Re: [PATCH] remove use_sg_chaining

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

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.

-- 
Jens Axboe