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Date:   Fri, 8 Feb 2019 14:49:44 -0800
From:   Andrew Morton <akpm@linux-foundation.org>
To:     Jan Kara <jack@suse.cz>
Cc:     Dave Chinner <david@fromorbit.com>, Roman Gushchin <guro@fb.com>,
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        "vdavydov.dev@gmail.com" <vdavydov.dev@gmail.com>
Subject: Re: [PATCH 1/2] Revert
 "mm: don't reclaim inodes with many attached pages"
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On Fri, 8 Feb 2019 13:50:49 +0100 Jan Kara <jack@suse.cz> wrote:

> > > Has anyone done significant testing with Rik's maybe-fix?
> > 
> > I will give it a spin with bonnie++ today. We'll see what comes out.
> 
> OK, I did a bonnie++ run with Rik's patch (on top of 4.20 to rule out other
> differences). This machine does not show so big differences in bonnie++
> numbers but the difference is still clearly visible. The results are
> (averages of 5 runs):
> 
> 		 Revert			Base			Rik
> SeqCreate del    78.04 (   0.00%)	98.18 ( -25.81%)	90.90 ( -16.48%)
> RandCreate del   87.68 (   0.00%)	95.01 (  -8.36%)	87.66 (   0.03%)
> 
> 'Revert' is 4.20 with "mm: don't reclaim inodes with many attached pages"
> and "mm: slowly shrink slabs with a relatively small number of objects"
> reverted. 'Base' is the kernel without any reverts. 'Rik' is a 4.20 with
> Rik's patch applied.
> 
> The numbers are time to do a batch of deletes so lower is better. You can see
> that the patch did help somewhat but it was not enough to close the gap
> when files are deleted in 'readdir' order.

OK, thanks.

I guess we need a rethink on Roman's fixes.   I'll queued the reverts.


BTW, one thing I don't think has been discussed (or noticed) is the
effect of "mm: don't reclaim inodes with many attached pages" on 32-bit
highmem machines.  Look why someone added that code in the first place:

: commit f9a316fa9099053a299851762aedbf12881cff42
: Author: Andrew Morton <akpm@digeo.com>
: Date:   Thu Oct 31 04:09:37 2002 -0800
: 
:     [PATCH] strip pagecache from to-be-reaped inodes
:     
:     With large highmem machines and many small cached files it is possible
:     to encounter ZONE_NORMAL allocation failures.  This can be demonstrated
:     with a large number of one-byte files on a 7G machine.
:     
:     All lowmem is filled with icache and all those inodes have a small
:     amount of highmem pagecache which makes them unfreeable.
:     
:     The patch strips the pagecache from inodes as they come off the tail of
:     the inode_unused list.
:     
:     I play tricks in there peeking at the head of the inode_unused list to
:     pick up the inode again after running iput().  The alternatives seemed
:     to involve more widespread changes.
:     
:     Or running invalidate_inode_pages() under inode_lock which would be a
:     bad thing from a scheduling latency and lock contention point of view.

I guess I shold have added a comment.  Doh.