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.
