Re: [PATCH 0/3] mm,thp,rmap: rework the use of subpages_mapcount

[Johannes Weiner <hannes@cmpxchg.org>]

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.