Re: [RFC PATCH] mm: Add reclaim type to memory.reclaim

[Yafang Shao <laoar.shao@gmail.com>]

On Tue, Feb 27, 2024 at 9:20 PM Michal Hocko <mhocko@suse.com> wrote:
>
> On Tue 27-02-24 18:06:05, Yafang Shao wrote:
> > On Tue, Feb 27, 2024 at 5:45 PM Michal Hocko <mhocko@suse.com> wrote:
> [...]
> > > > Reclaiming numerous dentries through force_empty can also lead to
> > > > potential issues, which is why we attempt to shrink the slab gradually
> > > > to mitigate them. However, it's important to note that the underlying
> > > > causes of the issues in force_empty and rmdir are not identical, as
> > > > they involve different locks.
> > >
> > > Please be more specific about those issues.
> >
> > Both of these issues stem from lock contention:
> >
> > - rmdir
> > When executing rmdir, the lock of the inode of the empty directory is
> > held. If this directory contains numerous negative dentries, this lock
> > is held for an extended duration. Consequently, if other processes
> > attempt to acquire this lock, they are blocked.
> > A simple reproducer involves:
> >
> > 1. Generating numerous negative dentries in an empty directory.
> > 2. Running `rmdir ~/test` and `ls ~/` concurrently.
>
> I fail to see how is this relevant to memcg reclaim


It appears there might still be some misunderstanding regarding the
issue at hand.

The numerous negative dentries are generated within a memcg. I
simplified the explanation by omitting the memcg context.

>
> > This setup demonstrates that ls takes a significant amount of time to
> > complete due to lock contention.
> >
> > - force_empty
> > Force_empty holds the lock of super_block->dentry_list. However, I
> > haven't yet had the opportunity to produce a specific example to
> > illustrate this issue.
>
> OK, get back to us once you can identify an actual problem. We might be

Pls. take a look at the force_empty->prune_dcache_sb->shrink_dentry_list.

> talking about different things here though. My question is directed at
> existing memcg interfaces to reclaim the memory. That would be legacy
> (and effectively deprecated) force_empty and memory.reclaim that we
> have.



-- 
Regards
Yafang