1 files changed, 46 insertions, 2 deletions

diff --git a/fs/nfs/file.c b/fs/nfs/file.c
index dfc89671dc94..5021b75d2d1e 100644
--- a/fs/nfs/file.c
+++ b/fs/nfs/file.c
@@ -328,6 +328,42 @@ nfs_file_fsync(struct file *file, struct dentry *dentry, int datasync)
 }
 
 /*
+ * Decide whether a read/modify/write cycle may be more efficient
+ * then a modify/write/read cycle when writing to a page in the
+ * page cache.
+ *
+ * The modify/write/read cycle may occur if a page is read before
+ * being completely filled by the writer.  In this situation, the
+ * page must be completely written to stable storage on the server
+ * before it can be refilled by reading in the page from the server.
+ * This can lead to expensive, small, FILE_SYNC mode writes being
+ * done.
+ *
+ * It may be more efficient to read the page first if the file is
+ * open for reading in addition to writing, the page is not marked
+ * as Uptodate, it is not dirty or waiting to be committed,
+ * indicating that it was previously allocated and then modified,
+ * that there were valid bytes of data in that range of the file,
+ * and that the new data won't completely replace the old data in
+ * that range of the file.
+ */
+static int nfs_want_read_modify_write(struct file *file, struct page *page,
+                        loff_t pos, unsigned len)
+{
+        unsigned int pglen = nfs_page_length(page);
+        unsigned int offset = pos & (PAGE_CACHE_SIZE - 1);
+        unsigned int end = offset + len;
+
+        if ((file->f_mode & FMODE_READ) &&      /* open for read? */
+            !PageUptodate(page) &&              /* Uptodate? */
+            !PagePrivate(page) &&               /* i/o request already? */
+            pglen &&                            /* valid bytes of file? */
+            (end < pglen || offset))            /* replace all valid bytes? */
+                return 1;
+        return 0;
+}
+
+/*
  * This does the "real" work of the write. We must allocate and lock the
  * page to be sent back to the generic routine, which then copies the
  * data from user space.
@@ -340,15 +376,16 @@ static int nfs_write_begin(struct file *file, struct address_space *mapping,
                         struct page **pagep, void **fsdata)
 {
         int ret;
-        pgoff_t index;
+        pgoff_t index = pos >> PAGE_CACHE_SHIFT;
         struct page *page;
-        index = pos >> PAGE_CACHE_SHIFT;
+        int once_thru = 0;
 
         dfprintk(PAGECACHE, "NFS: write_begin(%s/%s(%ld), %u@%lld)\n",
                 file->f_path.dentry->d_parent->d_name.name,
                 file->f_path.dentry->d_name.name,
                 mapping->host->i_ino, len, (long long) pos);
 
+start:
         /*
          * Prevent starvation issues if someone is doing a consistency
          * sync-to-disk
@@ -367,6 +404,13 @@ static int nfs_write_begin(struct file *file, struct address_space *mapping,
         if (ret) {
                 unlock_page(page);
                 page_cache_release(page);
+        } else if (!once_thru &&
+                   nfs_want_read_modify_write(file, page, pos, len)) {
+                once_thru = 1;
+                ret = nfs_readpage(file, page);
+                page_cache_release(page);
+                if (!ret)
+                        goto start;
         }
         return ret;
 }