Re: [PATCH] mm: release the spinlock on zap_pte_range

[Minchan Kim]

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-minchan@xxxxxxxxxx
> > >  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 <npiggin@xxxxxxx>
> > > > 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.