Re: [PATCH 3/3] netfs: fix test for whether we can skip read when writing beyond EOF

[Jeff Layton]

On Mon, 2021-06-14 at 14:20 +0100, David Howells wrote:
> From: Jeff Layton <jlayton@xxxxxxxxxx>
> 
> It's not sufficient to skip reading when the pos is beyond the EOF.
> There may be data at the head of the page that we need to fill in
> before the write.
> 
> Add a new helper function that corrects and clarifies the logic of
> when we can skip reads, and have it only zero out the part of the page
> that won't have data copied in for the write.
> 
> Finally, don't set the page Uptodate after zeroing. It's not up to date
> since the write data won't have been copied in yet.
> 
> [DH made the following changes:
> 
>  - Prefixed the new function with "netfs_".
> 
>  - Don't call zero_user_segments() for a full-page write.
> 
>  - Altered the beyond-last-page check to avoid a DIV instruction and got
>    rid of then-redundant zero-length file check.
> ]
> 
> Fixes: e1b1240c1ff5f ("netfs: Add write_begin helper")
> Reported-by: Andrew W Elble <aweits@xxxxxxx>
> Signed-off-by: Jeff Layton <jlayton@xxxxxxxxxx>
> Signed-off-by: David Howells <dhowells@xxxxxxxxxx>
> cc: ceph-devel@xxxxxxxxxxxxxxx
> Link: https://lore.kernel.org/r/20210613233345.113565-1-jlayton@xxxxxxxxxx/
> ---
> 
>  fs/netfs/read_helper.c |   49 +++++++++++++++++++++++++++++++++++-------------
>  1 file changed, 36 insertions(+), 13 deletions(-)
> 
> diff --git a/fs/netfs/read_helper.c b/fs/netfs/read_helper.c
> index 725614625ed4..70a5b1a19a50 100644
> --- a/fs/netfs/read_helper.c
> +++ b/fs/netfs/read_helper.c
> @@ -1011,12 +1011,42 @@ int netfs_readpage(struct file *file,
>  }
>  EXPORT_SYMBOL(netfs_readpage);
>  
> -static void netfs_clear_thp(struct page *page)
> +/**
> + * netfs_skip_page_read - prep a page for writing without reading first
> + * @page: page being prepared
> + * @pos: starting position for the write
> + * @len: length of write
> + *
> + * In some cases, write_begin doesn't need to read at all:
> + * - full page write
> + * - write that lies in a page that is completely beyond EOF
> + * - write that covers the the page from start to EOF or beyond it
> + *
> + * If any of these criteria are met, then zero out the unwritten parts
> + * of the page and return true. Otherwise, return false.
> + */
> +static noinline bool netfs_skip_page_read(struct page *page, loff_t pos, size_t len)
>  {
> -	unsigned int i;
> +	struct inode *inode = page->mapping->host;
> +	loff_t i_size = i_size_read(inode);
> +	size_t offset = offset_in_thp(page, pos);
> +
> +	/* Full page write */
> +	if (offset == 0 && len >= thp_size(page))
> +		return true;
> +
> +	/* pos beyond last page in the file */
> +	if (pos - offset >= i_size)
> +		goto zero_out;
> +
> +	/* Write that covers from the start of the page to EOF or beyond */
> +	if (offset == 0 && (pos + len) >= i_size)
> +		goto zero_out;
>  
> -	for (i = 0; i < thp_nr_pages(page); i++)
> -		clear_highpage(page + i);
> +	return false;
> +zero_out:
> +	zero_user_segments(page, 0, offset, offset + len, thp_size(page));
> +	return true;
>  }
>  
>  /**
> @@ -1024,7 +1054,7 @@ static void netfs_clear_thp(struct page *page)
>   * @file: The file to read from
>   * @mapping: The mapping to read from
>   * @pos: File position at which the write will begin
> - * @len: The length of the write in this page
> + * @len: The length of the write (may extend beyond the end of the page chosen)
>   * @flags: AOP_* flags
>   * @_page: Where to put the resultant page
>   * @_fsdata: Place for the netfs to store a cookie
> @@ -1061,8 +1091,6 @@ int netfs_write_begin(struct file *file, struct address_space *mapping,
>  	struct inode *inode = file_inode(file);
>  	unsigned int debug_index = 0;
>  	pgoff_t index = pos >> PAGE_SHIFT;
> -	int pos_in_page = pos & ~PAGE_MASK;
> -	loff_t size;
>  	int ret;
>  
>  	DEFINE_READAHEAD(ractl, file, NULL, mapping, index);
> @@ -1090,13 +1118,8 @@ int netfs_write_begin(struct file *file, struct address_space *mapping,
>  	 * within the cache granule containing the EOF, in which case we need
>  	 * to preload the granule.
>  	 */
> -	size = i_size_read(inode);
>  	if (!ops->is_cache_enabled(inode) &&
> -	    ((pos_in_page == 0 && len == thp_size(page)) ||
> -	     (pos >= size) ||
> -	     (pos_in_page == 0 && (pos + len) >= size))) {
> -		netfs_clear_thp(page);
> -		SetPageUptodate(page);
> +	    netfs_skip_page_read(page, pos, len)) {
>  		netfs_stat(&netfs_n_rh_write_zskip);
>  		goto have_page_no_wait;
>  	}
> 
> 

I've gone ahead and merged this into the ceph/testing branch since it is
a data corruptor. Once this patch goes into mainline, we'll drop that
patch and rebase onto that commit.
-- 
Jeff Layton <jlayton@xxxxxxxxxx>