mm: restore KERNEL_DS optimisations

Restore the KERNEL_DS optimisation, especially helpful to the 2copy write
path.

This may be a pretty questionable gain in most cases, especially after the
legacy 2copy write path is removed, but it doesn't cost much.

Signed-off-by: Nick Piggin <npiggin@suse.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

diff --git a/mm/filemap.c b/mm/filemap.c
index ec25ba1..ca04226 100644 (file)
--- a/mm/filemap.c
+++ b/mm/filemap.c
@@ -2061,7 +2061,7 @@ static ssize_t generic_perform_write_2copy(struct file *file,
                 * cannot take a pagefault with the destination page locked.
                 * So pin the source page to copy it.
                 */
-               if (!PageUptodate(page)) {
+               if (!PageUptodate(page) && !segment_eq(get_fs(), KERNEL_DS)) {
                        unlock_page(page);
 
                        src_page = alloc_page(GFP_KERNEL);
@@ -2186,6 +2186,13 @@ static ssize_t generic_perform_write(struct file *file,
        const struct address_space_operations *a_ops = mapping->a_ops;
        long status = 0;
        ssize_t written = 0;
+       unsigned int flags = 0;
+
+       /*
+        * Copies from kernel address space cannot fail (NFSD is a big user).
+        */
+       if (segment_eq(get_fs(), KERNEL_DS))
+               flags |= AOP_FLAG_UNINTERRUPTIBLE;
 
        do {
                struct page *page;
@@ -2217,7 +2224,7 @@ again:
                        break;
                }
 
-               status = a_ops->write_begin(file, mapping, pos, bytes, 0,
+               status = a_ops->write_begin(file, mapping, pos, bytes, flags,
                                                &page, &fsdata);
                if (unlikely(status))
                        break;