linux-kernel - Re: [GIT PULL] afs: Fixes

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Message-ID: <23134.1511649343@warthog.procyon.org.uk>
Date:   Sat, 25 Nov 2017 22:35:43 +0000
From:   David Howells <dhowells@...hat.com>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     dhowells@...hat.com, linux-afs@...r.kernel.org,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [GIT PULL] afs: Fixes

Linus Torvalds <torvalds@...ux-foundation.org> wrote:

> So I see in the commit message why afs needs to do this, but it's
> worth pointing out that it's
> 
>  (a) impossible to avoid the "inconsistent data" case for writable mmap'ed
>  pages

Doesn't clear_page_dirty_for_io() write-protect the PTE for the page to be
written out, in which case ->page_mkwrite() will get called again before the
page is redirtied?

>  (b) can cause some really nasty latency issues

True, but I think the most common case is a file being opened, written start
to finish and then closed.  Actually, the worst-handled thing I've seen is a
shell script appending a bunch of things to a file because ->flush() syncs the
file each time it is closed:-/

What would you recommend instead?  I'm currently trying and keep track of what
needs to be written so that I only write what's changed to the server, rather
than writing only whole pages.

David