linux-kernel - Re: [PATCH 08/19] ceph: address space operations

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Message-ID: <Pine.LNX.4.64.0907232145560.14156@cobra.newdream.net>
Date:	Thu, 23 Jul 2009 21:48:36 -0700 (PDT)
From:	Sage Weil <sage@...dream.net>
To:	Andi Kleen <andi@...stfloor.org>
cc:	linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 08/19] ceph: address space operations

On Thu, 23 Jul 2009, Andi Kleen wrote:
> > There are two other memory allocations during writeout: a vector of pages 
> > to be written, and the message we're sending to the OSD.  If I use a 
> > mempool for those to guarantee as least some writeout will occur, how do I 
> > safely defer when allocations do fail?  Will pdflush (or it's replacement) 
> > eventually come back and try ->writepages() again?
> 
> mempool allocs should never fail, just block for a long time until
> someone else frees. This means you need to ensure of course you
> always make forward progress.

Oh, that avoids the issue reasonably well then.

Using mempools for the message passing is going to be a bit trickier (need 
to avoid any deadlocks due to dependencies on other messages), but it's 
doable.  At least in the limited case of making writeback proceed.

sage
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