linux-kernel - Re: mkfs.f2fs gets stuck with "blk_update

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Date:	Sun, 7 Apr 2013 09:58:46 +0800
From:	Shaohua Li <shli@...nel.org>
To:	Max Filippov <jcmvbkbc@...il.com>
Cc:	LKML <linux-kernel@...r.kernel.org>, Jens Axboe <axboe@...nel.dk>,
	Jaegeuk Kim <jaegeuk.kim@...sung.com>,
	linux-f2fs-devel@...ts.sourceforge.net, slava@...eyko.com
Subject: Re: mkfs.f2fs gets stuck with "blk_update_request: bio idx 0 >= vcnt
 0" on 3.8

On Fri, Apr 05, 2013 at 06:18:10AM +0400, Max Filippov wrote:
> On Fri, Apr 5, 2013 at 5:53 AM, Shaohua Li <shli@...nel.org> wrote:
> > On Thu, Apr 04, 2013 at 06:00:18AM +0400, Max Filippov wrote:
> 
> [...]
> 
> >> the commit 0cfbcafcae8b7364b5fa96c2b26ccde7a3a296a9 'block: add plug
> >> for blkdev_issue_discard'
> >> have added merge opportunity for DISCARD requests. When I do
> >> mkfs.f2fs on a 5G partition (0xad8000 sectors) it submits two bios,
> >> one for 0x7fe000 sectors (0xffc00000 bytes) and another for
> >> 0x2da000 sectors (0x5b400000 bytes). Prior to that commit these
> >> bios weren't merged into one request. Now the second bio gets
> >> merged with the first, but the request's __data_len field is unsigned int
> >> and it gets wrapped to 0x5b000000 bytes instead of 0x15b000000
> >> in the bio_attempt_back_merge. Later this reduced size is passed to
> >> the blk_update_request causing KERN_ERR and not completed
> >> request. Reverting this commit fixes mkfs.f2fs for me.
> >
> > A workaround is setting limits.max_discard_sectors to a smaller value.
> 
> I'm not sure:
> 1) in my case max_discard_sectors is 0x7fe000 (0xffc00000 bytes,
>     which still fits into 32 bits) and
> 2) this parameter will only change size of individual discard requests for
>     the discarded range, but as long as these requests are done inside
>     the plug they will be merged anyway with an overflow if we try
>     to discard more than 4G at once.

No, merge in plug list checks max_discard_sectors. Please see
ll_back_merge_fn() in bio_attempt_back_merge(). Merge in unplug checks
max_discard_sectors too. I didn't see any chance a merge doesn't check
max_discard_sectors. Can you post a blktrace when the discard ioctl runs and
double check if max_discard_sectors is really 0x7fe000?
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