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Re: [PATCH 7/7] xfs: Don't use PF_MEMALLOC

To: KOSAKI Motohiro <kosaki.motohiro@xxxxxxxxxxxxxx>
Subject: Re: [PATCH 7/7] xfs: Don't use PF_MEMALLOC
From: Dave Chinner <david@xxxxxxxxxxxxx>
Date: Thu, 19 Nov 2009 09:16:42 +1100
Cc: LKML <linux-kernel@xxxxxxxxxxxxxxx>, linux-mm <linux-mm@xxxxxxxxx>, Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>, Christoph Hellwig <hch@xxxxxxxxxxxxx>, linux-fsdevel@xxxxxxxxxxxxxxx, xfs-masters@xxxxxxxxxxx, xfs@xxxxxxxxxxx
In-reply-to: <20091118153302.3E20.A69D9226@xxxxxxxxxxxxxx>
References: <20091117162235.3DEB.A69D9226@xxxxxxxxxxxxxx> <20091117221108.GK9467@xxxxxxxxxxxxxxxx> <20091118153302.3E20.A69D9226@xxxxxxxxxxxxxx>
User-agent: Mutt/1.5.18 (2008-05-17)
On Wed, Nov 18, 2009 at 05:56:46PM +0900, KOSAKI Motohiro wrote:
> > On Tue, Nov 17, 2009 at 04:23:43PM +0900, KOSAKI Motohiro 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.
> > 
> > The xfsbufd is a woken run by a registered memory shaker. i.e. it
> > runs when the system needs to reclaim memory. It forceѕ the
> > delayed write metadata buffers (of which there can be a lot) to disk
> > so that they can be reclaimed on IO completion. This IO submission
> > may require ѕome memory to be allocated to be able to free that
> > memory.
> > 
> > Hence, AFAICT the use of PF_MEMALLOC is valid here.
> 
> Thanks a lot. 
> I have one additional question, may I ask you?
> 
> How can we calculate maximum memory usage in xfsbufd?

It doesn't get calculated at the moment. It is very difficult to
calculate a usable size metric for it because there are multiple
caches (up to 3 per filesystem), and dentry/inode reclaim causes the
size of the cache to grow.  Hence the size of the cache is not
really something that can be considered a stable or predictable
input into a "reclaim now" calculation. As such we simply cause
xfsbufd run simultaneously with the shrinkers that cause it to
grow....

> I'm afraid that VM and XFS works properly but adding two makes memory exhaust.

I don't understand what you are trying to say here.

> And, I conclude XFS doesn't need sharing reservation memory with VM,
> it only need non failed allocation. right? IOW I'm prefer perter's
> suggestion.

Right. However, it is worth keeping in mind that this is a
performance critical path for inode reclaim.  Hence any throttling
of allocation will slow down the rate at which memory is freed by
the system....

Cheers,

Dave.
-- 
Dave Chinner
david@xxxxxxxxxxxxx

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