Re: [PATCH 14/14] mm,migration: Allow the migration ofPageSwapCache pages

[KAMEZAWA Hiroyuki]

On Thu, 1 Apr 2010 11:43:18 +0900
Minchan Kim <minchan.kim@xxxxxxxxx> wrote:

> On Wed, Mar 31, 2010 at 2:26 PM, KAMEZAWA Hiroyuki    /*
> >> diff --git a/mm/rmap.c b/mm/rmap.c
> >> index af35b75..d5ea1f2 100644
> >> --- a/mm/rmap.c
> >> +++ b/mm/rmap.c
> >> @@ -1394,9 +1394,11 @@ int rmap_walk(struct page *page, int (*rmap_one)(struct page *,
> >>
> >> Â Â Â if (unlikely(PageKsm(page)))
> >> Â Â Â Â Â Â Â return rmap_walk_ksm(page, rmap_one, arg);
> >> - Â Â else if (PageAnon(page))
> >> + Â Â else if (PageAnon(page)) {
> >> + Â Â Â Â Â Â if (PageSwapCache(page))
> >> + Â Â Â Â Â Â Â Â Â Â return SWAP_AGAIN;
> >> Â Â Â Â Â Â Â return rmap_walk_anon(page, rmap_one, arg);
> >
> > SwapCache has a condition as (PageSwapCache(page) && page_mapped(page) == true.
> >
> 
> In case of tmpfs, page has swapcache but not mapped.
> 
> > Please see do_swap_page(), PageSwapCache bit is cleared only when
> >
> > do_swap_page()...
> > Â Â Â swap_free(entry);
> > Â Â Â Âif (vm_swap_full() || (vma->vm_flags & VM_LOCKED) || PageMlocked(page))
> > Â Â Â Â Â Â Â Âtry_to_free_swap(page);
> >
> > Then, PageSwapCache is cleared only when swap is freeable even if mapped.
> >
> > rmap_walk_anon() should be called and the check is not necessary.
> 
> Frankly speaking, I don't understand what is Mel's problem, why he added
> Swapcache check in rmap_walk, and why do you said we don't need it.
> 
> Could you explain more detail if you don't mind?
> 
I may miss something.

unmap_and_move()
 1. try_to_unmap(TTU_MIGRATION)
 2. move_to_newpage
 3. remove_migration_ptes
	-> rmap_walk()

Then, to map a page back we unmapped we call rmap_walk().

Assume a SwapCache which is mapped, then, PageAnon(page) == true.

 At 1. try_to_unmap() will rewrite pte with swp_entry of SwapCache.
       mapcount goes to 0.
 At 2. SwapCache is copied to a new page.
 At 3. The new page is mapped back to the place. Now, newpage's mapcount is 0.
       Before patch, the new page is mapped back to all ptes.
       After patch, the new page is not mapped back because its mapcount is 0.

I don't think shared SwapCache of anon is not an usual behavior, so, the logic
before patch is more attractive.

If SwapCache is not mapped before "1", we skip "1" and rmap_walk will do nothing
because page->mapping is NULL.

Thanks,
-Kame
















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