Re: [PATCH v4 0/3] protect page cache from freeing inode

[Andrew Morton <akpm@linux-foundation.org>]

On Sun, 23 Feb 2020 04:31:31 -0500 Yafang Shao <laoar.shao@gmail.com> wrote:

> On my server there're some running MEMCGs protected by memory.{min, low},
> but I found the usage of these MEMCGs abruptly became very small, which
> were far less than the protect limit. It confused me and finally I
> found that was because of inode stealing.
> Once an inode is freed, all its belonging page caches will be dropped as
> well, no matter how may page caches it has. So if we intend to protect the
> page caches in a memcg, we must protect their host (the inode) first.
> Otherwise the memcg protection can be easily bypassed with freeing inode,
> especially if there're big files in this memcg.
> The inherent mismatch between memcg and inode is a trouble. One inode can
> be shared by different MEMCGs, but it is a very rare case. If an inode is
> shared, its belonging page caches may be charged to different MEMCGs.
> Currently there's no perfect solution to fix this kind of issue, but the
> inode majority-writer ownership switching can help it more or less.

What are the potential regression scenarios here?  Presumably a large
number of inodes pinned by small amounts of pagecache, consuming memory
and CPU during list scanning.  Anything else?

Have you considered constructing test cases to evaluate the impact of
such things?