linux-kernel - Re: [PATCH 4/4] mm: use the cached page for filemap

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Message-Id: <20181204145034.4b69bdea36506be45946f8c9@linux-foundation.org>
Date:   Tue, 4 Dec 2018 14:50:34 -0800
From:   Andrew Morton <akpm@...ux-foundation.org>
To:     Josef Bacik <josef@...icpanda.com>
Cc:     kernel-team@...com, hannes@...xchg.org,
        linux-kernel@...r.kernel.org, tj@...nel.org, david@...morbit.com,
        linux-fsdevel@...r.kernel.org, linux-mm@...ck.org, riel@...hat.com,
        jack@...e.cz
Subject: Re: [PATCH 4/4] mm: use the cached page for filemap_fault

On Fri, 30 Nov 2018 14:58:12 -0500 Josef Bacik <josef@...icpanda.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.