lkml.org 
[lkml]   [2008]   [Oct]   [6]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
Date
SubjectRe: [PATCH] BUG: nr_phys_segments cannot be less than nr_hw_segments
From
On Thu, 2 Oct 2008 19:13:57 +0200
Jens Axboe <jens.axboe@oracle.com> wrote:

> On Thu, Oct 02 2008, James Bottomley wrote:
> > On Thu, 2008-10-02 at 18:58 +0200, Jens Axboe wrote:
> > > On Thu, Oct 02 2008, James Bottomley wrote:
> > > > The bug would appear to be that we sometimes only look at q->max_sectors
> > > > when deciding on mergability. Either we have to insist on max_sectors
> > > > <= hw_max_sectors, or we have to start using min(q->max_sectors,
> > > > q->max_hw_sectors) for this.
> > >
> > > q->max_sectors MUST always be <= q->max_hw_sectors, otherwise we could
> > > be sending down requests that are too large for the device to handle. So
> > > that condition would be a big bug. The sysfs interface checks for this,
> > > and blk_queue_max_sectors() makes sure that is true as well.
> >
> > Yes, that seems always to be enforced. Perhaps there are other ways of
> > tripping this problem ... I'm still sure if it occurs it's because we do
> > a physical merge where a virtual merge is forbidden.
> >
> > > The fixes proposed still look weird. There is no phys vs hw segment
> > > constraints, the request must adhere to the limits set by both. It's
> > > mostly a moot point anyway, as 2.6.28 will get rid of the hw accounting
> > > anyway.
> >
> > Agree all three proposed fixes look wrong. However, if it's what I
> > think, just getting rid of hw accounting won't fix the problem because
> > we'll still have the case where a physical merge is forbidden by iommu
> > constraints ... this still needs to be accounted for.
> >
> > What we really need is a demonstration of what actually is going
> > wrong ...
>
> Yep, IIRC we both asked for that the last time as well... Nikanth?

Possibly, blk_phys_contig_segment might miscalculate
q->max_segment_size?

blk_phys_contig_segment does:

req->biotail->bi_size + next_req->bio->bi_size > q->max_segment_size;

But it's possible that req->biotail and the previous bio are supposed
be merged into one segment? Then we could create too large segment
here.


\
 
 \ /
  Last update: 2008-10-06 19:27    [W:0.076 / U:0.008 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site