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

[KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>]

On Thu, 1 Apr 2010 13:44:29 +0900
Minchan Kim <minchan.kim@gmail.com> wrote:

> On Thu, Apr 1, 2010 at 12:01 PM, KAMEZAWA Hiroyuki
> <kamezawa.hiroyu@jp.fujitsu.com> wrote:
> > On Thu, 1 Apr 2010 11:43:18 +0900
> > Minchan Kim <minchan.kim@gmail.com> wrote:
> >
> >> On Wed, Mar 31, 2010 at 2:26 PM, KAMEZAWA Hiroyuki A  A  A  /*
> >> >> 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 *,
> >> >>
> >> >> A  A  A  if (unlikely(PageKsm(page)))
> >> >> A  A  A  A  A  A  A  return rmap_walk_ksm(page, rmap_one, arg);
> >> >> - A  A  else if (PageAnon(page))
> >> >> + A  A  else if (PageAnon(page)) {
> >> >> + A  A  A  A  A  A  if (PageSwapCache(page))
> >> >> + A  A  A  A  A  A  A  A  A  A  return SWAP_AGAIN;
> >> >> A  A  A  A  A  A  A  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()...
> >> > A  A  A  swap_free(entry);
> >> > A  A  A  A if (vm_swap_full() || (vma->vm_flags & VM_LOCKED) || PageMlocked(page))
> >> > A  A  A  A  A  A  A  A try_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()
> > A 1. try_to_unmap(TTU_MIGRATION)
> > A 2. move_to_newpage
> > A 3. remove_migration_ptes
> > A  A  A  A -> rmap_walk()
> >
> > Then, to map a page back we unmapped we call rmap_walk().
> >
> > Assume a SwapCache which is mapped, then, PageAnon(page) == true.
> >
> > A At 1. try_to_unmap() will rewrite pte with swp_entry of SwapCache.
> > A  A  A  mapcount goes to 0.
> > A At 2. SwapCache is copied to a new page.
> > A At 3. The new page is mapped back to the place. Now, newpage's mapcount is 0.
> > A  A  A  Before patch, the new page is mapped back to all ptes.
> > A  A  A  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. I agree. We don't need the check.
> Then, my question is why Mel added the check in rmap_walk.
> He mentioned some BUG trigger and fixed things after this patch.
> What's it?
> Is it really related to this logic?
> I don't think so or we are missing something.
> 
Hmm. Consiering again.

Now.
	if (PageAnon(page)) {
		rcu_locked = 1;
		rcu_read_lock();
		if (!page_mapped(page)) {
			if (!PageSwapCache(page))
				goto rcu_unlock;
		} else {
			anon_vma = page_anon_vma(page);
			atomic_inc(&anon_vma->external_refcount);
		}


Maybe this is a fix.

==
	skip_remap = 0;
	if (PageAnon(page)) {
		rcu_read_lock();
		if (!page_mapped(page)) {
			if (!PageSwapCache(page))
				goto rcu_unlock;
			/*
			 * We can't convice this anon_vma is valid or not because
			 * !page_mapped(page). Then, we do migration(radix-tree replacement)
			 * but don't remap it which touches anon_vma in page->mapping.
			 */
			skip_remap = 1;
			goto skip_unmap;
		} else {
			anon_vma = page_anon_vma(page);
			atomic_inc(&anon_vma->external_refcount);
		}
	}	
	.....copy page, radix-tree replacement,....

	if (!rc && !skip_remap)
		 remove_migration_ptes(page, page);
==

Thanks,
-Kame











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