Re: [PATCH v3 07/21] fs: xfs: align args->minlen for forced allocation alignment

[John Garry]

On 06/06/2024 09:47, Dave Chinner wrote:
> On Wed, Jun 05, 2024 at 03:26:11PM +0100, John Garry wrote:
> > Hi Dave,
> >
> > I still think that there is a problem with this code or some other allocator
> > code which gives rise to unexpected -ENOSPC. I just highlight this code,
> > above, as I get an unexpected -ENOSPC failure here when the fs does have
> > many free (big enough) extents. I think that the problem may be elsewhere,
> > though.
> >
> > Initially we have a file like this:
> >
> >   EXT: FILE-OFFSET      BLOCK-RANGE      AG AG-OFFSET        TOTAL
> >     0: [0..127]:        62592..62719      0 (62592..62719)     128
> >     1: [128..895]:      hole                                   768
> >     2: [896..1023]:     63616..63743      0 (63616..63743)     128
> >     3: [1024..1151]:    64896..65023      0 (64896..65023)     128
> >     4: [1152..1279]:    65664..65791      0 (65664..65791)     128
> >     5: [1280..1407]:    68224..68351      0 (68224..68351)     128
> >     6: [1408..1535]:    76416..76543      0 (76416..76543)     128
> >     7: [1536..1791]:    62720..62975      0 (62720..62975)     256
> >     8: [1792..1919]:    60032..60159      0 (60032..60159)     128
> >     9: [1920..2047]:    63488..63615      0 (63488..63615)     128
> >    10: [2048..2303]:    63744..63999      0 (63744..63999)     256
> >
> > forcealign extsize is 16 4k fsb, so the layout looks ok.
> >
> > Then we truncate the file to 454 sectors (or 56.75 fsb). This gives:
> >
> > EXT: FILE-OFFSET      BLOCK-RANGE      AG AG-OFFSET        TOTAL
> >     0: [0..127]:        62592..62719      0 (62592..62719)     128
> >     1: [128..455]:      hole                                   328
> >
> > We have 57 fsb.
> >
> > Then I attempt to write from byte offset 232448 (454 sector) and a get a
> > write failure in xfs_bmap_select_minlen() returning -ENOSPC; at that point
> > the file looks like this:
>
> So you are doing an unaligned write of some size at EOF and EOF is
> not aligned to the extsize?

Correct

> >   EXT: FILE-OFFSET      BLOCK-RANGE      AG AG-OFFSET        TOTAL
> >     0: [0..127]:        62592..62719      0 (62592..62719)     128
> >     1: [128..447]:      hole                                   320
> >     2: [448..575]:      62720..62847      0 (62720..62847)     128
> >
> > That hole in ext #1 is 40 fsb, and not aligned with forcealign granularity.
> > This means that ext #2 is misaligned wrt forcealign granularity.
>
> OK, so the command to produce this would be something like this?
>
> # xfs_io -fd -c "truncate 0" \
> 	-c "chattr +<forcealign>" -c "extsize 64k" \
> 	-c "pwrite 0 64k -b 64k" -c "pwrite 448k 64k -b 64k" \
> 	-c "bmap -vvp" \
> 	-c "truncate 227k" \
> 	-c "bmap -vvp" \
> 	-c "pwrite 227k 64k -b 64k" \
> 	-c "bmap -vvp" \
> 	/mnt/scratch/testfile

No, unfortunately not. Well maybe not on a clean filesystem. In my 
stress test, something else is causing this. Probably heavy fragmentation.

> > This is strange.
> >
> > I notice that we when allocate ext #2, xfs_bmap_btalloc() returns
> > ap->blkno=7840, length=16, offset=56. I would expect offset % 16 == 0, which
> > it is not.
>
> IOWs, the allocation was not correctly rounded down to an aligned
> start offset.  What were the initial parameters passed to this
> allocation?

For xfs_bmap_btalloc() entry,

ap->offset=48, length=32, blkno=0, total=0, minlen=1, minleft=1, eof=1, 
wasdel=0, aeof=0, conv=0, datatype=5, flags=0x8

> i.e. why didn't it round the start offset down to 48?
> Answering that question will tell you where the bug is.

After xfs_bmap_compute_alignments() -> xfs_bmap_extsize_align(), 
ap->offset=48 - that seems ok.

Maybe the problem is in xfs_bmap_process_allocated_extent(). For the 
problematic case when calling that function:

args->fsbno=7840 args->len=16 ap->offset=48 orig_offset=56 orig_length=24

So, as the comment reads there, we could not satisfy the original length 
request, so we move up the position of the extent.

I assume that we just don't want to do that for forcealign, correct?

> Of course, if the allocation start is rounded down to 48, then
> the length should be rounded up to 32 to cover the entire range we
> are writing new data to.
>
> > In the following sub-io block zeroing, I note that we zero the front padding
> > from pos=196608 (or fsb 48 or sector 384) for len=35840, and back padding
> > from pos=263680 for len=64000 (upto sector 640 or fsb 80). That seems wrong,
> > as we are zeroing data in the ext #1 hole, right?
>
> The sub block zeroing is doing exactly the right thing - it is
> demonstrating the exact range that the force aligned allocation
> should have covered.

Agreed

> > Now the actual -ENOSPC comes from xfs_bmap_btalloc() -> ... ->
> > xfs_bmap_select_minlen() with initially blen=32 args->alignment=16
> > ap->minlen=1 args->maxlen=8. There xfs_bmap_btalloc() has ap->length=8
> > initially. This may be just a symptom.
>
> Yeah, now the allocator is trying to fix up the mess that the first unaligned
> allocation created, and it's tripping over ENOSPC because it's not
> allowed to do sub-extent size hint allocations when forced alignment
> is enabled....
>
> > I guess that there is something wrong in the block allocator for ext #2. Any
> > idea where to check?
>
> Start with tracing exactly what range iomap is requesting be
> allocated, and then follow that through into the allocator to work
> out why the offset being passed to the allocation never gets rounded
> down to be aligned. There's a mistake in the logic somewhere that is
> failing to apply the start alignment to the allocation request (i.e.
> the bug will be in the allocation setup code path. i.e. somewhere
> in the xfs_bmapi_write -> xfs_bmap_btalloc path *before* we get to
> the xfs_alloc_vextent...() calls.
As above, the problem seems in the processing fix-up.

Thanks,
John