Re: [PATCH v3] zram: support REQ_DISCARD

[Joonsoo Kim]

On Wed, Mar 12, 2014 at 01:33:18PM -0700, Andrew Morton wrote:
> On Wed, 12 Mar 2014 17:01:09 +0900 Joonsoo Kim <iamjoonsoo.kim@xxxxxxx> wrote:
> 
> > zram is ram based block device and can be used by backend of filesystem.
> > When filesystem deletes a file, it normally doesn't do anything on data
> > block of that file. It just marks on metadata of that file. This behavior
> > has no problem on disk based block device, but has problems on ram based
> > block device, since we can't free memory used for data block. To overcome
> > this disadvantage, there is REQ_DISCARD functionality. If block device
> > support REQ_DISCARD and filesystem is mounted with discard option,
> > filesystem sends REQ_DISCARD to block device whenever some data blocks are
> > discarded. All we have to do is to handle this request.
> > 
> > This patch implements to flag up QUEUE_FLAG_DISCARD and handle this
> > REQ_DISCARD request. With it, we can free memory used by zram if it isn't
> > used.
> > 
> > ...
> >
> > --- a/drivers/block/zram/zram_drv.c
> > +++ b/drivers/block/zram/zram_drv.c
> > @@ -541,6 +541,33 @@ static int zram_bvec_rw(struct zram *zram, struct bio_vec *bvec, u32 index,
> >  	return ret;
> >  }
> >  
> > +static void zram_bio_discard(struct zram *zram, u32 index,
> > +			     int offset, struct bio *bio)
> 
> A little bit of documentation here wouldn't hurt.  "index" and "offset"
> are pretty vague identifiers.  What do these args represent and what
> are their units.
> 
> > +{
> > +	size_t n = bio->bi_iter.bi_size;
> > +
> > +	/*
> > +	 * On some arch, logical block (4096) aligned request couldn't be
> > +	 * aligned to PAGE_SIZE, since their PAGE_SIZE aren't 4096.
> > +	 * Therefore we should handle this misaligned case here.
> > +	 */
> > +	if (offset) {
> > +		if (n < offset)
> > +			return;
> > +
> > +		n -= offset;
> > +		index++;
> > +	}
> > +
> > +	while (n >= PAGE_SIZE) {
> > +		write_lock(&zram->meta->tb_lock);
> > +		zram_free_page(zram, index);
> > +		write_unlock(&zram->meta->tb_lock);
> > +		index++;
> > +		n -= PAGE_SIZE;
> > +	}
> 
> We could take the lock a single time rather than once per page.  Was
> there a reason for doing it this way?  If so, that should be documented
> as well please - there is no way a reader can know the reason from this
> code.
> 
> 
> > +}
> > +
> >  static void zram_reset_device(struct zram *zram, bool reset_capacity)
> >  {
> >  	size_t index;
> > @@ -676,6 +703,12 @@ static void __zram_make_request(struct zram *zram, struct bio *bio)
> >  	offset = (bio->bi_iter.bi_sector &
> >  		  (SECTORS_PER_PAGE - 1)) << SECTOR_SHIFT;
> >  
> > +	if (unlikely(bio->bi_rw & REQ_DISCARD)) {
> > +		zram_bio_discard(zram, index, offset, bio);
> > +		bio_endio(bio, 0);
> > +		return;
> > +	}
> > +
> >  	bio_for_each_segment(bvec, bio, iter) {
> >  		int max_transfer_size = PAGE_SIZE - offset;
> >  
> > @@ -845,6 +878,17 @@ static int create_device(struct zram *zram, int device_id)
> >  					ZRAM_LOGICAL_BLOCK_SIZE);
> >  	blk_queue_io_min(zram->disk->queue, PAGE_SIZE);
> >  	blk_queue_io_opt(zram->disk->queue, PAGE_SIZE);
> > +	zram->disk->queue->limits.discard_granularity = PAGE_SIZE;
> > +	zram->disk->queue->limits.max_discard_sectors = UINT_MAX;
> > +	/*
> > +	 * We will skip to discard mis-aligned range, so we can't ensure
> > +	 * whether discarded region is zero or not.
> > +	 */
> 
> That's a bit hard to follow.  What is it that is misaligned, relative
> to what?
> 
> And where does this skipping occur?  zram_bio_discard() avoids
> discarding partial pages at the start and end of the bio (I think).  Is
> that what we're referring to here?  If so, what about the complete
> pages between the two partial pages - they are zeroed on read.  Will
> the code end up having to rezero those?
> 
> As you can tell, I'm struggling to understand what's going on here ;)
> Some additional description of how it all works would be nice.  Perferably
> as code comments so the information is permanent.

Hello, Andrew.

I applied all your comments in below patch. :)
Thanks for comment.

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