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>
