From mboxrd@z Thu Jan  1 00:00:00 1970
From: Andrew Morton <akpm@linux-foundation.org>
Subject: Re: [PATCH v7 0/5] vfs: Non-blockling buffered fs read (page cache
 only)
Date: Mon, 30 Mar 2015 13:37:58 -0700
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To: Jeremy Allison <jra@samba.org>
Cc: Christoph Hellwig <hch@infradead.org>, Milosz Tanski <milosz@adfin.com>, linux-kernel@vger.kernel.org, linux-fsdevel@vger.kernel.org, linux-aio@kvack.org, Mel Gorman <mgorman@suse.de>, Volker Lendecke <Volker.Lendecke@sernet.de>, Tejun Heo <tj@kernel.org>, Jeff Moyer <jmoyer@redhat.com>, Theodore Ts'o <tytso@mit.edu>, Al Viro <viro@zeniv.linux.org.uk>, linux-api@vger.kernel.org, Michael Kerrisk <mtk.manpages@gmail.com>, linux-arch@vger.kernel.org, Dave Chinner <david@fromorbit.com>

On Mon, 30 Mar 2015 13:32:27 -0700 Jeremy Allison <jra@samba.org> wrote:

> On Mon, Mar 30, 2015 at 01:26:25PM -0700, Andrew Morton wrote:
> > 
> > cons:
> > 
> > d) fincore() is more expensive
> > 
> > e) fincore() will very occasionally block
> 
> The above is the killer for Samba. If fincore
> returns true but when we schedule the pread
> we block, we're hosed.
> 
> Once we block, we're done serving clients on the main
> thread until this returns. That can cause unpredictable
> response times which can cause client timeouts.
> 
> A fincore+pread solution that blocks is simply unsafe
> to use for us. We'll have to stay with the threadpool :-(.

Finally.  Thanks ;)

This implies that the samba main thread also has to avoid any memory
allocations both direct and within syscall and pagefault - those will
occasionally exhibit similar worse-case latency. Is this done now?