On Wed, Dec 05, 2018 at 10:58:02AM +0800, Ming Lei wrote:
> On Tue, Dec 04, 2018 at 07:30:24PM -0700, Jens Axboe wrote:
> > On 12/4/18 7:27 PM, Ming Lei wrote:
> > > On Tue, Dec 04, 2018 at 07:16:11PM -0700, Jens Axboe wrote:
> > >> On 12/4/18 6:37 PM, Ming Lei wrote:
> > >>> On Tue, Dec 04, 2018 at 03:47:46PM -0700, Jens Axboe wrote:
> > >>>> If we attempt a direct issue to a SCSI device, and it returns BUSY,
> > >>>> then
> > >>>> we queue the request up normally. However, the SCSI layer may have
> > >>>> already setup SG tables etc for this particular command. If we later
> > >>>> merge with this request, then the old tables are no longer valid. Once
> > >>>> we issue the IO, we only read/write the original part of the request,
> > >>>> not the new state of it.
> > >>>>
> > >>>> This causes data corruption, and is most often noticed with the file
> > >>>> system complaining about the just read data being invalid:
> > >>>>
> > >>>> [ 235.934465] EXT4-fs error (device sda1): ext4_iget:4831: inode
> > >>>> #7142: comm dpkg-query: bad extra_isize 24937 (inode size 256)
> > >>>>
> > >>>> because most of it is garbage...
> > >>>>
> > >>>> This doesn't happen from the normal issue path, as we will simply defer
> > >>>> the request to the hardware queue dispatch list if we fail. Once it's
> > >>>> on
> > >>>> the dispatch list, we never merge with it.
> > >>>>
> > >>>> Fix this from the direct issue path by flagging the request as
> > >>>> REQ_NOMERGE so we don't change the size of it before issue.
> > >>>>
> > >>>> See also:
> > >>>> https://bugzilla.kernel.org/show_bug.cgi?id=201685
> > >>>>
> > >>>> Fixes: 6ce3dd6eec1 ("blk-mq: issue directly if hw queue isn't busy in
> > >>>> case of 'none'")
> > >>>> Signed-off-by: Jens Axboe <[email protected]>
> > >>>>
> > >>>> ---
> > >>>>
> > >>>> diff --git a/block/blk-mq.c b/block/blk-mq.c
> > >>>> index 3f91c6e5b17a..d8f518c6ea38 100644
> > >>>> --- a/block/blk-mq.c
> > >>>> +++ b/block/blk-mq.c
> > >>>> @@ -1715,6 +1715,15 @@ static blk_status_t
> > >>>> __blk_mq_issue_directly(struct blk_mq_hw_ctx *hctx,
> > >>>> break;
> > >>>> case BLK_STS_RESOURCE:
> > >>>> case BLK_STS_DEV_RESOURCE:
> > >>>> + /*
> > >>>> + * If direct dispatch fails, we cannot allow any
> > >>>> merging on
> > >>>> + * this IO. Drivers (like SCSI) may have set up
> > >>>> permanent state
> > >>>> + * for this request, like SG tables and mappings, and
> > >>>> if we
> > >>>> + * merge to it later on then we'll still only do IO to
> > >>>> the
> > >>>> + * original part.
> > >>>> + */
> > >>>> + rq->cmd_flags |= REQ_NOMERGE;
> > >>>> +
> > >>>> blk_mq_update_dispatch_busy(hctx, true);
> > >>>> __blk_mq_requeue_request(rq);
> > >>>> break;
> > >>>>
> > >>>
> > >>> Not sure it is enough to just mark it as NOMERGE, for example, driver
> > >>> may have setup the .special_vec for discard, and NOMERGE may not prevent
> > >>> request from entering elevator queue completely. Cause 'rq.rb_node' and
> > >>> 'rq.special_vec' share same space.
> > >>
> > >> We should rather limit the scope of the direct dispatch instead. It
> > >> doesn't make sense to do for anything but read/write anyway.
> > >
> > > discard is kind of write, and it isn't treated very specially in make
> > > request path, except for multi-range discard.
> >
> > The point of direct dispatch is to reduce latencies for requests,
> > discards are so damn slow on ALL devices anyway that it doesn't make any
> > sense to try direct dispatch to begin with, regardless of whether it
> > possible or not.
>
> SCSI MQ device may benefit from direct dispatch from reduced lock contention.
>
> >
> > >>> So how about inserting this request via blk_mq_request_bypass_insert()
> > >>> in case that direct issue returns BUSY? Then it is invariant that
> > >>> any request queued via .queue_rq() won't enter scheduler queue.
> > >>
> > >> I did consider this, but I didn't want to experiment with exercising
> > >> a new path for an important bug fix. You do realize that your original
> > >> patch has been corrupting data for months? I think a little caution
> > >> is in order here.
> > >
> > > But marking NOMERGE still may have a hole on re-insert discard request as
> > > mentioned above.
> >
> > What I said was further limit the scope of direct dispatch, which means
> > not allowing anything that isn't a read/write.
>
> IMO, the conservative approach is to take the one used in legacy io
> path, in which it is never allowed to re-insert queued request to
> scheduler queue except for requeue, however RQF_DONTPREP is cleared
> before requeuing request to scheduler.
>
> >
> > > Given we never allow to re-insert queued request to scheduler queue
> > > except for 6ce3dd6eec1, I think it is the correct thing to do, and the
> > > fix is simple too.
> >
> > As I said, it's not the time to experiment. This issue has been there
> > since 4.19-rc1. The alternative is yanking both those patches, and then
> > looking at it later when the direct issue path has been cleaned up
> > first.
>
> The issue should have been there from v4.1, especially after commit
> f984df1f0f7 ("blk-mq: do limited block plug for multiple queue case"),
> which is the 1st one to re-insert the queued request into scheduler
> queue.
But at that time, there isn't io scheduler for MQ, so in theory the
issue should be there since v4.11, especially 945ffb60c11d ("mq-deadline:
add blk-mq adaptation of the deadline IO scheduler").
Thanks,
Ming
