On Sun, Jan 20 2008, Boaz Harrosh wrote:
> On Sun, Jan 20 2008 at 21:24 +0200, James Bottomley <James.Bottomley@xxxxxxxxxxxxxxxxxxxxx> 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@xxxxxxxxxxxxxxxxxxxxx> wrote:
> >>> this patch depends on the sg branch of the block tree
> >>>
> >>> James
> >>>
> >>> ---
> >>> From: James Bottomley <James.Bottomley@xxxxxxxxxxxxxxxxxxxxx>
> >>> 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 ...)
> >
> > James
> >
>
> OK Thanks for confirming my concern, In modern life with devices like
> iSCSI that have ~0 as it's max_sector, swapping over that should be
> considered and configured carefully. Once with pNFS over
> blocks/objects it should be addressed. Perhaps with a FAIL_FAST
> semantics for users like pNFS to split up the requests if they fail
> with out-of-memory.
I'll have to disagree again, you can't expect users to know these sorts
of things ("sorry your system deadlocked, you should have known not to
increase max_sectors_kb for something you swap on"). Especially when
handling it correctly in scsi_init_io() is a few lines of change.
No excuse for not doing this correctly. At least for blk_fs_request()
requests, for blk_pc_request() failing is the only option.
--
Jens Axboe
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