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From:   Yang Shi <shy828301@gmail.com>
Date:   Wed, 24 Nov 2021 13:17:07 -0800
Message-ID: <CAHbLzkqmTks5dGn=yCykKSwqtmsCKVqLx_bU9D1=fTvEgX06xw@mail.gmail.com>
Subject: Re: [PATCH v2] mm: thp: update split_queue_len correctly
To:     Shakeel Butt <shakeelb@google.com>
Cc:     "Kirill A. Shutemov" <kirill@shutemov.name>,
        David Hildenbrand <david@redhat.com>,
        "Kirill A . Shutemov" <kirill.shutemov@linux.intel.com>,
        Zi Yan <ziy@nvidia.com>, Matthew Wilcox <willy@infradead.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        Linux MM <linux-mm@kvack.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
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On Wed, Nov 24, 2021 at 12:44 PM Shakeel Butt <shakeelb@google.com> wrote:
>
> On Wed, Nov 24, 2021 at 12:12 PM Kirill A. Shutemov
> <kirill@shutemov.name> 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?