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From:   "Huang\, Ying" <ying.huang@intel.com>
To:     Mel Gorman <mgorman@suse.de>
Cc:     Michal Hocko <mhocko@kernel.org>,
        David Hildenbrand <david@redhat.com>,
        Johannes Weiner <hannes@cmpxchg.org>,
        Matthew Wilcox <willy@infradead.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        <linux-mm@kvack.org>, <linux-kernel@vger.kernel.org>,
        Vlastimil Babka <vbabka@suse.cz>, Zi Yan <ziy@nvidia.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Dave Hansen <dave.hansen@linux.intel.com>,
        Minchan Kim <minchan@kernel.org>,
        Hugh Dickins <hughd@google.com>,
        Alexander Duyck <alexander.duyck@gmail.com>
Subject: Re: [RFC 0/3] mm: Discard lazily freed pages when migrating
References: <20200228094954.GB3772@suse.de>
        <87h7z76lwf.fsf@yhuang-dev.intel.com> <20200302151607.GC3772@suse.de>
        <87zhcy5hoj.fsf@yhuang-dev.intel.com>
        <20200303080945.GX4380@dhcp22.suse.cz>
        <87o8td4yf9.fsf@yhuang-dev.intel.com>
        <20200303085805.GB4380@dhcp22.suse.cz>
        <87ftep4pzy.fsf@yhuang-dev.intel.com>
        <20200304095802.GE16139@dhcp22.suse.cz>
        <87blpc2wxj.fsf@yhuang-dev.intel.com> <20200305104859.GG3772@suse.de>
Date:   Mon, 09 Mar 2020 13:26:29 +0800
In-Reply-To: <20200305104859.GG3772@suse.de> (Mel Gorman's message of "Thu, 5
        Mar 2020 10:48:59 +0000")
Message-ID: <874kuyyu7u.fsf@yhuang-dev.intel.com>
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Mel Gorman <mgorman@suse.de> writes:

> I think the only concrete outcome has been that userspace potentially
> benefits if the total number of MADV_FREE pages is reported
> globally. Even that is marginal as smaps has the information to tell
> the difference between high RSS due to a memory leak and high RSS
> usage due to MADV_FREE. The /proc/vmstats for MADV_FREE are of
> marginal benefit given that they do not tell us much about the current
> number of MADV_FREE pages in the system.

We can implement a counter for MADV_FREE that increases when we
ClearPageSwapBacked() and decrease when we SetPageSwapBacked() for an
anonymous page.  But this cannot count lazily freed pages correctly.
Because when a clean MDV_FREE page becomes dirty, there's no page fault
so we will not be notified.  And you have never run into the MADV_FREE
issues other than the memory leaking debugging...

Best Regards,
Huang, Ying