Re: [PATCH] mm/mlock: fix BUG_ON unlocked page for nolinear VMAs

[Sasha Levin]

On 12/18/2013 04:43 PM, Andrew Morton wrote:
> On Wed, 18 Dec 2013 17:23:03 +0800 Wanpeng Li <liwanp@xxxxxxxxxxxxxxxxxx> wrote:
>
> > > > diff --git a/mm/rmap.c b/mm/rmap.c
> > > > index 55c8b8d..1e24813 100644
> > > > --- a/mm/rmap.c
> > > > +++ b/mm/rmap.c
> > > > @@ -1347,6 +1347,7 @@ static int try_to_unmap_cluster(unsigned long cursor, unsigned int *mapcount,
> > > >   	unsigned long end;
> > > >   	int ret = SWAP_AGAIN;
> > > >   	int locked_vma = 0;
> > > > +	int we_locked = 0;
> > > >
> > > >   	address = (vma->vm_start + cursor) & CLUSTER_MASK;
> > > >   	end = address + CLUSTER_SIZE;
> > > > @@ -1385,9 +1386,15 @@ static int try_to_unmap_cluster(unsigned long cursor, unsigned int *mapcount,
> > > >   		BUG_ON(!page || PageAnon(page));
> > > >
> > > >   		if (locked_vma) {
> > > > -			mlock_vma_page(page);   /* no-op if already mlocked */
> > > > -			if (page == check_page)
> > > > +			if (page != check_page) {
> > > > +				we_locked = trylock_page(page);
> > >
> > > If it's not us who has the page already locked, but somebody else, he
> > > might unlock it at this point and then the BUG_ON in mlock_vma_page()
> > > will trigger again.
>
> yes, this patch is pretty weak.
>
> > Any better idea is appreciated. ;-)
>
> Remove the BUG_ON() from mlock_vma_page()?  Why was it added?
> isolate_lru_page() and putback_lru_page() and *might* require
> the page be locked, but I don't immediately see issues?

Ping? This BUG() is triggerable in 3.13-rc6 right now.


Thanks,
Sasha

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