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    SubjectRe: [PATCH V5 6/6] proc: show MADV_FREE pages info in smaps
    On Wed, Mar 01, 2017 at 06:49:56PM +0100, Michal Hocko wrote:
    > On Wed 01-03-17 09:37:10, Shaohua Li wrote:
    > > On Wed, Mar 01, 2017 at 02:36:24PM +0100, Michal Hocko wrote:
    > > > On Fri 24-02-17 13:31:49, Shaohua Li wrote:
    > > > > show MADV_FREE pages info of each vma in smaps. The interface is for
    > > > > diganose or monitoring purpose, userspace could use it to understand
    > > > > what happens in the application. Since userspace could dirty MADV_FREE
    > > > > pages without notice from kernel, this interface is the only place we
    > > > > can get accurate accounting info about MADV_FREE pages.
    > > >
    > > > I have just got to test this patchset and noticed something that was a
    > > > bit surprising
    > > >
    > > > madvise(mmap(len), len, MADV_FREE)
    > > > Size: 102400 kB
    > > > Rss: 102400 kB
    > > > Pss: 102400 kB
    > > > Shared_Clean: 0 kB
    > > > Shared_Dirty: 0 kB
    > > > Private_Clean: 102400 kB
    > > > Private_Dirty: 0 kB
    > > > Referenced: 0 kB
    > > > Anonymous: 102400 kB
    > > > LazyFree: 102368 kB
    > > >
    > > > It took me a some time to realize that LazyFree is not accurate because
    > > > there are still pages on the per-cpu lru_lazyfree_pvecs. I believe this
    > > > is an implementation detail which shouldn't be visible to the userspace.
    > > > Should we simply drain the pagevec? A crude way would be to simply
    > > > lru_add_drain_all after we are done with the given range. We can also
    > > > make this lru_lazyfree_pvecs specific but I am not sure this is worth
    > > > the additional code.
    > >
    > > Minchan's original patch includes a drain of pvec. I discard it because I think
    > > it's not worth the effort. There aren't too many memory in the per-cpu vecs.
    >
    > but multiply that by the number of CPUs.
    >
    > > Like what you said, I doubt this is noticeable to userspace.
    >
    > maybe I wasn't clear enough. I've noticed and I expect others would as
    > well. We really shouldn't leak implementation details like that. So I
    > _believe_ this should be fixed. Draining all pagevecs is rather coarse
    > but it is the simplest thing to do. If you do not want to fold this
    > into the original patch I can send a standalone one. Or do you have any
    > concerns about draining?

    No, no objection at all. Just doubt it's worthy. Looks nobody complains similar
    issue, For exmaple, deactivate_file_page does the similar thing, then the smaps
    'Referenced' could be inaccurate.

    Thanks,
    Shaohua

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