Re: [PATCH 01/14] fs: Support THPs in vfs_dedupe_file_range

From: "Darrick J. Wong" <darrick.wong@oracle.com>
To: "Matthew Wilcox (Oracle)" <willy@infradead.org>
Cc: linux-fsdevel@vger.kernel.org, linux-xfs@vger.kernel.org,
	linux-mm@kvack.org
Subject: Re: [PATCH 01/14] fs: Support THPs in vfs_dedupe_file_range
Date: Wed, 14 Oct 2020 09:12:16 -0700	[thread overview]
Message-ID: <20201014161216.GE9832@magnolia> (raw)
In-Reply-To: <20201014030357.21898-2-willy@infradead.org>

On Wed, Oct 14, 2020 at 04:03:44AM +0100, Matthew Wilcox (Oracle) wrote:
> We may get tail pages returned from vfs_dedupe_get_page().  If we do,
> we have to call page_mapping() instead of dereferencing page->mapping
> directly.  We may also deadlock trying to lock the page twice if they're
> subpages of the same THP, so compare the head pages instead.
> 
> Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
> ---
>  fs/read_write.c | 8 ++++++--
>  1 file changed, 6 insertions(+), 2 deletions(-)
> 
> diff --git a/fs/read_write.c b/fs/read_write.c
> index 19f5c4bf75aa..ead675fef582 100644
> --- a/fs/read_write.c
> +++ b/fs/read_write.c
> @@ -1604,6 +1604,8 @@ static struct page *vfs_dedupe_get_page(struct inode *inode, loff_t offset)
>   */
>  static void vfs_lock_two_pages(struct page *page1, struct page *page2)
>  {
> +	page1 = thp_head(page1);
> +	page2 = thp_head(page2);

Hmm, is this usage (calling thp_head() to extract the head page from an
arbitrary page reference) a common enough idiom that it doesn't need a
comment saying why we need the head page?

I'm asking that genuinely-- thp_head() is new to me but maybe it's super
obvious to everyone else?  Or at least the mm developers?  I suspect
that might be the case....?

Aside from that question, this looks fine to me.

Also, I was sort of thinking about sending a patch to Linus at the end
of the merge window moving all the remap/clone/dedupe common code to a
separate file to declutter fs/read_write.c and mm/filemap.c.  Does that
sound ok?

--D

>  	/* Always lock in order of increasing index. */
>  	if (page1->index > page2->index)
>  		swap(page1, page2);
> @@ -1616,6 +1618,8 @@ static void vfs_lock_two_pages(struct page *page1, struct page *page2)
>  /* Unlock two pages, being careful not to unlock the same page twice. */
>  static void vfs_unlock_two_pages(struct page *page1, struct page *page2)
>  {
> +	page1 = thp_head(page1);
> +	page2 = thp_head(page2);
>  	unlock_page(page1);
>  	if (page1 != page2)
>  		unlock_page(page2);
> @@ -1670,8 +1674,8 @@ static int vfs_dedupe_file_range_compare(struct inode *src, loff_t srcoff,
>  		 * someone is invalidating pages on us and we lose.
>  		 */
>  		if (!PageUptodate(src_page) || !PageUptodate(dest_page) ||
> -		    src_page->mapping != src->i_mapping ||
> -		    dest_page->mapping != dest->i_mapping) {
> +		    page_mapping(src_page) != src->i_mapping ||
> +		    page_mapping(dest_page) != dest->i_mapping) {
>  			same = false;
>  			goto unlock;
>  		}
> -- 
> 2.28.0
> 