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From:   "Huang\, Ying" <ying.huang@intel.com>
To:     Michal Hocko <mhocko@kernel.org>
Cc:     Mel Gorman <mgorman@suse.de>, 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>
        <20200304112653.GH16139@dhcp22.suse.cz>
Date:   Thu, 05 Mar 2020 09:45:11 +0800
In-Reply-To: <20200304112653.GH16139@dhcp22.suse.cz> (Michal Hocko's message
        of "Wed, 4 Mar 2020 12:26:53 +0100")
Message-ID: <8736an3788.fsf@yhuang-dev.intel.com>
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Michal Hocko <mhocko@kernel.org> writes:

> On Wed 04-03-20 19:15:20, Huang, Ying wrote:
>> Michal Hocko <mhocko@kernel.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