Re: [PATCH 4/4] block: Optionally snapshot page contents to provide stable pages during write

["Darrick J. Wong" <darrick.wong@oracle.com>]

On Mon, Dec 17, 2012 at 12:13:36AM +0800, Zheng Liu wrote:
> On Thu, Dec 13, 2012 at 12:08:11AM -0800, Darrick J. Wong wrote:
> > diff --git a/mm/bounce.c b/mm/bounce.c
> > index 0420867..fa11935 100644
> > --- a/mm/bounce.c
> > +++ b/mm/bounce.c
> > @@ -178,6 +178,38 @@ static void bounce_end_io_read_isa(struct bio *bio, int err)
> >  	__bounce_end_io_read(bio, isa_page_pool, err);
> >  }
> >  
> > +#ifdef CONFIG_NEED_BOUNCE_POOL
> > +static int might_snapshot_stable_page_write(struct bio **bio_orig)
> > +{
> > +	return bio_data_dir(*bio_orig) == WRITE;
> > +}
> > +
> > +static int should_snapshot_stable_pages(struct page *page, int rw)
> > +{
> > +	struct backing_dev_info *bdi;
> > +	struct address_space *mapping = page_mapping(page);
> > +
> > +	if (!mapping)
> > +		return 0;
> > +	bdi = mapping->backing_dev_info;
> > +	if (!bdi_cap_stable_pages_required(bdi))
> > +		return 0;
> > +
> > +	return mapping->host->i_sb->s_flags & MS_SNAP_STABLE &&
> > +	       rw == WRITE;
> > +}
> > +#else
> > +static int might_snapshot_stable_page_write(struct bio **bio_orig)
> > +{
> > +	return 0;
> > +}
> > +
> > +static int should_snapshot_static_pages(struct page *page, int rw)
>                               ^^^^^^
>                               It should be _stable_.

Good catch!  Thank you!

--D
> 
> Regards,
>                                                 - Zheng
> > +{
> > +	return 0;
> > +}
> > +#endif /* CONFIG_NEED_BOUNCE_POOL */
> > +
> >  static void __blk_queue_bounce(struct request_queue *q, struct bio **bio_orig,
> >  			       mempool_t *pool)
> >  {
> > @@ -192,7 +224,8 @@ static void __blk_queue_bounce(struct request_queue *q, struct bio **bio_orig,
> >  		/*
> >  		 * is destination page below bounce pfn?
> >  		 */
> > -		if (page_to_pfn(page) <= queue_bounce_pfn(q))
> > +		if (page_to_pfn(page) <= queue_bounce_pfn(q) &&
> > +		    !should_snapshot_stable_pages(page, rw))
> >  			continue;
> >  
> >  		/*
> > @@ -284,7 +317,8 @@ void blk_queue_bounce(struct request_queue *q, struct bio **bio_orig)
> >  	 * don't waste time iterating over bio segments
> >  	 */
> >  	if (!(q->bounce_gfp & GFP_DMA)) {
> > -		if (queue_bounce_pfn(q) >= blk_max_pfn)
> > +		if (queue_bounce_pfn(q) >= blk_max_pfn &&
> > +		    !might_snapshot_stable_page_write(bio_orig))
> >  			return;
> >  		pool = page_pool;
> >  	} else {
> --
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