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Re: TAKE - Locking fixes for the xfs I/O path

To: Marcelo Tosatti <marcelo@xxxxxxxxxxxxxxxx>
Subject: Re: TAKE - Locking fixes for the xfs I/O path
From: Steve Lord <lord@xxxxxxx>
Date: Wed, 24 Jan 2001 10:07:48 -0600
Cc: Steve Lord <lord@xxxxxxx>, linux-xfs@xxxxxxxxxxx
In-reply-to: Message from Marcelo Tosatti <marcelo@xxxxxxxxxxxxxxxx> of "Wed, 24 Jan 2001 12:00:02 -0200." <Pine.LNX.4.21.0101241157330.10409-100000@xxxxxxxxxxxxxxxxxxxxxx>
Sender: owner-linux-xfs@xxxxxxxxxxx
> 
> On Tue, 23 Jan 2001, Steve Lord wrote:
> 
> > Finally change the flags on memory allocations which happen under filesyste
> m
> > locks (usually the xfs inode lock) to use GFP_BUFFER rather than GFP_KERNEL
> .
> > This stops the memory reclaim threads from pushing back into the filesystem
> > again to free memory and deadlocking.
> > 
> > I have not yet managed to deadlock a system due to memory pressure with
> > these changes. dbench throughput also appears to improve.
> 
> Steve, 
> 
> Have to check if, under 2.4.1, with low memory machines under heavy IO no
> XFS allocations fails. 
> 
> We are not waiting for kswapd anymore, so the  !__GFP_IO allocations are
> more fragile.
> 

Oh Joy! So even though the request flags say it is ok to sleep for memory, it
can still fail? I was hoping we had found a way out of this hole. OK, I do
see that GFP_BUFFER users are expected to cope with failure, looks like it is
back to the drawing board here - xfs cannot cope with a non-robust memory
allocator. What we really need is an interface which says go get some memory,
and don't return until you have some, but do not bug me to free memory. The
problem with the GFP flags is that they are an extremely large hammer - i.e.
do not ask any filesystem for any memory is a bit over the top.

Steve

p.s. running with the try_to_swap_out change now.



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