linux-kernel - Re: [PATCH 4.4 50/92] mm: filemap: avoid unnecessary calls to lock_page when waiting for IO to complete during a read

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Date:   Thu, 24 May 2018 13:28:41 +0200
From:   Greg KH <gregkh@...uxfoundation.org>
To:     Hugh Dickins <hughd@...gle.com>
Cc:     Jan Kara <jack@...e.cz>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        stable <stable@...r.kernel.org>,
        Mel Gorman <mgorman@...hsingularity.net>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Mel Gorman <mgorman@...e.de>
Subject: Re: [PATCH 4.4 50/92] mm: filemap: avoid unnecessary calls to
 lock_page when waiting for IO to complete during a read

On Thu, May 24, 2018 at 04:17:12AM -0700, Hugh Dickins wrote:
>   Thu, May 24, 2018 at 4:06 AM Greg Kroah-Hartman
> <gregkh@...uxfoundation.org>
> wrote:
> 
> > On Thu, May 24, 2018 at 12:50:11PM +0200, Jan Kara wrote:
> > > On Thu 24-05-18 11:38:27, Greg Kroah-Hartman wrote:
> > > > 4.4-stable review patch.  If anyone has any objections, please let me
> know.
> > >
> > > Just one objection: Why does stable care about this (and the previous
> > > patch)? I've checked the stable queue and I don't see anything that
> would
> > > have these patches as a prerequisite. And on their own, they are only
> > > cleanups without substantial gains.
> 
> > There's a small gain here:
> 
> > > > paralleldd
> > > >                                     4.4.0                 4.4.0
> > > >                                   vanilla             avoidlock
> > > > Amean    Elapsd-1          5.28 (  0.00%)        5.15 (  2.50%)
> > > > Amean    Elapsd-4          5.29 (  0.00%)        5.17 (  2.12%)
> > > > Amean    Elapsd-7          5.28 (  0.00%)        5.18 (  1.78%)
> > > > Amean    Elapsd-12         5.20 (  0.00%)        5.33 ( -2.50%)
> > > > Amean    Elapsd-21         5.14 (  0.00%)        5.21 ( -1.41%)
> > > > Amean    Elapsd-30         5.30 (  0.00%)        5.12 (  3.38%)
> > > > Amean    Elapsd-48         5.78 (  0.00%)        5.42 (  6.21%)
> > > > Amean    Elapsd-79         6.78 (  0.00%)        6.62 (  2.46%)
> > > > Amean    Elapsd-110        9.09 (  0.00%)        8.99 (  1.15%)
> > > > Amean    Elapsd-128       10.60 (  0.00%)       10.43 (  1.66%)
> > > >
> > > > The impact is small but intuitively, it makes sense to avoid
> unnecessary
> > > > calls to lock_page.
> 
> > Yes, it's small, but it's marked in the SLES kernel as "needs to be
> > merged into stable", so obviously it matters to someone :)
> 
> Hmm. I had the same reaction to these two as Jan, but assumed that they
> made applying later patches easier, and didn't take the trouble he did to
> find that's not so.
> 
> I've no wish to be disputatious, but it does seem that the definition of
> "stable" has changed, and not necessarily for the better, if it's now a
> home for small gains: I thought we left those to upstream.

This is in the SLES kernel for a reason, and again, it's in the section
that says "this should be pushed to stable".  So if it's good enough for
the SLES kernel, why isn't it good enough for all users of this kernel
tree?

If you all think it should be dropped in both places, that's fine with
me :)

thanks,

greg k-h