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From: Andrew Morton <akpm@linux-foundation.org>
To: Johannes Weiner <hannes@cmpxchg.org>
Cc: linux-fsdevel@vger.kernel.org, linux-mm@kvack.org,
	linux-kernel@vger.kernel.org, Dave Chinner <david@fromorbit.com>,
	Yafang Shao <laoar.shao@gmail.com>,
	Michal Hocko <mhocko@suse.com>, Roman Gushchin <guro@fb.com>,
	Linus Torvalds <torvalds@linux-foundation.org>,
	Al Viro <viro@zeniv.linux.org.uk>,
	kernel-team@fb.com
Subject: Re: [PATCH] vfs: keep inodes with page cache off the inode shrinker LRU
Date: Wed, 13 May 2020 14:15:19 -0700
Message-ID: <20200513141519.061f8fca4788cd02b4d7068f@linux-foundation.org> (raw)
In-Reply-To: <20200512212936.GA450429@cmpxchg.org>

On Tue, 12 May 2020 17:29:36 -0400 Johannes Weiner <hannes@cmpxchg.org> wrote:

> 
> ...
>
> Solution
> 
> This patch fixes the aging inversion described above on
> !CONFIG_HIGHMEM systems, without reintroducing the problems associated
> with excessive shrinker LRU rotations, by keeping populated inodes off
> the shrinker LRUs entirely.
> 
> Currently, inodes are kept off the shrinker LRU as long as they have
> an elevated i_count, indicating an active user. Unfortunately, the
> page cache cannot simply hold an i_count reference, because unlink()
> *should* result in the inode being dropped and its cache invalidated.
> 
> Instead, this patch makes iput_final() consult the state of the page
> cache and punt the LRU linking to the VM if the inode is still
> populated; the VM in turn checks the inode state when it depopulates
> the page cache, and adds the inode to the LRU if necessary.
> 
> This is not unlike what we do for dirty inodes, which are moved off
> the LRU permanently until writeback completion puts them back on (iff
> still unused). We can reuse the same code -- inode_add_lru() - here.
> 
> This is also not unlike page reclaim, where the lower VM layer has to
> negotiate state with the higher VFS layer. Follow existing precedence
> and handle the inversion as much as possible on the VM side:
> 
> - introduce an I_PAGES flag that the VM maintains under the i_lock, so
>   that any inode code holding that lock can check the page cache state
>   without having to lock and inspect the struct address_space

Maintaining the same info in two places is a hassle.  Is this
optimization worthwhile?

> - introduce inode_pages_set() and inode_pages_clear() to maintain the
>   inode LRU state from the VM side, then update all cache mutators to
>   use them when populating the first cache entry or clearing the last
> 
> With this, the concept of "inodesteal" - where the inode shrinker
> drops page cache - is relegated to CONFIG_HIGHMEM systems only. The VM
> is in charge of the cache, the shrinker in charge of struct inode.

How tested is this on highmem machines?

> Footnotes
> 
> - For debuggability, add vmstat counters that track the number of
>   times a new cache entry pulls a previously unused inode off the LRU
>   (pginoderescue), as well as how many times existing cache deferred
>   an LRU addition. Keep the pginodesteal/kswapd_inodesteal counters
>   for backwards compatibility, but they'll just show 0 now.
> 
> - Fix /proc/sys/vm/drop_caches to drop shadow entries from the page
>   cache. Not doing so has always been a bit strange, but since most
>   people drop cache and metadata cache together, the inode shrinker
>   would have taken care of them before - no more, so do it VM-side.
> 
> ...
>
>  14 files changed, 208 insertions(+), 34 deletions(-)

Patch is surprisingly large.



  parent reply index

Thread overview: 58+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-02-11 17:55 Johannes Weiner
2020-02-11 18:20 ` Johannes Weiner
2020-02-11 19:05 ` Rik van Riel
2020-02-11 19:31   ` Johannes Weiner
2020-02-11 23:44     ` Andrew Morton
2020-02-12  0:28       ` Linus Torvalds
2020-02-12  0:47         ` Andrew Morton
2020-02-12  1:03           ` Linus Torvalds
2020-02-12  8:50             ` Russell King - ARM Linux admin
2020-02-13  9:50               ` Lucas Stach
2020-02-13 16:52               ` Arnd Bergmann
2020-02-15 11:25                 ` Geert Uytterhoeven
2020-02-15 16:59                   ` Arnd Bergmann
2020-02-16  9:44                     ` Geert Uytterhoeven
2020-02-16 19:54                       ` Chris Paterson
2020-02-16 20:38                         ` Arnd Bergmann
2020-02-20 14:35                           ` Chris Paterson
2020-02-26 18:04                 ` santosh.shilimkar
2020-02-26 21:01                   ` Arnd Bergmann
2020-02-26 21:11                     ` santosh.shilimkar
2020-03-06 20:34                       ` Nishanth Menon
2020-03-07  1:08                         ` santosh.shilimkar
2020-03-08 10:58                         ` Arnd Bergmann
2020-03-08 14:19                           ` Russell King - ARM Linux admin
2020-03-09 13:33                             ` Arnd Bergmann
2020-03-09 14:04                               ` Russell King - ARM Linux admin
2020-03-09 15:04                                 ` Arnd Bergmann
2020-03-10  9:16                                   ` Michal Hocko
2020-03-09 15:59                           ` Catalin Marinas
2020-03-09 16:09                             ` Russell King - ARM Linux admin
2020-03-09 16:57                               ` Catalin Marinas
2020-03-09 19:46                               ` Arnd Bergmann
2020-03-11 14:29                                 ` Catalin Marinas
2020-03-11 16:59                                   ` Arnd Bergmann
2020-03-11 17:26                                     ` Catalin Marinas
2020-03-11 22:21                                       ` Arnd Bergmann
2020-02-12  3:58         ` Matthew Wilcox
2020-02-12  8:09         ` Michal Hocko
2020-02-17 13:31         ` Pavel Machek
2020-02-12 16:35       ` Johannes Weiner
2020-02-12 18:26         ` Andrew Morton
2020-02-12 18:52           ` Johannes Weiner
2020-02-12 12:25 ` Yafang Shao
2020-02-12 16:42   ` Johannes Weiner
2020-02-13  1:47     ` Yafang Shao
2020-02-13 13:46       ` Johannes Weiner
2020-02-14  2:02         ` Yafang Shao
2020-02-13 18:34 ` [PATCH v2] " Johannes Weiner
2020-02-14 16:53 ` [PATCH] " kbuild test robot
2020-02-14 21:30 ` kbuild test robot
2020-02-14 21:30 ` [PATCH] vfs: fix boolreturn.cocci warnings kbuild test robot
2020-05-12 21:29 ` [PATCH] vfs: keep inodes with page cache off the inode shrinker LRU Johannes Weiner
2020-05-13  1:32   ` Yafang Shao
2020-05-13 13:00     ` Johannes Weiner
2020-05-13 21:15   ` Andrew Morton [this message]
2020-05-14 11:27     ` Johannes Weiner
2020-05-14  2:24   ` Andrew Morton
2020-05-14 10:37     ` Johannes Weiner

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