linux-kernel - Re: [PATCH] Should GFP_ATOMIC fail when we're below low watermark?

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Date:	Mon, 20 Aug 2007 10:59:36 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Nigel Cunningham <nigel@...el.suspend2.net>
Cc:	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] Should GFP_ATOMIC fail when we're below low watermark?

On Mon, 2007-08-20 at 18:38 +1000, Nigel Cunningham wrote:
> Hi.
> 
> On Monday 20 August 2007 12:43:50 Peter Zijlstra wrote:
> > On Mon, 2007-08-20 at 11:38 +1000, Nigel Cunningham wrote:
> > > Hi all.
> > > 
> > > In current git (and for a while now), an attempt to allocate memory with 
> > > GFP_ATOMIC will fail if we're below the low watermark level. The only way 
> to 
> > > access that memory that I can see (not that I've looked that hard) is to 
> have 
> > > PF_MEMALLOC set (ie from kswapd). I'm wondering if this behaviour is 
> correct. 
> > > Shouldn't GFP_ATOMIC allocations ignore watermarks too? How about 
> GFP_KERNEL?
> > > 
> > > The following patch is a potential fix for GFP_ATOMIC.
> > 
> > Sorry, no.
> > 
> > GFP_ATOMIC must fail when below the watermark. GFP_KERNEL has __GFP_WAIT
> > and hence can sleep and wait for reclaim so that should not be a problem
> > (usually).
> > 
> > GFP_ATOMIC may not access the reserves because the reserves are needed
> > to get out of OOM deadlocks within the VM. Consider the fact that
> > freeing memory needs memory - if there is no memory free, you cannot
> > free memory and you're pretty much stuck.
> 
> I guess, then, the question should be whether the watermark values are 
> appropriate. Do we need high order allocations watermarked if this is the 
> only purpose, particularly considering that whatever memory is allocated for 
> this purpose is essentially useless 99.9% of the time?

Could you perhaps explain what you're trying to do? No matter what we
do, GFP_ATOMIC will fail eventually, there is only so much one can do
without blocking.

As for higher order allocations, until we have a full online defrag
solution those too can fail at any moment (even with __GFP_WAIT).

-
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@...r.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/