On Wed, Jul 31, 2019 at 09:21:01AM +0200, Michal Hocko wrote: > On Wed 31-07-19 14:44:47, Minchan Kim wrote: > > On Tue, Jul 30, 2019 at 02:57:51PM +0200, Michal Hocko wrote: > > > [Cc Nick - the email thread starts > > > http://lkml.kernel.org/r/20190729071037.241581-1-minc...@kernel.org > > > A very brief summary is that mark_page_accessed seems to be quite > > > expensive and the question is whether we still need it and why > > > SetPageReferenced cannot be used instead. More below.] > > > > > > On Tue 30-07-19 21:39:35, Minchan Kim wrote: > [...] > > > > commit bf3f3bc5e73 > > > > Author: Nick Piggin <npig...@suse.de> > > > > Date: Tue Jan 6 14:38:55 2009 -0800 > > > > > > > > mm: don't mark_page_accessed in fault path > > > > > > > > Doing a mark_page_accessed at fault-time, then doing > > > > SetPageReferenced at > > > > unmap-time if the pte is young has a number of problems. > > > > > > > > mark_page_accessed is supposed to be roughly the equivalent of a > > > > young pte > > > > for unmapped references. Unfortunately it doesn't come with any > > > > context: > > > > after being called, reclaim doesn't know who or why the page was > > > > touched. > > > > > > > > So calling mark_page_accessed not only adds extra lru or > > > > PG_referenced > > > > manipulations for pages that are already going to have pte_young > > > > ptes anyway, > > > > but it also adds these references which are difficult to work with > > > > from the > > > > context of vma specific references (eg. MADV_SEQUENTIAL pte_young > > > > may not > > > > wish to contribute to the page being referenced). > > > > > > > > Then, simply doing SetPageReferenced when zapping a pte and finding > > > > it is > > > > young, is not a really good solution either. SetPageReferenced does > > > > not > > > > correctly promote the page to the active list for example. So after > > > > removing > > > > mark_page_accessed from the fault path, several > > > > mmap()+touch+munmap() would > > > > have a very different result from several read(2) calls for > > > > example, which > > > > is not really desirable. > > > > > > Well, I have to say that this is rather vague to me. Nick, could you be > > > more specific about which workloads do benefit from this change? Let's > > > say that the zapped pte is the only referenced one and then reclaim > > > finds the page on inactive list. We would go and reclaim it. But does > > > that matter so much? Hot pages would be referenced from multiple ptes > > > very likely, no? > > > > As Nick mentioned in the description, without mark_page_accessed in > > zapping part, repeated mmap + touch + munmap never acticated the page > > while several read(2) calls easily promote it. > > And is this really a problem? If we refault the same page then the > refaults detection should catch it no? In other words is the above still > a problem these days?
I admit we have been not fair for them because read(2) syscall pages are easily promoted regardless of zap timing unlike mmap-based pages. However, if we remove the mark_page_accessed in the zap_pte_range, it would make them more unfair in that read(2)-accessed pages are easily promoted while mmap-based page should go through refault to be promoted. I also want to remove the costly overhead from the hot path but couldn't come up with nice solution.
