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

[Matthew Wilcox <willy@infradead.org>]

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.

> 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.