Messages in this thread |  | | | From | Mike Snitzer <> | | Date | Mon, 27 Sep 2010 01:12:39 -0400 | | Subject | Re: [PATCH] block: prevent merges of discard and write requests |
| |
On Mon, Sep 27, 2010 at 12:59 AM, Jens Axboe <axboe@kernel.dk> wrote: > On 2010-09-27 12:30, Kyle McMartin wrote: >> On Sat, Sep 25, 2010 at 12:40:48PM +0200, Jens Axboe wrote: >>> On 2010-09-25 12:36, Adrian Hunter wrote: >>>> Add logic to prevent two I/O requests being merged if >>>> only one of them is a discard. Ditto secure discard. >>>> >>>> Without this fix, it is possible for write requests >>>> to transform into discard requests. For example: >>>> >>>> Submit bio 1 to discard 8 sectors from sector n >>>> Submit bio 2 to write 8 sectors from sector n + 16 >>>> Submit bio 3 to write 8 sectors from sector n + 8 >>>> >>>> Bio 1 becomes request 1. Bio 2 becomes request 2. >>>> Bio 3 is merged with request 2, and then subsequently >>>> request 2 is merged with request 1 resulting in just >>>> one I/O request which discards all 24 sectors. >>> >>> Wow, that's a disaster. We can now have requests in the >>> same direction and of the same type (fs), but not mergeable. >>> >>> I would move the check up above the position calculations. >>> I will apply this and upstream it right away. Thanks a lot! >>> >> >> Jens, is this (the REQ_DISCARD hunk) required for stable as well? It >> appears there's not much change relating to merging requests between >> HEAD and v2.6.35, so I assume it is? > > No, 2.6.35 and earlier is safe, it's only 2.6.36-rc that is > affected by this bug.
I'm not so sure... I think 2.6.35 is affected. Jens, what do you hold to be the regression point?
The initial discard merge implementation (commit e17fc0a1ccf88) always set REQ_SOFTBARRIER for discards. As such such non-ioctl blkdev_issue_discard() discards were always !rq_mergeable().
As commit e17fc0a1ccf88 shared: it is always possible to send a discard without the use of a barrier (but such callers were assumed to be safe, e.g. mkfs.* discarding entire device via ioctl?).
But things really broke once we started playing games with barrier flags associated with discards. The regression point (relative to discard merging) seems to have occurred when we got away from using REQ_SOFTBARRIER with commit: fbbf055692aeb "block: fix DISCARD_BARRIER requests". Which was still committed to v2.6.35...
So long story short: it seems stable's 2.6.35.y needs this fix too.
Of course I could easily be missing something.. if I got any of this wrong please correct me ;)
Thanks, Mike -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
|  |