linux-kernel - Re: [PATCH v2 14/25] iomap: Support arbitrarily many blocks per page

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Message-ID: <20200212080510.GA24497@infradead.org>
Date:   Wed, 12 Feb 2020 00:05:10 -0800
From:   Christoph Hellwig <hch@...radead.org>
To:     Matthew Wilcox <willy@...radead.org>
Cc:     linux-fsdevel@...r.kernel.org, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 14/25] iomap: Support arbitrarily many blocks per page

On Tue, Feb 11, 2020 at 08:18:34PM -0800, Matthew Wilcox wrote:
> From: "Matthew Wilcox (Oracle)" <willy@...radead.org>
> 
> Size the uptodate array dynamically.  Now that this array is protected
> by a spinlock, we can use bitmap functions to set the bits in this array
> instead of a loop around set_bit().
> 
> Signed-off-by: Matthew Wilcox (Oracle) <willy@...radead.org>
> ---
>  fs/iomap/buffered-io.c | 27 +++++++++------------------
>  1 file changed, 9 insertions(+), 18 deletions(-)
> 
> diff --git a/fs/iomap/buffered-io.c b/fs/iomap/buffered-io.c
> index c551a48e2a81..5e5a6b038fc3 100644
> --- a/fs/iomap/buffered-io.c
> +++ b/fs/iomap/buffered-io.c
> @@ -22,14 +22,14 @@
>  #include "../internal.h"
>  
>  /*
> - * Structure allocated for each page when block size < PAGE_SIZE to track
> + * Structure allocated for each page when block size < page size to track
>   * sub-page uptodate status and I/O completions.
>   */
>  struct iomap_page {
>  	atomic_t		read_count;
>  	atomic_t		write_count;
>  	spinlock_t		uptodate_lock;
> -	DECLARE_BITMAP(uptodate, PAGE_SIZE / 512);
> +	unsigned long		uptodate[];
>  };
>  
>  static inline struct iomap_page *to_iomap_page(struct page *page)
> @@ -45,15 +45,14 @@ static struct iomap_page *
>  iomap_page_create(struct inode *inode, struct page *page)
>  {
>  	struct iomap_page *iop = to_iomap_page(page);
> +	unsigned int n, nr_blocks = i_blocks_per_page(inode, page);
>  
> -	if (iop || i_blocks_per_page(inode, page) <= 1)
> +	if (iop || nr_blocks <= 1)
>  		return iop;
>  
> -	iop = kmalloc(sizeof(*iop), GFP_NOFS | __GFP_NOFAIL);
> -	atomic_set(&iop->read_count, 0);
> -	atomic_set(&iop->write_count, 0);
> +	n = BITS_TO_LONGS(nr_blocks);
> +	iop = kzalloc(struct_size(iop, uptodate, n), GFP_NOFS | __GFP_NOFAIL);

Nit: I don't really hink we need the n variable here, we can just
opencode the BITS_TO_LONGS in the struct_size call.

>  	struct inode *inode = page->mapping->host;
>  	unsigned first = off >> inode->i_blkbits;
> -	unsigned last = (off + len - 1) >> inode->i_blkbits;
> -	bool uptodate = true;
> +	unsigned count = len >> inode->i_blkbits;
>  	unsigned long flags;
> -	unsigned int i;
>  
>  	spin_lock_irqsave(&iop->uptodate_lock, flags);
> -	for (i = 0; i < i_blocks_per_page(inode, page); i++) {
> -		if (i >= first && i <= last)
> -			set_bit(i, iop->uptodate);
> -		else if (!test_bit(i, iop->uptodate))
> -			uptodate = false;
> -	}
> -
> -	if (uptodate)
> +	bitmap_set(iop->uptodate, first, count);
> +	if (bitmap_full(iop->uptodate, i_blocks_per_page(inode, page)))
>  		SetPageUptodate(page);

Switching to the bitmap helpers might be worthwhile prep patch that
can go in directly now that we're having the uptodate_lock.