linux-kernel - Re: [RFC 0/3] mm: Discard lazily freed pages when migrating

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Date:   Tue, 03 Mar 2020 19:36:57 +0800
From:   "Huang\, Ying" <ying.huang@...el.com>
To:     Michal Hocko <mhocko@...nel.org>
Cc:     David Hildenbrand <david@...hat.com>,
        Matthew Wilcox <willy@...radead.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        <linux-mm@...ck.org>, <linux-kernel@...r.kernel.org>,
        Mel Gorman <mgorman@...e.de>, Vlastimil Babka <vbabka@...e.cz>,
        Zi Yan <ziy@...dia.com>, Peter Zijlstra <peterz@...radead.org>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Minchan Kim <minchan@...nel.org>,
        "Johannes Weiner" <hannes@...xchg.org>,
        Hugh Dickins <hughd@...gle.com>,
        "Alexander Duyck" <alexander.duyck@...il.com>
Subject: Re: [RFC 0/3] mm: Discard lazily freed pages when migrating

Michal Hocko <mhocko@...nel.org> writes:

> On Tue 03-03-20 09:30:28, Huang, Ying wrote:
> [...]
>> Yes.  mmap() can control whether to populate the underlying physical
>> pages.
>
> right because many usecases benefit from it. They simply know that the
> mapping will be used completely and it is worth saving overhead for #PF.
> See. there is a clear justification for that policy.
>
>> But for migrating MADV_FREE pages, there's no control, all pages
>> will be populated again always by default.  Maybe we should avoid to do
>> that in some situations too.
>
> Now let's have a look here. It is the userspace that decided to mark
> MADV_FREE pages. It is under its full control which pages are to be
> freed lazily. If the userspace wants to move those pages then it is
> likely aware they have been MADV_FREE, right? If the userspace wanted to
> save migration overhead then it could either chose to not migrate those
> pages or simply unmap them right away. So in the end we are talking
> about saving munmap/MAMDV_DONTNEED or potentially more move_pages calls
> to skip over MADV_FREE holes. Which is all nice but is there any
> userspace that really does care? Because this is a fundamental question
> here and it doesn't make much sense to discuss this left to right unless
> this is clear.

Although I don't agree with you, I don't want to continue.  Because I
feel that the discussion may be too general to go anywhere.  I admit
that I go to the general side firstly, sorry about that.

Best Regards,
Huang, Ying