On Fri, 2008-11-21 at 17:50 -0500, Jeff Layton wrote:
> On Fri, 21 Nov 2008 14:38:18 -0600
> "Steve French" <smfrench@xxxxxxxxx> wrote:
>
> > Fix attached.
> >
> > Shaggy/Jeff/Nick etc. do you want to review/ack it since it is late in the rc?
> >
>
> Talking with Steve on IRC, we thought it might be better to optimize
> away the read when possible. I think this patch should do it. We skip
> the read if the write starts past the current end of the file, or if
> the offset into the page of the beginning of the write is 0 and we're
> writing past the current end of the file. In those situations we just
> zero out the rest of the page.
>
> Combined patch inlined below. I also took the liberty of adding a page
> pointer to make the code look a little cleaner.
>
> Thoughts?
>
> Signed-off-by: Jeff Layton <jlayton@xxxxxxxxxx>
> Signed-off-by: Steve French <smfrench@xxxxxxxxxx>
> ---
> fs/cifs/file.c | 42 ++++++++++++++++++++++++++++++------------
> 1 files changed, 30 insertions(+), 12 deletions(-)
>
> diff --git a/fs/cifs/file.c b/fs/cifs/file.c
> index b691b89..1d252b8 100644
> --- a/fs/cifs/file.c
> +++ b/fs/cifs/file.c
> @@ -2062,26 +2062,43 @@ static int cifs_write_begin(struct file *file, struct address_space *mapping,
> {
> pgoff_t index = pos >> PAGE_CACHE_SHIFT;
> loff_t offset = pos & (PAGE_CACHE_SIZE - 1);
> + loff_t page_start = pos & PAGE_MASK;
> + loff_t i_size;
> + struct page *page;
> + char *buf;
> + int rc = 0;
>
> cFYI(1, ("write_begin from %lld len %d", (long long)pos, len));
>
> - *pagep = __grab_cache_page(mapping, index);
> - if (!*pagep)
> - return -ENOMEM;
> + page = __grab_cache_page(mapping, index);
> + if (!page) {
> + rc = -ENOMEM;
> + goto out;
> + }
>
> - if (PageUptodate(*pagep))
> - return 0;
> + if (PageUptodate(page))
> + goto out;
>
> /* If we are writing a full page it will be up to date,
> no need to read from the server */
> if (len == PAGE_CACHE_SIZE && flags & AOP_FLAG_UNINTERRUPTIBLE)
> - return 0;
> -
> - if ((file->f_flags & O_ACCMODE) != O_WRONLY) {
> - int rc;
> + goto out;
>
> + i_size = i_size_read(page->mapping->host);
> + if (pos >= i_size || ((offset == 0) && ((pos + len) >= i_size))) {
I don't think the (pos >= i_size) part of this test is correct. We
would still need to preserve the data between the start of the page and
i_size.
The second part of the test makes sense.
> + /*
> + * optimize away the read when the unwritten part of the page
> + * isn't expected to have any existing file data. Just zero
> + * out the unused parts.
> + */
> + buf = kmap(page);
> + memset(buf, 0, offset);
> + memset(buf + offset + len, 0, PAGE_CACHE_SIZE - (offset + len));
> + kunmap(page);
> + SetPageUptodate(page);
> + } else if ((file->f_flags & O_ACCMODE) != O_WRONLY) {
> /* might as well read a page, it is fast enough */
> - rc = cifs_readpage_worker(file, *pagep, &offset);
> + cifs_readpage_worker(file, page, &page_start);
>
> /* we do not need to pass errors back
> e.g. if we do not have read access to the file
> @@ -2093,8 +2110,9 @@ static int cifs_write_begin(struct file *file, struct address_space *mapping,
> racing with this read? In any case
> this will be written out by write_end so is fine */
> }
> -
> - return 0;
> +out:
> + *pagep = page;
> + return rc;
> }
>
> const struct address_space_operations cifs_addr_ops = {
--
David Kleikamp
IBM Linux Technology Center
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