Re: [PATCH] Check for compound pages in set_page_dirty()

[Hugh Dickins <hugh@veritas.com>]

On Wed, 18 Jul 2007, Jens Axboe wrote:
> 
> We have these checks scattered, makes sense to put them in
> set_page_dirty() instead. This also fixes a bug where __bio_unmap_user()
> does set_page_dirty_lock() without checking for a compound page, instead
> of adding one more check we move it to set_page_dirty().
> 
> Signed-off-by: Jens Axboe <jens.axboe@oracle.com>
> 
> diff --git a/fs/bio.c b/fs/bio.c
> index cd888f9..ff96cd9 100644
> --- a/fs/bio.c
> +++ b/fs/bio.c
> @@ -902,7 +902,7 @@ void bio_set_pages_dirty(struct bio *bio)
>  	for (i = 0; i < bio->bi_vcnt; i++) {
>  		struct page *page = bvec[i].bv_page;
>  
> -		if (page && !PageCompound(page))
> +		if (page)
>  			set_page_dirty_lock(page);
>  	}
>  }
> diff --git a/fs/direct-io.c b/fs/direct-io.c
> index 52bb263..72195bc 100644
> --- a/fs/direct-io.c
> +++ b/fs/direct-io.c
> @@ -426,7 +426,7 @@ static int dio_bio_complete(struct dio *dio, struct bio *bio)
>  		for (page_no = 0; page_no < bio->bi_vcnt; page_no++) {
>  			struct page *page = bvec[page_no].bv_page;
>  
> -			if (dio->rw == READ && !PageCompound(page))
> +			if (dio->rw == READ)
>  				set_page_dirty_lock(page);
>  			page_cache_release(page);
>  		}
> diff --git a/mm/page-writeback.c b/mm/page-writeback.c
> index 886ea0d..3c590b9 100644
> --- a/mm/page-writeback.c
> +++ b/mm/page-writeback.c
> @@ -861,8 +861,12 @@ EXPORT_SYMBOL(redirty_page_for_writepage);
>   */
>  int fastcall set_page_dirty(struct page *page)
>  {
> -	struct address_space *mapping = page_mapping(page);
> +	struct address_space *mapping;
> +	
> +	if (unlikely(PageCompound(page)))
> +		return 0;
>  
> +	mapping = page_mapping(page);
>  	if (likely(mapping)) {
>  		int (*spd)(struct page *) = mapping->a_ops->set_page_dirty;
>  #ifdef CONFIG_BLOCK

I'd prefer it if we just remove those two tests from fs without
adding one into set_page_dirty at all; though I've not tested
that recently (replying before remembering the easiest way to
do so), and others may disagree.

The real reason for those tests was that pre-2.6.16 a compound page
stored its destructor in page[1].mapping: which went badly wrong if
that constituent page ever ended up being passed to set_page_dirty.

I moved it somewhere safer in 41d78ba55037468e6c86c53e3076d1a74841de39
but didn't have the courage to remove those tests you're now removing:
the earlier we skip out from that case, the more efficiently it's
handled, I didn't want to slow those paths down in case they were
important to someone.

So I'd be glad to see those tests now gone without replacement.

Hugh