Re: [PATCH 13/19] pnfs/blocklayout: correctly decrement extent length

[NeilBrown <neilb@suse.de>]

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On Thu, 21 Aug 2014 11:09:29 -0500 Christoph Hellwig <hch@lst.de> wrote:

> When we do non-page sized reads we can underflow the extent_length variable
> and read incorrect data.  Fix the extent_length calculation and change to
> defensive <= checks for the extent length in the read and write path.
> 
> Signed-off-by: Christoph Hellwig <hch@lst.de>

Hi Christoph,
 I was reviewing this patch for possible backport.
As 'extent_length' is sector_t, it is unsigned (either u64 or unsigned long).

So comparing "<= 0" has the same effect as comparing "== 0".
So the new checks are not "defensive".

That doesn't mean they are wrong, but they could be misleading...

There may be nothing that needs to be done here, but I thought I should let
you know.

NeilBrown

> ---
>  fs/nfs/blocklayout/blocklayout.c | 7 ++++---
>  1 file changed, 4 insertions(+), 3 deletions(-)
> 
> diff --git a/fs/nfs/blocklayout/blocklayout.c b/fs/nfs/blocklayout/blocklayout.c
> index 5427ae7..87a633d 100644
> --- a/fs/nfs/blocklayout/blocklayout.c
> +++ b/fs/nfs/blocklayout/blocklayout.c
> @@ -272,7 +272,7 @@ bl_read_pagelist(struct nfs_pgio_header *hdr)
>  	isect = (sector_t) (f_offset >> SECTOR_SHIFT);
>  	/* Code assumes extents are page-aligned */
>  	for (i = pg_index; i < hdr->page_array.npages; i++) {
> -		if (!extent_length) {
> +		if (extent_length <= 0) {
>  			/* We've used up the previous extent */
>  			bl_put_extent(be);
>  			bl_put_extent(cow_read);
> @@ -303,6 +303,7 @@ bl_read_pagelist(struct nfs_pgio_header *hdr)
>  			f_offset += pg_len;
>  			bytes_left -= pg_len;
>  			isect += (pg_offset >> SECTOR_SHIFT);
> +			extent_length -= (pg_offset >> SECTOR_SHIFT);
>  		} else {
>  			pg_offset = 0;
>  			pg_len = PAGE_CACHE_SIZE;
> @@ -333,7 +334,7 @@ bl_read_pagelist(struct nfs_pgio_header *hdr)
>  			}
>  		}
>  		isect += (pg_len >> SECTOR_SHIFT);
> -		extent_length -= PAGE_CACHE_SECTORS;
> +		extent_length -= (pg_len >> SECTOR_SHIFT);
>  	}
>  	if ((isect << SECTOR_SHIFT) >= header->inode->i_size) {
>  		hdr->res.eof = 1;
> @@ -797,7 +798,7 @@ next_page:
>  	/* Middle pages */
>  	pg_index = header->args.pgbase >> PAGE_CACHE_SHIFT;
>  	for (i = pg_index; i < header->page_array.npages; i++) {
> -		if (!extent_length) {
> +		if (extent_length <= 0) {
>  			/* We've used up the previous extent */
>  			bl_put_extent(be);
>  			bl_put_extent(cow_read);


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