Re: fsx-linux failing with latest cifs-2.6 git tree

[Jeff Layton <jlayton@redhat.com>]

On Fri, 21 Nov 2008 23:02:48 +0000
Dave Kleikamp <shaggy@linux.vnet.ibm.com> wrote:
> 
> 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.
> 

Thanks Dave, good catch.

I think this is what we want. Skip the read if the entire page is
beyond the end of file, or if the write will clobber all of the data
that would be read in.

Signed-off-by: Jeff Layton <jlayton@redhat.com>
Signed-off-by: Steve French <smfrench@us.ibm.com>
---
 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..0c7ef2a 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 (page_start >= i_size || (offset == 0 && (pos + len) >= i_size)) {
+		/*
+		 * 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 = {
-- 
1.5.5.1