linux-kernel - Re: [LSF/MM TOPIC] Page flags, can we free up space ?

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Date:   Wed, 30 Jan 2019 12:51:57 +0100
From:   David Hildenbrand <david@...hat.com>
To:     Jerome Glisse <jglisse@...hat.com>,
        lsf-pc@...ts.linux-foundation.org
Cc:     linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        Michal Hocko <mhocko@...nel.org>
Subject: Re: [LSF/MM TOPIC] Page flags, can we free up space ?

On 22.01.19 21:17, Jerome Glisse wrote:
> So lattely i have been looking at page flags and we are using 6 flags
> for memory reclaim and compaction:
> 
>     PG_referenced
>     PG_lru
>     PG_active
>     PG_workingset
>     PG_reclaim
>     PG_unevictable
> 
> On top of which you can add the page anonymous flag (anonymous or
> share memory)
>     PG_anon // does not exist, lower bit of page->mapping
> 
> And also the movable flag (which alias with KSM)
>     PG_movable // does not exist, lower bit of page->mapping


I would really like to see an easier way to spot if a page is movable.

__PageMovable() can produce way to many false positives.

movable will usually not be paired with other flags you mentioned as of now.

If many of these flags are not used in combination, we could merge some
of the flags into a number field. Valid combinations would get a number
assigned.

To keep it simple, only flags that are completely exclusive might be a
candidate. But not sure if we really have many of these.

> 
> 
> So i would like to explore if there is a way to express the same amount
> of information with less bits. My methodology is to exhaustively list
> all the possible states (valid combination of above flags) and then to
> see how we change from one state to another (what event trigger the change
> like mlock(), page being referenced, ...) and under which rules (ie do we
> hold the page lock, zone lock, ...).
> 
> My hope is that there might be someway to use less bits to express the
> same thing. I am doing this because for my work on generic page write
> protection (ie KSM for file back page) which i talk about last year and
> want to talk about again ;) I will need to unalias the movable bit from
> KSM bit.
> 
> 
> Right now this is more a temptative ie i do not know if i will succeed,
> in any case i can report on failure or success and discuss my finding to
> get people opinions on the matter.
> 
> 
> I think everyone interested in mm will be interested in this topic :)
> 
> Cheers,
> Jérôme
> 


-- 

Thanks,

David / dhildenb