netdev - Re: [PATCH mmotm] mm: alloc_large_system

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Date:	Fri, 01 May 2009 13:46:55 +0200
From:	Eric Dumazet <dada1@...mosbay.com>
To:	Hugh Dickins <hugh@...itas.com>
CC:	Mel Gorman <mel@....ul.ie>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Andi Kleen <andi@...stfloor.org>,
	David Miller <davem@...emloft.net>, netdev@...r.kernel.org,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [PATCH mmotm] mm: alloc_large_system_hash check order

Hugh Dickins a écrit :
> On Thu, 30 Apr 2009, Mel Gorman wrote:
>> On Wed, Apr 29, 2009 at 10:09:48PM +0100, Hugh Dickins wrote:
>>> On an x86_64 with 4GB ram, tcp_init()'s call to alloc_large_system_hash(),
>>> to allocate tcp_hashinfo.ehash, is now triggering an mmotm WARN_ON_ONCE on
>>> order >= MAX_ORDER - it's hoping for order 11.  alloc_large_system_hash()
>>> had better make its own check on the order.

Well, I dont know why, since alloc_large_system_hash() already take
care of retries, halving size between each tries.

>>>
>>> Signed-off-by: Hugh Dickins <hugh@...itas.com>
>> Looks good
>>
>> Reviewed-by: Mel Gorman <mel@....ul.ie>
> 
> Thanks.
> 
>> As I was looking there, it seemed that alloc_large_system_hash() should be
>> using alloc_pages_exact() instead of having its own "give back the spare
>> pages at the end of the buffer" logic. If alloc_pages_exact() was used, then
>> the check for an order >= MAX_ORDER can be pushed down to alloc_pages_exact()
>> where it may catch other unwary callers.
>>
>> How about adding the following patch on top of yours?
> 
> Well observed, yes indeed.  In fact, it even looks as if, shock horror,
> alloc_pages_exact() was _plagiarized_ from alloc_large_system_hash().
> Blessed be the GPL, I'm sure we can skip the lengthy lawsuits!

As a matter of fact, I was planning to call my lawyer, so I'll reconsider
this and save some euros, thanks !

;)

It makes sense to use a helper function if it already exist, of course !

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