linux-kernel - Re: [PATCH v4 14/36] iomap: Support large pages in iomap_adjust_read

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Message-ID: <20200521222438.GT2005@dread.disaster.area>
Date:   Fri, 22 May 2020 08:24:38 +1000
From:   Dave Chinner <david@...morbit.com>
To:     Matthew Wilcox <willy@...radead.org>
Cc:     linux-fsdevel@...r.kernel.org, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v4 14/36] iomap: Support large pages in
 iomap_adjust_read_range

On Fri, May 15, 2020 at 06:16:34AM -0700, Matthew Wilcox wrote:
> From: "Matthew Wilcox (Oracle)" <willy@...radead.org>
> 
> Pass the struct page instead of the iomap_page so we can determine the
> size of the page.  Use offset_in_thp() instead of offset_in_page() and use
> thp_size() instead of PAGE_SIZE.
> 
> Signed-off-by: Matthew Wilcox (Oracle) <willy@...radead.org>
> ---
>  fs/iomap/buffered-io.c | 15 ++++++++-------
>  1 file changed, 8 insertions(+), 7 deletions(-)
> 
> diff --git a/fs/iomap/buffered-io.c b/fs/iomap/buffered-io.c
> index 4a79061073eb..423ffc9d4a97 100644
> --- a/fs/iomap/buffered-io.c
> +++ b/fs/iomap/buffered-io.c
> @@ -83,15 +83,16 @@ iomap_page_release(struct page *page)
>   * Calculate the range inside the page that we actually need to read.
>   */
>  static void
> -iomap_adjust_read_range(struct inode *inode, struct iomap_page *iop,
> +iomap_adjust_read_range(struct inode *inode, struct page *page,
>  		loff_t *pos, loff_t length, unsigned *offp, unsigned *lenp)
>  {
> +	struct iomap_page *iop = to_iomap_page(page);
>  	loff_t orig_pos = *pos;
>  	loff_t isize = i_size_read(inode);
>  	unsigned block_bits = inode->i_blkbits;
>  	unsigned block_size = (1 << block_bits);
> -	unsigned poff = offset_in_page(*pos);
> -	unsigned plen = min_t(loff_t, PAGE_SIZE - poff, length);
> +	unsigned poff = offset_in_thp(page, *pos);
> +	unsigned plen = min_t(loff_t, thp_size(page) - poff, length);
>  	unsigned first = poff >> block_bits;
>  	unsigned last = (poff + plen - 1) >> block_bits;
>  
> @@ -129,7 +130,7 @@ iomap_adjust_read_range(struct inode *inode, struct iomap_page *iop,
>  	 * page cache for blocks that are entirely outside of i_size.
>  	 */
>  	if (orig_pos <= isize && orig_pos + length > isize) {
> -		unsigned end = offset_in_page(isize - 1) >> block_bits;
> +		unsigned end = offset_in_thp(page, isize - 1) >> block_bits;
>  
>  		if (first <= end && last > end)
>  			plen -= (last - end) * block_size;
> @@ -256,7 +257,7 @@ iomap_readpage_actor(struct inode *inode, loff_t pos, loff_t length, void *data,
>  	}
>  
>  	/* zero post-eof blocks as the page may be mapped */
> -	iomap_adjust_read_range(inode, iop, &pos, length, &poff, &plen);
> +	iomap_adjust_read_range(inode, page, &pos, length, &poff, &plen);
>  	if (plen == 0)
>  		goto done;
>  
> @@ -571,7 +572,6 @@ static int
>  __iomap_write_begin(struct inode *inode, loff_t pos, unsigned len, int flags,
>  		struct page *page, struct iomap *srcmap)
>  {
> -	struct iomap_page *iop = iomap_page_create(inode, page);
>  	loff_t block_size = i_blocksize(inode);
>  	loff_t block_start = pos & ~(block_size - 1);
>  	loff_t block_end = (pos + len + block_size - 1) & ~(block_size - 1);
> @@ -580,9 +580,10 @@ __iomap_write_begin(struct inode *inode, loff_t pos, unsigned len, int flags,
>  
>  	if (PageUptodate(page))
>  		return 0;
> +	iomap_page_create(inode, page);

What problem does this fix? i.e. if we can get here with an
uninitialised page, why isn't this a separate bug fix. I don't see
anything in this patch that actually changes behaviour, and there's
nothing in the commit description to tell me why this is here,
so... ???

Cheers,

Dave.
-- 
Dave Chinner
david@...morbit.com