On Thu, 1 Apr 2010 13:44:29 +0900
Minchan Kim <minchan.kim@xxxxxxxxx> wrote:
> On Thu, Apr 1, 2010 at 12:01 PM, KAMEZAWA Hiroyuki
> <kamezawa.hiroyu@xxxxxxxxxxxxxx> wrote:
> > 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 *,
> >> >>
> >> >> if (unlikely(PageKsm(page)))
> >> >> return rmap_walk_ksm(page, rmap_one, arg);
> >> >> - else if (PageAnon(page))
> >> >> + else if (PageAnon(page)) {
> >> >> + if (PageSwapCache(page))
> >> >> + return SWAP_AGAIN;
> >> >> 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()...
> >> > swap_free(entry);
> >> > if (vm_swap_full() || (vma->vm_flags & VM_LOCKED) || PageMlocked(page))
> >> > 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()
> > 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. 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
--
To unsubscribe, send a message with 'unsubscribe linux-mm' in
the body to majordomo@xxxxxxxxxx For more info on Linux MM,
see: http://www.linux-mm.org/ .
Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>