Re: [PATCH] remove use_sg_chaining

[Boaz Harrosh <bharrosh@panasas.com>]

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

Boaz