Re: [PATCH v3 26/30 fix] mm: mglru: do not call update_lru_size() during reparenting

[Qi Zheng]

On 1/21/26 12:19 PM, Harry Yoo wrote:
> On Wed, Jan 21, 2026 at 12:53:28PM +0900, Harry Yoo wrote:
> > On Thu, Jan 15, 2026 at 06:44:44PM +0800, Qi Zheng wrote:
> > > From: Qi Zheng <zhengqi.arch@xxxxxxxxxxxxx>
> > >
> > > Only non-hierarchical lruvec_stats->state_local needs to be reparented,
> > > so handle it in reparent_state_local(), and remove the unreasonable
> > > update_lru_size() call in __lru_gen_reparent_memcg().
> >
> > Hmm well, how are the hierarchical statistics consistent when pages are
> > reparented from an "active" gen to an "inactive" gen, or the other way around?

Oh, I completely forgot about that. If update_lru_size() is not called
during the rreparenting, this issue should be considered separately.

> > They'll become inconsistent when those pages are reclaimed or
> > moved between generations?
>
> FYI we've observed this while testing downstream implementation
> as it led to MemAvailable being unreasonably high due to inconsistent
> statistics.
>
> The solution was, if lru_gen_is_active(child, gen) and
> lru_gen_is_active(parent, gen) do not match, # of pages being
> reparented must be subtracted from the child's statistics
> (and up to the root, as it's hierarchical), and added to the parent's
> statistics for the generation.

Make sense, will fix it in v4.

Thanks!

>