Re: [PATCH] remove use_sg_chaining

[Jens Axboe <jens.axboe@oracle.com>]

On Sun, Jan 20 2008, Jens Axboe wrote:
> On Sun, Jan 20 2008, Boaz Harrosh wrote:
> > On Sun, Jan 20 2008 at 21:29 +0200, Jens Axboe <jens.axboe@oracle.com> wrote:
> > > On Sun, Jan 20 2008, James Bottomley wrote:
> > >> On Sun, 2008-01-20 at 21:18 +0200, Boaz Harrosh wrote:
> > >>> On Tue, Jan 15 2008 at 19:52 +0200, James Bottomley <James.Bottomley@HansenPartnership.com> wrote:
> > >>>> this patch depends on the sg branch of the block tree
> > >>>>
> > >>>> James
> > >>>>
> > >>>> ---
> > >>>> From: James Bottomley <James.Bottomley@HansenPartnership.com>
> > >>>> Date: Tue, 15 Jan 2008 11:11:46 -0600
> > >>>> Subject: remove use_sg_chaining
> > >>>>
> > >>>> With the sg table code, every SCSI driver is now either chain capable
> > >>>> or broken, so there's no need to have a check in the host template.
> > >>>>
> > >>>> Also tidy up the code by moving the scatterlist size defines into the
> > >>>> SCSI includes and permit the last entry of the scatterlist pools not
> > >>>> to be a power of two.
> > >>>> ---
> > >>> I have a theoretical problem that BUGed me from the beginning.
> > >>>
> > >>> Could it happen that a memory critical IO, (that is needed to free
> > >>> memory), be collected into an sg-chained large IO, and the allocation 
> > >>> of the multiple sg-pool-allocations fail, thous dead locking on
> > >>> out-of-memory? Is there a mechanism in place that will split large IO's 
> > >>> into smaller chunks in the event of out-of-memory condition in prep_fn?
> > >>>
> > >>> Is it possible to call blk_rq_map_sg() with less then what is present
> > >>> at request to only map the starting portion?
> > >> Obviously, that's why I was worrying about mempool size and default
> > >> blocks a while ago.
> > >>
> > >> However, the deadlock only occurs if the device is swap or backing a
> > >> filesystem with memory mapped files.  The use cases for this are really
> > >> tapes and other entities that need huge buffers.  That's why we're
> > >> keeping the system sector size at 1024 unless you alter it through sysfs
> > >> (here gun, there foot ...)
> > > 
> > > Alternatively (and much safer, imho), we allow blk_rq_map_sg() return
> > > smaller than nr_phys_segments and just ensure that the request is
> > > continued nicely through the normal 'request if residual' logic.
> > > 
> > Thats a grate Idea. I will Q it on my todo list. Thanks
> 
> ok good, thanks :-)

btw, the above is full of typos, my apologies. it should read "requeue
if residual", but I guess you already guessed as much.

-- 
Jens Axboe