On Thu, Aug 21, 2014 at 9:09 AM, Christoph Hellwig <hch@xxxxxx> wrote:
> Like all block based filesystems, the pNFS block layout driver can't read
> or write at a byte granularity and thus has to perform read-modify-write
> cycles on writes smaller than this granularity.
>
> Add a flag so that the core NFS code always reads a whole page when
> starting a smaller write, so that we can do it in the place where the VFS
> expects it instead of doing in very deadlock prone way in the writeback
> handler.
>
> Note that in theory we could do less than page size reads here for disks
> that have a smaller sector size which are server by a server with a smaller
> pnfs block size. But so far that doesn't seem like a worthwhile
> optimization.
>
> Signed-off-by: Christoph Hellwig <hch@xxxxxx>
> ---
> fs/nfs/file.c | 9 +++++++++
> fs/nfs/pnfs.h | 1 +
> 2 files changed, 10 insertions(+)
>
> diff --git a/fs/nfs/file.c b/fs/nfs/file.c
> index 524dd80..58566e9 100644
> --- a/fs/nfs/file.c
> +++ b/fs/nfs/file.c
> @@ -36,6 +36,7 @@
> #include "internal.h"
> #include "iostat.h"
> #include "fscache.h"
> +#include "pnfs.h"
>
> #include "nfstrace.h"
>
> @@ -323,10 +324,18 @@ nfs_file_fsync(struct file *file, loff_t start, loff_t end, int datasync)
> static int nfs_want_read_modify_write(struct file *file, struct page *page,
> loff_t pos, unsigned len)
> {
> + struct nfs_server *server = NFS_SERVER(file->f_mapping->host);
> unsigned int pglen = nfs_page_length(page);
> unsigned int offset = pos & (PAGE_CACHE_SIZE - 1);
> unsigned int end = offset + len;
>
> + if (server->pnfs_curr_ld &&
> + (server->pnfs_curr_ld->flags & PNFS_READ_WHOLE_PAGE)) {
> + if (!PageUptodate(page))
> + return 1;
> + return 0;
> + }
> +
> if ((file->f_mode & FMODE_READ) && /* open for read? */
> !PageUptodate(page) && /* Uptodate? */
> !PagePrivate(page) && /* i/o request already? */
> diff --git a/fs/nfs/pnfs.h b/fs/nfs/pnfs.h
> index 16cd14f..302b279 100644
> --- a/fs/nfs/pnfs.h
> +++ b/fs/nfs/pnfs.h
> @@ -72,6 +72,7 @@ enum layoutdriver_policy_flags {
> /* Should the pNFS client commit and return the layout upon a setattr */
> PNFS_LAYOUTRET_ON_SETATTR = 1 << 0,
> PNFS_LAYOUTRET_ON_ERROR = 1 << 1,
> + PNFS_READ_WHOLE_PAGE = 1 << 2,
> };
>
> struct nfs4_deviceid_node;
>
This doesn't look as if it will compile without IS_ENABLED(CONFIG_NFS_V4).
--
Trond Myklebust
Linux NFS client maintainer, PrimaryData
trond.myklebust@xxxxxxxxxxxxxxx
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