Re: [PATCH] fs: drop_caches: skip dropping pagecache which is always dirty

[Matthew Wilcox]

On Wed, Jul 20, 2022 at 06:02:32AM +0000, CGEL wrote:
> On Wed, Jul 20, 2022 at 04:02:40AM +0100, Matthew Wilcox wrote:
> > On Wed, Jul 20, 2022 at 02:21:19AM +0000, cgel.zte@xxxxxxxxx wrote:
> > > From: Yang Yang <yang.yang29@xxxxxxxxxx>
> > > 
> > > Pagecache of some kind of fs has PG_dirty bit set once it was
> > > allocated, so it can't be dropped. These fs include ramfs and
> > > tmpfs. This can make drop_pagecache_sb() more efficient.
> > 
> > Why do we want to make drop_pagecache_sb() more efficient?
> 
> Some users may use drop_caches besides testing or debugging.

This is a terrible reason.

> For example, some systems will create a lot of pagecache when boot up
> while reading bzImage, ramdisk, docker images etc. Most of this pagecache
> is useless after boot up. It may has a longterm negative effects for the
> workload when trigger page reclaim. It is especially harmful when trigger
> direct_reclaim or we need allocate pages in atomic context. So users may
> chose to drop_caches after boot up.

If that's actually a problem, work on fixing that.