Re: [PATCH 0/2] mm: skip memcg for certain address space

Michal Hocko
2024-07-18T08:09:32Z

On Thu 18-07-24 17:27:05, Qu Wenruo wrote:
>
>
> 在 2024/7/18 16:55, Michal Hocko 写道:
> > On Thu 18-07-24 09:17:42, Vlastimil Babka (SUSE) wrote:
> > > On 7/18/24 12:38 AM, Qu Wenruo wrote:
> > [...]
> > > > Does the folio order has anything related to the problem or just a
> > > > higher order makes it more possible?
> > >
> > > I didn't spot anything in the memcg charge path that would depend on the
> > > order directly, hm. Also what kernel version was showing these soft lockups?
> >
> > Correct. Order just defines the number of charges to be reclaimed.
> > Unlike the page allocator path we do not have any specific requirements
> > on the memory to be released.
>
> So I guess the higher folio order just brings more pressure to trigger the
> problem?

It increases the reclaim target (in number of pages to reclaim). That
might contribute but we are cond_resched-ing in shrink_node_memcgs and
also down the path in shrink_lruvec etc. So higher target shouldn't
cause soft lockups unless we have a bug there - e.g. not triggering any
of those paths with empty LRUs and looping somewhere. Not sure about
MGLRU state of things TBH.

> > > > And finally, even without the hang problem, does it make any sense to
> > > > skip all the possible memcg charge completely, either to reduce latency
> > > > or just to reduce GFP_NOFAIL usage, for those user inaccessible inodes?
> >
> > Let me just add to the pile of questions. Who does own this memory?
>
> A special inode inside btrfs, we call it btree_inode, which is not
> accessible out of the btrfs module, and its lifespan is the same as the
> mounted btrfs filesystem.

But the memory charge is attributed to the caller unless you tell
otherwise. So if this is really an internal use and you use a shared
infrastructure which expects the current task to be owner of the charged
memory then you need to wrap the initialization into set_active_memcg
scope.

--
Michal Hocko
SUSE Labs