Re: [PATCH 1/2] Revert "mm: don't reclaim inodes with many attached pages"

From: Andrew Morton
Date: Fri Feb 08 2019 - 17:49:50 EST

On Fri, 8 Feb 2019 13:50:49 +0100 Jan Kara <jack@xxxxxxx> 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@xxxxxxxxx>
: 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.