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

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Message-ID: <4e5ebad50611282317r55c22228qa5333306ccfff28e@mail.gmail.com>
Date:	Wed, 29 Nov 2006 15:17:13 +0800
From:	"Sonic Zhang" <sonic.adi@...il.com>
To:	"Nick Piggin" <nickpiggin@...oo.com.au>
Cc:	Aubrey <aubreylee@...il.com>,
	"Peter Zijlstra" <a.p.zijlstra@...llo.nl>,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: The VFS cache is not freed when there is not enough free memory to allocate

Forward to the mailing list.

Sonic Zhang wrote:
> 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.

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?

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

Thanks,
Nick
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