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

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Date:   Thu, 05 Mar 2020 09:45:11 +0800
From:   "Huang\, Ying" <ying.huang@...el.com>
To:     Michal Hocko <mhocko@...nel.org>
Cc:     Mel Gorman <mgorman@...e.de>, David Hildenbrand <david@...hat.com>,
        Johannes Weiner <hannes@...xchg.org>,
        Matthew Wilcox <willy@...radead.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        <linux-mm@...ck.org>, <linux-kernel@...r.kernel.org>,
        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>,
        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 Wed 04-03-20 19:15:20, Huang, Ying wrote:
>> Michal Hocko <mhocko@...nel.org> writes:
>> 
>> > On Tue 03-03-20 19:49:53, Huang, Ying wrote:
> [...]
>> >> Because the penalty difference is so large, I
>> >> think it may be a good idea to always put clean MADV_FREE pages at the
>> >> tail of the inactive file LRU list?
>> >
>> > You are again making assumptions without giving any actual real
>> > examples. Reconstructing MADV_FREE pages cost can differ a lot.
>> 
>> In which situation the cost to reconstruct MADV_FREE pages can be higher
>> than the cost to allocate file cache page and read from disk?  Heavy
>> contention on mmap_sem?
>
> Allocating a page might be really costly but even without that.

File cache page needs to be allocated too.

> Reconstructnig the content of the page can be really high and actually
> much larger than an IO to get a page from the storage. Consider
> decompression or some other transformations to get the content.
> Userspace has means to detect that the content is still up-to-date as
> already has been mentioned so all those steps can be avoided.

Although I don't believe someone has really done this.  It's possible at
least in theory.

Best Regards,
Huang, Ying