On Wed, Apr 19, 2017 at 09:08:54AM -0700, Darrick J. Wong wrote:
> On Wed, Apr 19, 2017 at 09:40:20AM -0500, Eric Sandeen wrote:
> > On 4/19/17 7:32 AM, Brian Foster wrote:
> > > On Tue, Apr 18, 2017 at 05:29:27PM -0700, Darrick J. Wong wrote:
> > >> On Tue, Apr 18, 2017 at 07:14:03PM -0500, Eric Sandeen wrote:
> > >>> On 4/18/17 7:09 PM, Darrick J. Wong wrote:
> > >>>> On Tue, Apr 18, 2017 at 12:55:44PM -0400, Brian Foster wrote:
> > >>>>> On Mon, Apr 17, 2017 at 05:12:36PM -0500, Eric Sandeen wrote:
> > >>>>>> On 4/17/17 3:57 PM, Brian Foster wrote:
> > >>>>>>> On Thu, Apr 13, 2017 at 01:45:43PM -0500, Eric Sandeen wrote:
> > >>>>> ...
> > >>>>>>>
> > >>>>>>> This fix seems fine to me, but I'm wondering if this code may have
> > >>>>>>> issues with other kinds of misalignment between the directory blocks and
> > >>>>>>> underlying bmap extents as well. For example, what happens if we end up
> > >>>>>>> with something like the following on an 8k dir fsb fs?
> > >>>>>>>
> > >>>>>>> 0:[0,xxx,3,0]
> > >>>>>>> 1:[3,xxx,1,0]
> > >>>>>>>
> > >>>>>>> ... or ...
> > >>>>>>>
> > >>>>>>> 0:[0,xxx,3,0]
> > >>>>>>> 1:[3,xxx,3,0]
> > >>>>>>
> > >>>>>> Well, as far as that goes it won't be an issue; for 8k dir block sizes
> > >>>>>> we will allocate an extent map with room for 10 extents, and we'll go
> > >>>>>> well beyond the above extents which cross directory block boundaries.
> > >>>>>>
> > >>>>>>> ...
> > >>>>>>> N:[...]
> > >>>>>>>
> > >>>>>>> Am I following correctly that we may end up assuming the wrong mapping
> > >>>>>>> for the second dir fsb and/or possibly skipping blocks?
> > >>>>>>
> > >>>>>> As far as I can tell, this code is only managing the read-ahead state
> > >>>>>> by looking at these cached extents. We keep track of our position within
> > >>>>>> that allocated array of mappings - this bug just stepped off the end
> > >>>>>> while doing so.
> > >>>>>>
> > >>>>>> Stopping at the correct point should keep all of the state consistent
> > >>>>>> and correct.
> > >>>>>>
> > >>>>>> But yeah, it's kind of hairy & hard to read, IMHO.
> > >>>>>>
> > >>>>>> Also as far as I can tell, we handle such discontiguities correctly,
> > >>>>>> other than the bug I found. If you see something that looks suspicious,
> > >>>>>> I'm sure I could tweak my test case to craft a specific situation if
> > >>>>>> there's something you'd like to see tested...
> > >>>>>>
> > >>>>>
> > >>>>> Background: Eric and I chatted a bit on irc to rectify that what I'm
> > >>>>> calling out above is a different issue from what is fixed by this patch.
> > >>>>>
> > >>>>> Eric,
> > >>>>>
> > >>>>> I managed to construct a directory that looks like this:
> > >>>>>
> > >>>>> EXT: FILE-OFFSET BLOCK-RANGE AG AG-OFFSET TOTAL
> > >>>>> 0: [0..7]: 88..95 0 (88..95) 8
> > >>>>> 1: [8..15]: 80..87 0 (80..87) 8
> > >>>>> 2: [16..39]: 168..191 0 (168..191) 24
> > >>>>> 3: [40..63]: 5242952..5242975 1 (72..95) 24
> > >>>>>
> > >>>>> The fs has 8k directory fsbs. Dir fsb offset 0 spans extents 0 and 1,
> > >>>>> offset 1 (which corresponds to the 512b range 16-31 above) is covered
> > >>>>> completely by extent 2 and dir offset 2 (range 32-47) spans extents 2
> > >>>>> and 3. An ls of this directory produces this:
> > >>>>>
> > >>>>> XFS (dm-3): Metadata corruption detected at xfs_dir3_data_reada_verify+0x42/0x80 [xfs], xfs_dir3_data_reada block 0x500058
> > >>>>> XFS (dm-3): Unmount and run xfs_repair
> > >>>>> XFS (dm-3): First 64 bytes of corrupted metadata buffer:
> > >>>>> ffffbcb901c44000: 00 00 00 00 00 00 00 64 0f 78 78 78 78 78 78 78 .......d.xxxxxxx
> > >>>>> ffffbcb901c44010: 78 78 78 78 2e 38 38 36 01 00 00 00 00 00 10 00 xxxx.886........
> > >>>>> ffffbcb901c44020: 00 00 00 00 00 00 00 64 0f 78 78 78 78 78 78 78 .......d.xxxxxxx
> > >>>>> ffffbcb901c44030: 78 78 78 78 2e 38 38 37 01 00 00 00 00 00 10 20 xxxx.887.......
> > >>>>>
> > >>>>> ... which is yelling about block 184 (dir fsb 2). The fs is otherwise
> > >>>>> clean according to xfs_repair.
> > >>>>>
> > >>>>> I _think_ something like the appended diff deals with it, but this is
> > >>>>> lightly tested only and could definitely use more eyes.
> > >>>>>
> > >>>>> Brian
> > >>>>>
> > >>>>> --- 8< ---
> > >>>>>
> > >>>>> diff --git a/fs/xfs/xfs_dir2_readdir.c b/fs/xfs/xfs_dir2_readdir.c
> > >>>>> index ad9396e..9fa379d 100644
> > >>>>> --- a/fs/xfs/xfs_dir2_readdir.c
> > >>>>> +++ b/fs/xfs/xfs_dir2_readdir.c
> > >>>>> @@ -404,7 +404,8 @@ xfs_dir2_leaf_readbuf(
> > >>>>> * Read-ahead a contiguous directory block.
> > >>>>> */
> > >>>>> if (i > mip->ra_current &&
> > >>>>> - map[mip->ra_index].br_blockcount >= geo->fsbcount) {
> > >>>>> + (map[mip->ra_index].br_blockcount - mip->ra_offset) >=
> > >>>>> + geo->fsbcount) {
> > >>>>> xfs_dir3_data_readahead(dp,
> > >>>>> map[mip->ra_index].br_startoff + mip->ra_offset,
> > >>>>> XFS_FSB_TO_DADDR(dp->i_mount,
> > >>>>> @@ -432,7 +433,7 @@ xfs_dir2_leaf_readbuf(
> > >>>>> * The rest of this extent but not more than a dir
> > >>>>> * block.
> > >>>>> */
> > >>>>> - length = min_t(int, geo->fsbcount,
> > >>>>> + length = min_t(int, geo->fsbcount - j,
> > >>>>
> > >>>> Looks ok to me to make Eric's bugfix complete.
> > >>>
> > >>> Brian, thanks for crafting the image to expose this. :)
> > >>>
> > >>> I've been otherwise occupied today, sorry - technically this fixes a separate
> > >>> issue, yes? So 2 patches, 2 bugfixes AFAICT?
> > >>
> > >> Ok. I don't mind taking two patches since you're right, the first patch
> > >> saves us from running off the end of the shadow bmap array, and the
> > >> second one fixes bookkeepping problems when a dir extent starts midway
> > >> through a large dirblock.
> > >>
> > >
> > > Yeah, these are definitely separate issues and Eric's original patch
> > > here is probably higher priority on its own.
> > >
> > > But I'm not clear on the plan here.. do we want this additional patch or
> > > are we planning just to rewrite the readahead bits (or do we want to do
> > > both)? I guess having a bugfix patch in the tree is useful from a stable
> > > perspective... hm?
> > >
> > > Brian
> >
> > Yeah.
> >
> > There's 20 years of the old code in the field with these bugs - I think 2
> > straightforward fixes have value even if we might want to rewrite it in the
> > future.
>
> Assuming 4.11 is released on Sunday, I was figuring 4.12 for the two
> fixes + backporting to older kernels, and explore the rewrite for 4.13.
>
Ok, sounds good to me. I'll get this one posted once it survives a bit
more testing. Thanks.
Brian
> --D
>
> >
> > -Eric
> >
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