsterba@suse.com Signed-off-by: Ben Hutchings ben@decadent.org.uk --- fs/btrfs/free-space-cache.c | 6 ++++++ 1 file changed, 6 insertions(+)
--- a/fs/btrfs/free-space-cache.c +++ b/fs/btrfs/free-space-cache.c @@ -360,6 +360,12 @@ static int io_ctl_prepare_pages(struct i 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");