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Date:   Tue, 22 Nov 2022 01:55:39 -0500
From:   Johannes Weiner <hannes@cmpxchg.org>
To:     Matthew Wilcox <willy@infradead.org>
Cc:     Shakeel Butt <shakeelb@google.com>,
        Hugh Dickins <hughd@google.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        "Kirill A. Shutemov" <kirill@shutemov.name>,
        David Hildenbrand <david@redhat.com>,
        Vlastimil Babka <vbabka@suse.cz>, Peter Xu <peterx@redhat.com>,
        Yang Shi <shy828301@gmail.com>,
        John Hubbard <jhubbard@nvidia.com>,
        Mike Kravetz <mike.kravetz@oracle.com>,
        Sidhartha Kumar <sidhartha.kumar@oracle.com>,
        Muchun Song <songmuchun@bytedance.com>,
        Miaohe Lin <linmiaohe@huawei.com>,
        Naoya Horiguchi <naoya.horiguchi@linux.dev>,
        Mina Almasry <almasrymina@google.com>,
        James Houghton <jthoughton@google.com>,
        Zach O'Keefe <zokeefe@google.com>,
        linux-kernel@vger.kernel.org, linux-mm@kvack.org
Subject: Re: [PATCH 0/3] mm,thp,rmap: rework the use of subpages_mapcount
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On Tue, Nov 22, 2022 at 05:57:42AM +0000, Matthew Wilcox wrote:
> On Mon, Nov 21, 2022 at 01:52:23PM -0500, Johannes Weiner wrote:
> > That leaves clearing writeback. This can't hold the page lock due to
> > the atomic context, so currently we need to take lock_page_memcg() as
> > the lock of last resort.
> > 
> > I wonder if we can have cgroup take the xalock instead: writeback
> > ending on file pages always acquires the xarray lock. Swap writeback
> > currently doesn't, but we could make it so (swap_address_space).
> > 
> > The only thing that gives me pause is the !mapping check in
> > __folio_end_writeback. File and swapcache pages usually have mappings,
> > and truncation waits for writeback to finish before axing
> > page->mapping. So AFAICS this can only happen if we call end_writeback
> > on something that isn't under writeback - in which case the test_clear
> > will fail and we don't update the stats anyway. But I want to be sure.
> > 
> > Does anybody know from the top of their heads if a page under
> > writeback could be without a mapping in some weird cornercase?
> 
> I can't think of such a corner case.  We should always wait for
> writeback to finish before removing the page from the page cache;
> the writeback bit used to be (and kind of still is) an implicit
> reference to the page, which means that we can't remove the page
> cache's reference to the page without waiting for writeback.

Great, thanks!

> > If we could ensure that the NR_WRITEBACK decs are always protected by
> > the xalock, we could grab it from mem_cgroup_move_account(), and then
> > kill lock_page_memcg() altogether.
> 
> I'm not thrilled by this idea, but I'm not going to veto it.

Ok, I'm also happy to drop this one.

Certainly, the rmap one is the lowest-hanging fruit. I have the patch
rebased against Hugh's series in mm-unstable; I'll wait for that to
settle down, and then send an updated version to Andrew.