On 1/15/20 12:32 PM, Filipe Manana wrote:
On Tue, Jan 7, 2020 at 7:43 PM Josef Bacik <josef@xxxxxxxxxxxxxx> wrote:
v1->v2:
- fixed a bug in 'btrfs: use the file extent tree infrastructure' that would
result in 0 length files because btrfs_truncate_inode_items() was clearing the
file extent map when we fsync'ed multiple times. Validated this with a
modified fsx and generic/521 that reproduced the problem, those modifications
were sent up as well.
- dropped the RFC
----------------- Original Message -----------------------
We've historically had this problem where you could flush a targeted section of
an inode and end up with a hole between extents without a hole extent item.
This of course makes fsck complain because this is not ok for a file system that
doesn't have NO_HOLES set. Because this is a well understood problem I and
others have been ignoring fsck failures during certain xfstests (generic/475 for
example) because they would regularly trigger this edge case.
However this isn't a great behavior to have, we should really be taking all fsck
failures seriously, and we could potentially ignore fsck legitimate fsck errors
because we expect it to be this particular failure.
In order to fix this we need to keep track of where we have valid extent items,
and only update i_size to encompass that area. This unfortunately means we need
a new per-inode extent_io_tree to keep track of the valid ranges. This is
relatively straightforward in practice, and helpers have been added to manage
this so that in the case of a NO_HOLES file system we just simply skip this work
altogether.
I've been hammering on this for a week now and I'm pretty sure its ok, but I'd
really like Filipe to take a look and I still have some longer running tests
going on the series. All of our boxes internally are btrfs and the box I was
testing on ended up with a weird RPM db corruption that was likely from an
earlier, broken version of the patch. However I cannot be 100% sure that was
the case, so I'm giving it a few more days of testing before I'm satisfied
there's not some weird thing that RPM does that xfstests doesn't cover.
This has gone through several iterations of xfstests already, including many
loops of generic/475 for validation to make sure it was no longer failing. So
far so good, but for something like this wider testing will definitely be
necessary. Thanks,
So a comment that applies to the whole patchset.
On power failures we can now end up with non-prealloc extents beyond
the disk_i_size after mounting the filesystem.
Not entirely sure if it will give any potential problems other then
non-reclaimed space for a long time (unless the file is truncated or
written to or beyond the extent's offset), have you tested this
scenario?
I suppose the test cases from fstests that use dm's log writes target
exercise this easily.
Yeah I've run it through xfstests a bunch and none of the log writes things blew up.
Keep in mind that this scenario can already happen, just not as easily. The
original btrfs_ordered_update_i_size() would only update i_size if the previous
ordered extent had completed. If it hadn't you would end up with a normal
extent past i_size, and if you crashed at the right time you would be in this
spot. This patch only makes that case more likely to happen if you happen to do
something like sync_file_range() in the middle of the dirty range. Thanks,
Josef
