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

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Message-ID: <20190904125226.GV3838@dhcp22.suse.cz>
Date:   Wed, 4 Sep 2019 14:52:26 +0200
From:   Michal Hocko <mhocko@...nel.org>
To:     sunqiuyang <sunqiuyang@...wei.com>
Cc:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>
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.

-- 
Michal Hocko
SUSE Labs