linux-kernel - Re: Prevent inode/dentry trashing?

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Message-ID: <YNnPAJ4HNoN6g6T9@blackbook>
Date:   Mon, 28 Jun 2021 15:30:40 +0200
From:   Michal Koutný <mkoutny@...e.com>
To:     "Enrico Weigelt, metux IT consult" <lkml@...ux.net>
Cc:     Philipp Hahn <pmhahn+lkml@...ahn.de>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "cgroups@...r.kernel.org" <cgroups@...r.kernel.org>
Subject: Re: Prevent inode/dentry trashing?

On Mon, Jun 28, 2021 at 11:40:39AM +0200, "Enrico Weigelt, metux IT consult" <lkml@...ux.net> wrote:
> Could you please tell a bit more how this really works ?
> (maybe some pointers to the code)

When cgroup's consumption is about to cross the configured limit,
reclaim is started
https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/mm/memcontrol.c?id=62fb9874f5da54fdb243003b386128037319b219#n2579

that may evict old entries
https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/mm/vmscan.c?id=62fb9874f5da54fdb243003b386128037319b219#n2852

and if there's still no success freeing some space the dentry allocation
can fail
https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/mm/slab.h?id=62fb9874f5da54fdb243003b386128037319b219#n277

(This describes just one code path, the subject isn't always a dentry.)

> I'm curios what happens if those cache objects are used by different
> cgroups - are they accounted to multiple times (once per cgroup) ?
> What happens when one cgroup using some cache object reaching its limit,
> wile another one does not ?

That's explained here
https://www.kernel.org/doc/html/v5.13/admin-guide/cgroup-v2.html#memory-ownership

Michal

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