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Date:   Mon, 2 Mar 2020 15:25:49 +0100
From:   Michal Hocko <mhocko@kernel.org>
To:     "Huang, Ying" <ying.huang@intel.com>
Cc:     David Hildenbrand <david@redhat.com>,
        Matthew Wilcox <willy@infradead.org>,
        Andrew Morton <akpm@linux-foundation.org>, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org, Mel Gorman <mgorman@suse.de>,
        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>,
        Johannes Weiner <hannes@cmpxchg.org>,
        Hugh Dickins <hughd@google.com>,
        Alexander Duyck <alexander.duyck@gmail.com>
Subject: Re: [RFC 0/3] mm: Discard lazily freed pages when migrating
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On Mon 02-03-20 22:12:53, Huang, Ying wrote:
> Michal Hocko <mhocko@kernel.org> writes:
[...]
> > And MADV_FREE pages are a kind of cache as well. If the target node is
> > short on memory then those will be reclaimed as a cache so a
> > pro-active freeing sounds counter productive as you do not have any
> > idea whether that cache is going to be used in future. In other words
> > you are not going to free a clean page cache if you want to use that
> > memory as a migration target right? So you should make a clear case
> > about why MADV_FREE cache is less important than the clean page cache
> > and ideally have a good justification backed by real workloads.
> 
> Clean page cache still have valid contents, while clean MADV_FREE pages
> has no valid contents.  So penalty of discarding the clean page cache is
> reading from disk, while the penalty of discarding clean MADV_FREE pages
> is just page allocation and zeroing.

And "just page allocation and zeroing" overhead is the primary
motivation to keep the page in memory. It is a decision of the workload
to use MADV_FREE because chances are that this will speed things up. All
that with a contract that the memory goes away under memory pressure so
with a good workload/memory sizing you do not really lose that
optimization. Now you want to make decision on behalf of the consumer of
the MADV_FREE memory.

> I understand that MADV_FREE is another kind of cache and has its value.
> But in the original implementation, during migration, we have already
> freed the original "cache", then reallocate the cache elsewhere and
> copy.  This appears more like all pages are populated in mmap() always.
> I know there's value to populate all pages in mmap(), but does that need
> to be done always by default?

It is not. You have to explicitly request MAP_POPULATE to initialize
mmap.

-- 
Michal Hocko
SUSE Labs