Re: [PATCH RFC v2 1/9] mm: optimize do_wp_page() for exclusive pages in the swapcache

[Matthew Wilcox]

On Wed, Jan 26, 2022 at 10:55:49AM +0100, David Hildenbrand wrote:
> Liang Zhang reported [1] that the current COW logic in do_wp_page() is
> sub-optimal when it comes to swap+read fault+write fault of anonymous
> pages that have a single user, visible via a performance degradation in
> the redis benchmark. Something similar was previously reported [2] by
> Nadav with a simple reproducer.
> 
> Let's optimize for pages that have been added to the swapcache but only
> have an exclusive owner. Try removing the swapcache reference if there is
> hope that we're the exclusive user.
> 
> We will fail removing the swapcache reference in two scenarios:
> (1) There are additional swap entries referencing the page: copying
>     instead of reusing is the right thing to do.
> (2) The page is under writeback: theoretically we might be able to reuse
>     in some cases, however, we cannot remove the additional reference
>     and will have to copy.
> 
> Further, we might have additional references from the LRU pagevecs,
> which will force us to copy instead of being able to reuse. We'll try
> handling such references for some scenarios next. Concurrent writeback
> cannot be handled easily and we'll always have to copy.
> 
> While at it, remove the superfluous page_mapcount() check: it's
> implicitly covered by the page_count() for ordinary anon pages.
> 
> [1] https://lkml.kernel.org/r/20220113140318.11117-1-zhangliang5@xxxxxxxxxx
> [2] https://lkml.kernel.org/r/0480D692-D9B2-429A-9A88-9BBA1331AC3A@xxxxxxxxx
> 
> Reported-by: Liang Zhang <zhangliang5@xxxxxxxxxx>
> Reported-by: Nadav Amit <nadav.amit@xxxxxxxxx>
> Signed-off-by: David Hildenbrand <david@xxxxxxxxxx>

Reviewed-by: Matthew Wilcox (Oracle) <willy@xxxxxxxxxxxxx>