linux-kernel - Re: Vanilla-Kernel 3 - page allocation failure

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Date:	Tue, 18 Oct 2011 09:32:51 -0200
From:	Thadeu Lima de Souza Cascardo <cascardo@...ux.vnet.ibm.com>
To:	Philipp Herz - Profihost AG <p.herz@...fihost.ag>
Cc:	linux-kernel@...r.kernel.org
Subject: Re: Vanilla-Kernel 3 - page allocation failure

On Tue, Oct 18, 2011 at 12:25:03PM +0200, Philipp Herz - Profihost AG wrote:
> After updating kernel (x86_64) to stable version 3 there are a few
> messages appearing in the kernel log such as
> 
> kworker/0:1: page allocation failure: order:1, mode:0x20
> mysql: page allocation failure: order:1, mode:0x20
> php5: page allocation failure: order:1, mode:0x20
> 
> Searching the net showed that these messages are known to occur since 2004.
> 
> Some people were able to get rid of them by setting
> /proc/sys/vm/min_free_kbytes to a high enough value. This does not
> help in our case.
> 
> 
> Is there a kernel comand line argument to avoid these messages?
> 
> As of mm/page_alloc.c these messages are marked to be only warning
> messages and would not appear if 'gpf_mask' was set to __GFP_NOWARN
> in function warn_alloc_failed.
> 
> How does this mask get set? Is it set by the "external" process
> knocking at the memory manager?
> 

Hello, Philipp.

This happens when kernel tries to allocate memory, sometimes in response
to some request by the user space, but also in other contexts. For
example, an interrupt by a network driver may try to allocate memory. In
this context, it will use GFP_ATOMIC as a mask, for example. The most
usual flags in the kernel are GFP_KERNEL and GFP_ATOMIC.

> What is the magic behind the 'order' and 'mode'?
> 

The order is the binary log of the number of pages requested. So, order 1
allocations are 2 pages, order 4 would be 16 pages, for example.

The mode is, in fact, gfp_flags. 0x20 is GFP_ATOMIC. This kind of
allocation cannot do IO or access the filesystem. Also, it cannot wait
for reclaim memory from cache.

This warning is usually followed by some statistics about memory use
in your system. Please post it to give more information about this
situation.

I have watched some of this happen when lots of cache is used by some
filesystems. Perhaps, some tweaking of the vm sysctl options may help,
but I can point any magic tweaking right now.

Regards,
Cascardo.

> I'm not a subscriber, so please CC me a copy of messages related to
> the subject. I'm not sure if I can help much by looking at the
> inside of the kernel, but I will try my best to answer any questions
> concerning this issue.
> 
> Best regards, Philipp
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