linux-kernel - Re: [PATCH V5 6/6] proc: show MADV

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Date:   Wed, 1 Mar 2017 14:36:24 +0100
From:   Michal Hocko <mhocko@...nel.org>
To:     Shaohua Li <shli@...com>
Cc:     linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        Kernel-team@...com, minchan@...nel.org, hughd@...gle.com,
        hannes@...xchg.org, riel@...hat.com, mgorman@...hsingularity.net,
        akpm@...ux-foundation.org
Subject: Re: [PATCH V5 6/6] proc: show MADV_FREE pages info in smaps

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.
---
diff --git a/mm/madvise.c b/mm/madvise.c
index dc5927c812d3..d2c318db16c9 100644
--- a/mm/madvise.c
+++ b/mm/madvise.c
@@ -474,7 +474,7 @@ static int madvise_free_single_vma(struct vm_area_struct *vma,
 	madvise_free_page_range(&tlb, vma, start, end);
 	mmu_notifier_invalidate_range_end(mm, start, end);
 	tlb_finish_mmu(&tlb, start, end);
-
+	lru_add_drain_all();
 	return 0;
 }
 
-- 
Michal Hocko
SUSE Labs