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Date: Tue, 4 Dec 2018 14:50:34 -0800
From: Andrew Morton <akpm@linux-foundation.org>
To: Josef Bacik <josef@toxicpanda.com>
Cc: kernel-team@fb.com, hannes@cmpxchg.org, linux-kernel@vger.kernel.org,
 tj@kernel.org, david@fromorbit.com, linux-fsdevel@vger.kernel.org,
 linux-mm@kvack.org, riel@redhat.com, jack@suse.cz
Subject: Re: [PATCH 4/4] mm: use the cached page for filemap_fault
Message-Id: <20181204145034.4b69bdea36506be45946f8c9@linux-foundation.org>
In-Reply-To: <20181130195812.19536-5-josef@toxicpanda.com>
References: <20181130195812.19536-1-josef@toxicpanda.com>
	<20181130195812.19536-5-josef@toxicpanda.com>
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On Fri, 30 Nov 2018 14:58:12 -0500 Josef Bacik <josef@toxicpanda.com> wrote:

> If we drop the mmap_sem we have to redo the vma lookup which requires
> redoing the fault handler.  Chances are we will just come back to the
> same page, so save this page in our vmf->cached_page and reuse it in the
> next loop through the fault handler.
> 

Is this really worthwhile?  Rerunning the fault handler is rare (we
hope) and a single pagecache lookup is fast.

Some performance testing results would be helpful here.  It's
practically obligatory when claiming a performance improvement.