linux-kernel - Re: [PATCH 2/7] mmc: Don't use PF

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Date:	Tue, 17 Nov 2009 20:58:04 +0900 (JST)
From:	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>
To:	Alan Cox <alan@...rguk.ukuu.org.uk>
Cc:	kosaki.motohiro@...fujitsu.com,
	LKML <linux-kernel@...r.kernel.org>,
	linux-mm <linux-mm@...ck.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	linux-mmc@...r.kernel.org
Subject: Re: [PATCH 2/7] mmc: Don't use PF_MEMALLOC

> On Tue, 17 Nov 2009 16:17:50 +0900 (JST)
> KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com> wrote:
> 
> > Non MM subsystem must not use PF_MEMALLOC. Memory reclaim need few
> > memory, anyone must not prevent it. Otherwise the system cause
> > mysterious hang-up and/or OOM Killer invokation.
> 
> So now what happens if we are paging and all our memory is tied up for
> writeback to a device or CIFS etc which can no longer allocate the memory
> to complete the write out so the MM can reclaim ?

Probably my answer is not so simple. sorry.

reason1: MM reclaim does both dropping clean memory and writing out dirty pages.
reason2: if all memory is exhausted, maybe we can't recover it. it is
fundamental limitation of Virtual Memory subsystem. and, min-watermark is
decided by number of system physcal memory, but # of I/O issue (i.e. # of
pages of used by writeback thread) is mainly decided # of devices. 
then, we can't gurantee min-watermark is sufficient on any systems.
Only reasonable solution is mempool like reservation, I think.
IOW, any reservation memory shouldn't share unrelated subsystem. otherwise
we lost any gurantee.

So, I think we need to hear why many developer don't use mempool,
instead use PF_MEMALLOC.

> Am I missing something or is this patch set not addressing the case where
> the writeback thread needs to inherit PF_MEMALLOC somehow (at least for
> the I/O in question and those blocking it)

Yes, probably my patchset isn't perfect. honestly I haven't understand
why so many developer prefer to use PF_MEMALLOC.

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