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

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Date:	Fri, 15 Jan 2016 12:30:15 +0900
From:	Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>
To:	Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>
Cc:	Minchan Kim <minchan@...nel.org>,
	Junil Lee <junil0814.lee@....com>,
	Andrew Morton <akpm@...ux-foundation.org>, ngupta@...are.org,
	linux-mm@...ck.org, linux-kernel@...r.kernel.org,
	Sergey Senozhatsky <sergey.senozhatsky@...il.com>
Subject: Re: [PATCH] zsmalloc: fix migrate_zspage-zs_free race condition

On (01/15/16 12:27), Sergey Senozhatsky wrote:
> > > @@ -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
> object until the class is locked anyway, and migration is happening with
			  UNlocked

> 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