Re: [patch -mmotm] mm: invoke oom killer for __GFP_NOFAIL

[Andrew Morton]

On Sat, 9 May 2009 15:46:39 -0700 (PDT)
David Rientjes <rientjes@xxxxxxxxxx> wrote:

> The oom killer must be invoked regardless of the order if the allocation
> is __GFP_NOFAIL, otherwise it will loop forever when reclaim fails to
> free some memory.

Sigh.  We're supposed to be deleting __GFP_NOFAIL.  I added it as a way
of easily finding lame error-handling-challenged callers which need to
be fixed up.  So of course we went and added lots more callers.

y:/usr/src/linux-2.6.30-rc5> grep -rl GFP_NOFAIL . 
./arch/x86/xen/mmu.c
./arch/sparc/kernel/mdesc.c
./mm/page_alloc.c
./mm/failslab.c
./block/cfq-iosched.c
./fs/bio-integrity.c
./fs/ntfs/ChangeLog
./fs/ntfs/malloc.h
./fs/reiserfs/journal.c
./fs/gfs2/meta_io.c
./fs/gfs2/rgrp.c
./fs/gfs2/dir.c
./fs/gfs2/log.c
./fs/jbd/transaction.c
./fs/jbd/journal.c
./fs/jbd2/transaction.c
./fs/jbd2/journal.c
./drivers/net/cxgb3/cxgb3_main.c
./drivers/net/cxgb3/cxgb3_offload.c
./include/linux/slab.h
./include/linux/gfp.h

JBD (and hence JBD2) are the original sinners.

That net driver should be taught to just handle the allocation failure,
please.


It's super-uber-bad to be using __GFP_NOFAIL in an IO scheduler!  But maybe
that's just a brainfart:

			/*
			 * Inform the allocator of the fact that we will
			 * just repeat this allocation if it fails, to allow
			 * the allocator to do whatever it needs to attempt to
			 * free memory.
			 */

If "we will just repeat this allocation" means what it says then we
should use __GFP_NORETRY here, then retry the allocation if it failed. 
But a) this risks getting stuck in a hot loop in CFQ and b) we really
really don't want to be looping infinitely for memory relcaim down in
the guts of the block layer!

>From my reading, this function is called from get_request_wait(), via

	rq = get_request(q, rw_flags, bio, GFP_NOIO);

so we can't even do pageout here.

Jens, this all looks quite risky.
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