linux-kernel - Re: [PATCH] zsmalloc: fix migrate_zspage-zs

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Message-ID: <20160115044916.GB11203@bbox>
Date:	Fri, 15 Jan 2016 13:49:16 +0900
From:	Minchan Kim <minchan@...nel.org>
To:	Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>
CC:	Junil Lee <junil0814.lee@....com>,
	Andrew Morton <akpm@...ux-foundation.org>, ngupta@...are.org,
	linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] zsmalloc: fix migrate_zspage-zs_free race condition

On Fri, Jan 15, 2016 at 12:27:12PM +0900, Sergey Senozhatsky wrote:
> Cc Andrew,
> 
> On (01/15/16 11:35), Minchan Kim wrote:
> [..]
> > > Signed-off-by: Junil Lee <junil0814.lee@....com>
> > > ---
> > >  mm/zsmalloc.c | 1 +
> > >  1 file changed, 1 insertion(+)
> > > 
> > > diff --git a/mm/zsmalloc.c b/mm/zsmalloc.c
> > > index e7414ce..bb459ef 100644
> > > --- a/mm/zsmalloc.c
> > > +++ b/mm/zsmalloc.c
> > > @@ -1635,6 +1635,7 @@ static int migrate_zspage(struct zs_pool *pool, struct size_class *class,
> > >  		free_obj = obj_malloc(d_page, class, handle);
> > >  		zs_object_copy(free_obj, used_obj, class);
> > >  		index++;
> > > +		free_obj |= BIT(HANDLE_PIN_BIT);
> > >  		record_obj(handle, free_obj);
> > 
> > I think record_obj should store free_obj to *handle with masking off least bit.
> > IOW, how about this?
> > 
> > record_obj(handle, obj)
> > {
> >         *(unsigned long)handle = obj & ~(1<<HANDLE_PIN_BIT);
> > }
> 
> [just a wild idea]
> 
> or zs_free() can take spin_lock(&class->lock) earlier, it cannot free the

Earlier? What do you mean? For getting right class, we should get a stable
handle so we couldn't get class lock first than handle lock.
If I misunderstand, please elaborate a bit.


> object until the class is locked anyway, and migration is happening with
> the locked class. extending class->lock scope in zs_free() thus should
> not affect the perfomance. so it'll be either zs_free() is touching the
> object or the migration, not both.
> 
> 	-ss