On Fri, Jul 27, 2018 at 11:19:10AM -0400, Chuck Lever wrote:
> I've given up on the idea of zero-copy handling of SYMLINK on the
> server side. This is because the Linux VFS symlink API requires the
> symlink pathname to be in a NUL-terminated kmalloc'd buffer. The
> NUL-termination is going to be problematic (watching out for
> landing on a page boundary and dealing with a 4096-byte pathname).
>
> I don't believe that SYMLINK creation is on a performance path or is
> requested frequently enough that it will cause noticeable CPU cache
> pollution due to data copies.
Sounds fine.
The limits here are weird. In the v2 case the maximum length is
NFS_MAXPATHLEN == 1024. OK, I see, I guess that limit was imposed by
the NFSv2 protocol. In the v3 case it's NFS3_MAXPATHLEN == PATH_MAX ==
4096. But we were imposing an artificial max of 4095 just so we could
fit the result in one page with null termination.
OK, makes sense to me (though I can't recall anyone actually complaining
about that limit).
--b.
>
> There will be two places where a transport callout will be necessary
> to fill in the rqstp: one will be in the svc_fill_symlink_pathname()
> helper that is used by NFSv2 and NFSv3, and the other will be in
> nfsd4_decode_create().
>
> Signed-off-by: Chuck Lever <chuck.lever@xxxxxxxxxx>
> ---
> fs/nfsd/nfs3proc.c | 2 +
> fs/nfsd/nfsproc.c | 2 +
> include/linux/sunrpc/svc.h | 3 +-
> net/sunrpc/svc.c | 67 ++++++++++++++++----------------------------
> 4 files changed, 31 insertions(+), 43 deletions(-)
>
> diff --git a/fs/nfsd/nfs3proc.c b/fs/nfsd/nfs3proc.c
> index 8d1c2d1a..9eb8086 100644
> --- a/fs/nfsd/nfs3proc.c
> +++ b/fs/nfsd/nfs3proc.c
> @@ -290,6 +290,7 @@
> RETURN_STATUS(nfserr_nametoolong);
>
> argp->tname = svc_fill_symlink_pathname(rqstp, &argp->first,
> + page_address(rqstp->rq_arg.pages[0]),
> argp->tlen);
> if (IS_ERR(argp->tname))
> RETURN_STATUS(nfserrno(PTR_ERR(argp->tname)));
> @@ -303,6 +304,7 @@
> fh_init(&resp->fh, NFS3_FHSIZE);
> nfserr = nfsd_symlink(rqstp, &resp->dirfh, argp->fname, argp->flen,
> argp->tname, &resp->fh);
> + kfree(argp->tname);
> RETURN_STATUS(nfserr);
> }
>
> diff --git a/fs/nfsd/nfsproc.c b/fs/nfsd/nfsproc.c
> index a6faee5..0d20fd1 100644
> --- a/fs/nfsd/nfsproc.c
> +++ b/fs/nfsd/nfsproc.c
> @@ -454,6 +454,7 @@
> return nfserr_nametoolong;
>
> argp->tname = svc_fill_symlink_pathname(rqstp, &argp->first,
> + page_address(rqstp->rq_arg.pages[0]),
> argp->tlen);
> if (IS_ERR(argp->tname))
> return nfserrno(PTR_ERR(argp->tname));
> @@ -466,6 +467,7 @@
> nfserr = nfsd_symlink(rqstp, &argp->ffh, argp->fname, argp->flen,
> argp->tname, &newfh);
>
> + kfree(argp->tname);
> fh_put(&argp->ffh);
> fh_put(&newfh);
> return nfserr;
> diff --git a/include/linux/sunrpc/svc.h b/include/linux/sunrpc/svc.h
> index 43f88bd..73e130a 100644
> --- a/include/linux/sunrpc/svc.h
> +++ b/include/linux/sunrpc/svc.h
> @@ -499,7 +499,8 @@ unsigned int svc_fill_write_vector(struct svc_rqst *rqstp,
> struct page **pages,
> struct kvec *first, size_t total);
> char *svc_fill_symlink_pathname(struct svc_rqst *rqstp,
> - struct kvec *first, size_t total);
> + struct kvec *first, void *p,
> + size_t total);
>
> #define RPC_MAX_ADDRBUFLEN (63U)
>
> diff --git a/net/sunrpc/svc.c b/net/sunrpc/svc.c
> index 2194ed5..d13e05f 100644
> --- a/net/sunrpc/svc.c
> +++ b/net/sunrpc/svc.c
> @@ -1577,65 +1577,48 @@ unsigned int svc_fill_write_vector(struct svc_rqst *rqstp, struct page **pages,
> * svc_fill_symlink_pathname - Construct pathname argument for VFS symlink call
> * @rqstp: svc_rqst to operate on
> * @first: buffer containing first section of pathname
> + * @p: buffer containing remaining section of pathname
> * @total: total length of the pathname argument
> *
> - * Returns pointer to a NUL-terminated string, or an ERR_PTR. The buffer is
> - * released automatically when @rqstp is recycled.
> + * The VFS symlink API demands a NUL-terminated pathname in mapped memory.
> + * Returns pointer to a NUL-terminated string, or an ERR_PTR. Caller must free
> + * the returned string.
> */
> char *svc_fill_symlink_pathname(struct svc_rqst *rqstp, struct kvec *first,
> - size_t total)
> + void *p, size_t total)
> {
> - struct xdr_buf *arg = &rqstp->rq_arg;
> - struct page **pages;
> - char *result;
> -
> - /* VFS API demands a NUL-terminated pathname. This function
> - * uses a page from @rqstp as the pathname buffer, to enable
> - * direct placement. Thus the total buffer size is PAGE_SIZE.
> - * Space in this buffer for NUL-termination requires that we
> - * cap the size of the returned symlink pathname just a
> - * little early.
> - */
> - if (total > PAGE_SIZE - 1)
> - return ERR_PTR(-ENAMETOOLONG);
> + size_t len, remaining;
> + char *result, *dst;
>
> - /* Some types of transport can present the pathname entirely
> - * in rq_arg.pages. If not, then copy the pathname into one
> - * page.
> - */
> - pages = arg->pages;
> - WARN_ON_ONCE(arg->page_base != 0);
> - if (first->iov_base == 0) {
> - result = page_address(*pages);
> - result[total] = '\0';
> - } else {
> - size_t len, remaining;
> - char *dst;
> + result = kmalloc(total + 1, GFP_KERNEL);
> + if (!result)
> + return ERR_PTR(-ESERVERFAULT);
>
> - result = page_address(*(rqstp->rq_next_page++));
> - dst = result;
> - remaining = total;
> + dst = result;
> + remaining = total;
>
> - len = min_t(size_t, total, first->iov_len);
> + len = min_t(size_t, total, first->iov_len);
> + if (len) {
> memcpy(dst, first->iov_base, len);
> dst += len;
> remaining -= len;
> + }
>
> - /* No more than one page left */
> - if (remaining) {
> - len = min_t(size_t, remaining, PAGE_SIZE);
> - memcpy(dst, page_address(*pages), len);
> - dst += len;
> - }
> -
> - *dst = '\0';
> + if (remaining) {
> + len = min_t(size_t, remaining, PAGE_SIZE);
> + memcpy(dst, p, len);
> + dst += len;
> }
>
> - /* Sanity check: we don't allow the pathname argument to
> + *dst = '\0';
> +
> + /* Sanity check: Linux doesn't allow the pathname argument to
> * contain a NUL byte.
> */
> - if (strlen(result) != total)
> + if (strlen(result) != total) {
> + kfree(result);
> return ERR_PTR(-EINVAL);
> + }
> return result;
> }
> EXPORT_SYMBOL_GPL(svc_fill_symlink_pathname);
--
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