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 merge with this request, then the old tables are no longer valid.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 <axboe@xxxxxxxxx> > > >>>> > > >>>> --- > > >>>> > > >>>> 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
