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

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Date:	Fri, 15 Jan 2016 12:27:12 +0900
From:	Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>
To:	Minchan Kim <minchan@...nel.org>
Cc:	Junil Lee <junil0814.lee@....com>,
	Andrew Morton <akpm@...ux-foundation.org>, ngupta@...are.org,
	sergey.senozhatsky.work@...il.com, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] zsmalloc: fix migrate_zspage-zs_free race condition

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
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