Re: [PATCH v2] mm: thp: update split_queue_len correctly

[Yang Shi]

On Wed, Nov 24, 2021 at 12:44 PM Shakeel Butt <shakeelb@xxxxxxxxxx> wrote:
>
> On Wed, Nov 24, 2021 at 12:12 PM Kirill A. Shutemov
> <kirill@xxxxxxxxxxxxx> wrote:
> >
> > On Tue, Nov 23, 2021 at 11:09:16AM -0800, Shakeel Butt wrote:
> > > The deferred THPs are split on memory pressure through shrinker
> > > callback and splitting of THP during reclaim can fail for several
> > > reasons like unable to lock the THP, under writeback or unexpected
> > > number of pins on the THP. Such pages are put back on the deferred split
> > > list for consideration later. However kernel does not update the
> > > deferred queue size on putting back the pages whose split was failed.
> > > This patch fixes that.
> >
> > Hm. No. split_huge_page_to_list() updates the queue size on split success.
> >
>
> Right. This is really convoluted. split_huge_page_to_list() is just
> assuming that if the given page is on a deferred list then it must be
> on the list returned by get_deferred_split_queue(page). The
> interaction of move_charge and deferred split seems broken.

Because memcg code doesn't move charge for PTE mapped THP at all. See
the below comment from mem_cgroup_move_charge_pte_range():

"We can have a part of the split pmd here. Moving it can be done but
it would be too convoluted so simply ignore such a partial THP and
keep it in original memcg. There should be somebody mapping the head."

BTW, did you run into any problem related to this?

>
> Andrew, can you please drop this patch?