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SubjectRe: IO errors after "block: remove bio get nr vec s()"
On 2015-12-20 23:44, Kent Overstreet wrote:
> On Sun, Dec 20, 2015 at 07:18:01PM +0100, Christoph Hellwig wrote:
>> On Sun, Dec 20, 2015 at 09:51:14AM -0800, Linus Torvalds wrote:
>> > Kent, Jens, Christoph et al,
>> ie please see this bugzilla:
>> >o
>> > httpps://
>> >
>> > where Artem Tashkinov bisected his problems with 4.3 down to commit
>> > b54ffb73cadc ("block: remove bio_get_nr_vecs()") that you've all
>> > signed off on.
>> Artem,
>> can you re-check the commits around this series again? I would be
>> extremtly surprised if it's really this particular commit and not
>> one just before it causing the problem - it just allocates bios
>> to the biggest possible instead of only allocating up to what
>> bio_add_page would accept.
> pretty sure it's something with how blk_bio_segment_split() decides
> what
> segments are mergable and not. bio_get_nr_vecs() was just returning
> nr_pages ==
> queue_max_segments (ignoring sectors for the moment) - so wait, wtf?
> that's
> basically assuming no segment merging can ever happen, if it does then
> this was
> causing us to send smaller requests to the device than we could have
> been.
> so actually two possibilities I can see:
> - in blk_bio_segment_split(), something's screwed up with how it
> decides what
> segments are going to be mergable or not. but I don't think that's
> likely
> since it's doing the exact same thing the rest of the segment
> merging code
> does.
> - or, the driver was lying in its queue limits, using
> queue_max_segments for
> "the maximum number of pages I can possibly take", and that bug
> lurked
> undiscovered because of the screwed-upness in bio_get_nr_vecs().
> Offhand I don't know where to start digging in the driver code to look
> into the
> second theory though. Tejun, you got any ideas?

Here's an actual bisect log which Linus was missing:

git bisect start
# bad: [6a13feb9c82803e2b815eca72fa7a9f5561d7861] Linux 4.3
git bisect bad 6a13feb9c82803e2b815eca72fa7a9f5561d7861
# good: [64291f7db5bd8150a74ad2036f1037e6a0428df2] Linux 4.2
git bisect good 64291f7db5bd8150a74ad2036f1037e6a0428df2
# bad: [807249d3ada1ff28a47c4054ca4edd479421b671] Merge branch
'upstream' of git://
git bisect bad 807249d3ada1ff28a47c4054ca4edd479421b671
# good: [102178108e2246cb4b329d3fb7872cd3d7120205] Merge tag
'armsoc-drivers' of
git bisect good 102178108e2246cb4b329d3fb7872cd3d7120205
# good: [62da98656b62a5ca57f22263705175af8ded5aa1] netfilter:
nf_conntrack: make nf_ct_zone_dflt built-in
git bisect good 62da98656b62a5ca57f22263705175af8ded5aa1
# good: [f1a3c0b933e7ff856223d6fcd7456d403e54e4e5] Merge tag
'devicetree-for-4.3' of
git bisect good f1a3c0b933e7ff856223d6fcd7456d403e54e4e5
# bad: [9cbf22b37ae0592dea809cb8d424990774c21786] Merge tag 'dlm-4.3' of
git bisect bad 9cbf22b37ae0592dea809cb8d424990774c21786
# good: [8bdc69b764013a9b5ebeef7df8f314f1066c5d79] Merge branch
'for-4.3' of git://
git bisect good 8bdc69b764013a9b5ebeef7df8f314f1066c5d79
# good: [df910390e2db07a76c87f258475f6c96253cee6c] Merge tag 'scsi-misc'
of git://
git bisect good df910390e2db07a76c87f258475f6c96253cee6c
# bad: [d975f309a8b250e67b66eabeb56be6989c783629] Merge branch
'for-4.3/sg' of git://
git bisect bad d975f309a8b250e67b66eabeb56be6989c783629
# bad: [89e2a8404e4415da1edbac6ca4f7332b4a74fae2] crypto/omap-sham:
remove an open coded access to ->page_link
git bisect bad 89e2a8404e4415da1edbac6ca4f7332b4a74fae2
# good: [0e28997ec476bad4c7dbe0a08775290051325f53] btrfs: remove bio
splitting and merge_bvec_fn() calls
git bisect good 0e28997ec476bad4c7dbe0a08775290051325f53
# bad: [2ec3182f9c20a9eef0dacc0512cf2ca2df7be5ad] Documentation: update
notes in biovecs about arbitrarily sized bios
git bisect bad 2ec3182f9c20a9eef0dacc0512cf2ca2df7be5ad
# good: [7140aafce2fc14c5af02fdb7859b6bea0108be3d] md/raid5: get rid of
git bisect good 7140aafce2fc14c5af02fdb7859b6bea0108be3d
# good: [6cf66b4caf9c71f64a5486cadbd71ab58d0d4307] fs: use helper
bio_add_page() instead of open coding on bi_io_vec
git bisect good 6cf66b4caf9c71f64a5486cadbd71ab58d0d4307
# bad: [b54ffb73cadcdcff9cc1ae0e11f502407e3e2e4c] block: remove
git bisect bad b54ffb73cadcdcff9cc1ae0e11f502407e3e2e4c

And like he said since the step before the last one was good and the
very last one was bad there was no way I could have made a mistake.

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