RE: [PATCH 1/1] mm/migrate: fix list corruption in migration of non-LRU movable pages

[sunqiuyang]

________________________________________
From: Michal Hocko [mhocko@xxxxxxxxxx]
Sent: Wednesday, September 04, 2019 20:52
To: sunqiuyang
Cc: linux-kernel@xxxxxxxxxxxxxxx; linux-mm@xxxxxxxxx
Subject: Re: [PATCH 1/1] mm/migrate: fix list corruption in migration of non-LRU movable pages

On Wed 04-09-19 12:19:11, sunqiuyang wrote:
> > Do not top post please
> >
> > On Wed 04-09-19 07:27:25, sunqiuyang wrote:
> > > isolate_migratepages_block() from another thread may try to isolate the page again:
> > >
> > > for (; low_pfn < end_pfn; low_pfn++) {
> > >   /* ... */
> > >   page = pfn_to_page(low_pfn);
> > >  /* ... */
> > >   if (!PageLRU(page)) {
> > >     if (unlikely(__PageMovable(page)) && !PageIsolated(page)) {
> > >         /* ... */
> > >         if (!isolate_movable_page(page, isolate_mode))
> > >           goto isolate_success;
> > >       /*... */
> > > isolate_success:
> > >      list_add(&page->lru, &cc->migratepages);
> > >
> > > And this page will be added to another list.
> > > Or, do you see any reason that the page cannot go through this path?
> >
> > The page shouldn't be __PageMovable after the migration is done. All the
> > state should have been transfered to the new page IIUC.
> >
>
> I don't see where page->mapping is modified after the migration is done.
>
> Actually, the last comment in move_to_new_page() says,
> "Anonymous and movable page->mapping will be cleard by
> free_pages_prepare so don't reset it here for keeping
> the type to work PageAnon, for example. "
>
> Or did I miss something? Thanks,

This talks about mapping rather than flags stored in the mapping.
I can see that in tree migration handlers (z3fold_page_migrate,
vmballoon_migratepage via balloon_page_delete, zs_page_migrate via
reset_page) all reset the movable flag. I am not sure whether that is a
documented requirement or just a coincidence. Maybe it should be
documented. I would like to hear from Minchan.

---
I checked the three migration handlers and only found __ClearPageMovable,
which clears registered address_space val with keeping PAGE_MAPPING_MOVABLE flag,
so the page should still be __PageMovable when caught by another migration thread. Right?

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--
Michal Hocko
SUSE Labs