Re: [PATCH] mm: account lazily freed anon pages in NR_FILE_PAGES

[Michal Hocko <mhocko@suse.com>]

On Thu 05-11-20 16:22:52, Michal Hocko wrote:
> On Thu 05-11-20 22:16:10, Yafang Shao wrote:
> > On Thu, Nov 5, 2020 at 9:35 PM Michal Hocko <mhocko@suse.com> wrote:
> > >
> > > On Thu 05-11-20 21:10:12, Yafang Shao wrote:
> > > > The memory utilization (Used / Total) is used to monitor the memory
> > > > pressure by us. If it is too high, it means the system may be OOM sooner
> > > > or later when swap is off, then we will make adjustment on this system.
> > > >
> > > > However, this method is broken since MADV_FREE is introduced, because
> > > > these lazily free anonymous can be reclaimed under memory pressure while
> > > > they are still accounted in NR_ANON_MAPPED.
> > > >
> > > > Furthermore, since commit f7ad2a6cb9f7 ("mm: move MADV_FREE pages into
> > > > LRU_INACTIVE_FILE list"), these lazily free anonymous pages are moved
> > > > from anon lru list into file lru list. That means
> > > > (Inactive(file) + Active(file)) may be much larger than Cached in
> > > > /proc/meminfo. That makes our users confused.
> > > >
> > > > So we'd better account the lazily freed anonoymous pages in
> > > > NR_FILE_PAGES as well.
> > >
> > > Can you simply subtract lazyfree pages in the userspace?
> > 
> > Could you pls. tell me how to subtract lazyfree pages in the userspace?
> > Pls. note that we can't use (pglazyfree - pglazyfreed) because
> > pglazyfreed is only counted in the regular reclaim path while the
> > process exit path is not counted, that means we have to introduce
> > another counter like LazyPage....
> 
> OK, I see your concern. I thought that we do update counters on a
> regular unmap. I do not see any reason why we shouldn't. It is indeed
> bad that we cannot tell the current number of lazy free pages by no
> means. Was this deliberate Minchan?

http://lkml.kernel.org/r/20201105162219.GG744831@cmpxchg.org has
explained this. I completely forgot about the fact that those pages can
be reused and lose their madvise status and we will learn about that
only when checking the page.

Thanks Johannes for clarification.
-- 
Michal Hocko
SUSE Labs