Re: [patch 1/3] vmscan: factor out page reference checks

[Minchan Kim <minchan.kim@gmail.com>]

On Wed, Feb 24, 2010 at 12:40 AM, Johannes Weiner <hannes@cmpxchg.org> wrote:
> Hello Minchan,
>
> On Tue, Feb 23, 2010 at 11:44:14PM +0900, Minchan Kim wrote:
>> On Tue, 2010-02-23 at 15:21 +0100, Johannes Weiner wrote:
>> > Hello Minchan,
>> >
>> > On Tue, Feb 23, 2010 at 10:38:23PM +0900, Minchan Kim wrote:
>>
>> <snip>
>>
>> > > >
>> > > >                 if (PageDirty(page)) {
>> > > > -                       if (sc->order <= PAGE_ALLOC_COSTLY_ORDER && referenced)
>> > > > +                       if (references == PAGEREF_RECLAIM_CLEAN)
>> > >
>> > > How equal PAGEREF_RECLAIM_CLEAN and sc->order <= PAGE_ALLOC_COSTLY_ORDER
>> > > && referenced by semantic?
>> >
>> > It is encoded in page_check_references().  When
>> >     sc->order <= PAGE_ALLOC_COSTLY_ORDER && referenced
>> > it returns PAGEREF_RECLAIM_CLEAN.
>> >
>> > So
>> >
>> >     - PageDirty() && order < COSTLY && referenced
>> >     + PageDirty() && references == PAGEREF_RECLAIM_CLEAN
>> >
>> > is an equivalent transformation.  Does this answer your question?
>>
>> Hmm. I knew it. My point was PAGEREF_RECLAIM_CLEAN seems to be a little
>> awkward. I thought PAGEREF_RECLAIM_CLEAN means if the page was clean, it
>> can be reclaimed.
>
> But you were thinking right, it is exactly what it means!  If
> the state is PAGEREF_RECLAIM_CLEAN, reclaim the page if it is clean:
>
>        if (PageDirty(page)) {
>                if (references == PAGEREF_RECLAIM_CLEAN)
>                        goto keep_locked;       /* do not reclaim */
>                ...
>        }
>
>> I think it would be better to rename it with represent "Although it's
>> referenced page recently, we can reclaim it if VM try to reclaim high
>> order page".
>
> I changed it to PAGEREF_RECLAIM_LUMPY and PAGEREF_RECLAIM, but I felt
> it made it worse.  It's awkward that we have to communicate that state
> at all, maybe it would be better to do
>
>        if (PageDirty(page) && referenced_page)
>                return PAGEREF_KEEP;
>
> in page_check_references()?  But doing PageDirty() twice is also kinda
> lame.
>
> I don't know.  Can we leave it like that for now?

I hope as it is if we don't have any better idea and you don't feel it strong.
But let's listen to other's opinion. maybe they have a good idea.

Thanks, Hannes.



-- 
Kind regards,
Minchan Kim

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