Re: [PATCH RESEND] zram: correct offset usage in zram_bio_discard

[Sergey Senozhatsky]

Hello Minchan,

On (04/24/14 11:06), Minchan Kim wrote:
> Hello,
> 
> On Wed, Apr 23, 2014 at 04:41:15PM +0800, Weijie Yang wrote:
> > We want to skip the physical block(PAGE_SIZE) which is partially
> > covered by the discard bio, so we check the remaining size and
> > subtract it if there is a need to goto the next physical block.
> > 
> > The current offset usage in zram_bio_discard is incorrect, it will
> > cause its upper filesystem breakdown.
> > Consider the following scenario:
> > on some architecture or config, PAGE_SIZE is 64K for example,
> > filesystem is set up on zram disk without PAGE_SIZE aligned,
> > a discard bio leads to a offset = 4K and size=72K,
> > normally, it should not really discard any physical block as it
> > partially cover two physical blocks.
> > However, with the current offset usage, it will discard the second
> > physical block and free its memory, which will cause filesystem breakdown.
> > 
> > This patch corrects the offset usage in zram_bio_discard.
> 
> Nice catch.
> Surely we need fix but I'd like to go further. Let's think.
> How do you find that? Real problem or code review?
> I'd like to know how much that happens in real practice because if it's
> a lot, it means discard support is just an overhead without any benefit.
> 
> If you just found it with code review(ie, don't have any data),
> would you mind adding some stat like 'num_discard/failed_discard' so
> we can keep an eye on that. Although it's for rare case, I think it's worth.
> Because someone would do swapon zram with --discard,
> it could make same problem due to early page freeing of zram-swap to
> avoid duplicating VM-owned memory and ZRAM-owned memory.
> 
> We can guide to zram-swap user not to use swapon with --discard but
> I don't want it because swap_slot_free_notify is really mess which
> violates layering so I hope replace it with discard finally so such
> statistics really help us to drive that way more quickly.
> 

I second this. if we could drop swap_slot_free_notify that'd be nice.

	-ss

> > 
> > Signed-off-by: Weijie Yang <weijie.yang@xxxxxxxxxxx>
> > Acked-by: Joonsoo Kim <iamjoonsoo.kim@xxxxxxx>
> > ---
> >  drivers/block/zram/zram_drv.c |    4 ++--
> >  1 file changed, 2 insertions(+), 2 deletions(-)
> > 
> > diff --git a/drivers/block/zram/zram_drv.c b/drivers/block/zram/zram_drv.c
> > index 9849b52..48eccb3 100644
> > --- a/drivers/block/zram/zram_drv.c
> > +++ b/drivers/block/zram/zram_drv.c
> > @@ -572,10 +572,10 @@ static void zram_bio_discard(struct zram *zram, u32 index,
> >  	 * skipping this logical block is appropriate here.
> >  	 */
> >  	if (offset) {
> > -		if (n < offset)
> > +		if (n <= (PAGE_SIZE - offset))
> >  			return;
> >  
> > -		n -= offset;
> > +		n -= (PAGE_SIZE - offset);
> >  		index++;
> >  	}
> >  
> > -- 
> > 1.7.10.4
> > 
> > 
> > --
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> 
> -- 
> Kind regards,
> Minchan Kim
> 
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