linux-kernel - Re: The VFS cache is not freed when there is not enough free memory to allocate

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Date:	Wed, 29 Nov 2006 17:27:52 +0800
From:	Aubrey <aubreylee@...il.com>
To:	"Nick Piggin" <nickpiggin@...oo.com.au>
Cc:	"Sonic Zhang" <sonic.adi@...il.com>,
	"Peter Zijlstra" <a.p.zijlstra@...llo.nl>,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org,
	vapier.adi@...il.com
Subject: Re: The VFS cache is not freed when there is not enough free memory to allocate

On 11/29/06, Sonic Zhang <sonic.adi@...il.com> wrote:
> Forward to the mailing list.
>
> > On 11/27/06, Nick Piggin <nickpiggin@...oo.com.au> wrote:
>
>
> >> I haven't actually written any nommu userspace code, but it is obvious
> >> that you must try to keep malloc to <= PAGE_SIZE (although order 2 and
> >> even 3 allocations seem to be reasonable, from process context)... Then
> >> you would use something a bit more advanced than a linear array to store
> >> data (a pagetable-like radix tree would be a nice, easy idea).
> >>
> >
> > But, even we split the 8M memory into 2048 x 4k blocks, we still face
> > this failure. The key problem is that available memory is small than
> > 2048 x 4k, while there are still a lot of VFS cache. The VFS cache can
> > be freed, but kernel allocation function ignores it. See the new test
> > application.
>
>
> Which kernel allocation function? If you can provide more details I'd
> like to get to the bottom of this.

I posted it here, I think you missed it. So forwarded it to you.

>
> Because the anonymous memory allocation in mm/nommu.c is all allocated
> with GFP_KERNEL from process context, and in that case, the allocator
> should not fail but call into page reclaim which in turn will free VFS
> caches.
>
>
>
> > What's a better way to free the VFS cache in memory allocator?
>
>
> It should be freeing it for you, so I'm not quite sure what is going
> on. Can you send over the kernel messages you see when the allocation
> fails?

I don't think so. The kernel doesn't attempt to free it. The log is
included in the mail I forwarded to you.

>
> Also, do you happen to know of a reasonable toolchain + emulator setup
> that I could test the nommu kernel with?

A project named skyeye.
http://www.skyeye.org/index.shtml

-Aubrey
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