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SubjectRe: [PATCH 06/10] ksm: identify PageKsm pages
On Wed, 22 Jul 2009 13:54:06 +0100 (BST)
Hugh Dickins <hugh.dickins@tiscali.co.uk> wrote:

> On Tue, 21 Jul 2009, Rik van Riel wrote:
> > Andrea Arcangeli wrote:
> > > > - if (PageAnon(old_page)) {
> > > > + if (PageAnon(old_page) && !PageKsm(old_page)) {
> > > > if (!trylock_page(old_page)) {
> > > > page_cache_get(old_page);
> > > > pte_unmap_unlock(page_table, ptl);
> > >
> > > What exactly does it buy to have PageAnon return 1 on ksm pages,
> > > besides requiring the above additional check (that if we stick to the
> > > above code, I would find safer to move inside reuse_swap_page).
> >
> > I guess that if they are to remain unswappable, they
> > should go onto the unevictable list.
>
> The KSM pages are not put on any LRU, so wouldn't be slowing vmscan
> down with futile scans: isn't the unevictable list for pages which
> belong to another LRU once they become evictable again?
>
> (At this instant I've forgotten why there's an unevictable list at
> all - somewhere in vmscan.c which is accustomed to dealing with
> pages on lists, so easier to have them on a list than not?)
>
I forget, too. But in short thinking, Unevictable pages should be
on LRU (marked as PG_lru) for isolating page (from LRU) called by
page migration etc.

isolate_lru_page()
-> put page on private list
-> do some work
-> putback_lru_page()

sequence is useful at handling pages in a list.
Because mlock/munclock can be called arbitrarily, unevicatable lru
works enough good for making above kinds of code simpler.

Thanks,
-Kame




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