Re: [PATCH] sunrpc: account for xdr->page_base in xdr_alloc_bvec

[Jeff Layton <jlayton@kernel.org>]

On Mon, 2023-08-14 at 10:32 -0400, Jeff Layton wrote:
> I've been seeing a regression in mainline (v6.5-rc) kernels where
> unaligned reads were returning corrupt data.
> 
> 9d96acbc7f37 added a routine to allocate and populate a bvec array that
> can be used to back an iov_iter. When it does this, it always sets the
> offset in the first bvec to zero, even when the xdr->page_base is
> non-zero.
> 
> The old code in svc_tcp_sendmsg used to account for this, as it was
> sending the pages one at a time anyway, but now that we just hand the
> iov to the network layer, we need to ensure that the bvecs are properly
> initialized.
> 
> Fix xdr_alloc_bvec to set the offset in the first bvec to the offset
> indicated by xdr->page_base, and then 0 in all subsequent bvecs.
> 
> Fixes: 9d96acbc7f37 ("SUNRPC: Add a bvec array to struct xdr_buf for use with iovec_iter()")

We might need a different fixes tag here. While I think xdr_alloc_bvec
ought to be where we account for this, the actual patch that broke
things is this one:

    5df5dd03a8f7 sunrpc: Use sendmsg(MSG_SPLICE_PAGES) rather then sendpage

The old code accounted for the fact that the first bvec always had a zero offset.


> Signed-off-by: Jeff Layton <jlayton@kernel.org>
> ---
> NB: This is only lightly tested so far, but it seems to fix the pynfs
> regressions I've been seeing.
> ---
>  net/sunrpc/xdr.c | 4 +++-
>  1 file changed, 3 insertions(+), 1 deletion(-)
> 
> diff --git a/net/sunrpc/xdr.c b/net/sunrpc/xdr.c
> index 2a22e78af116..d0f5fc8605b8 100644
> --- a/net/sunrpc/xdr.c
> +++ b/net/sunrpc/xdr.c
> @@ -144,6 +144,7 @@ int
>  xdr_alloc_bvec(struct xdr_buf *buf, gfp_t gfp)
>  {
>  	size_t i, n = xdr_buf_pagecount(buf);
> +	unsigned int offset = offset_in_page(buf->page_base);
>  
>  	if (n != 0 && buf->bvec == NULL) {
>  		buf->bvec = kmalloc_array(n, sizeof(buf->bvec[0]), gfp);
> @@ -151,7 +152,8 @@ xdr_alloc_bvec(struct xdr_buf *buf, gfp_t gfp)
>  			return -ENOMEM;
>  		for (i = 0; i < n; i++) {
>  			bvec_set_page(&buf->bvec[i], buf->pages[i], PAGE_SIZE,
> -				      0);
> +				      offset);
> +			offset = 0;
>  		}
>  	}
>  	return 0;
> 
> ---
> base-commit: 2ccdd1b13c591d306f0401d98dedc4bdcd02b421
> change-id: 20230814-sendpage-b04874eed249
> 
> Best regards,

-- 
Jeff Layton <jlayton@kernel.org>