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>

1 files changed, 9 insertions, 2 deletions

diff --git a/mm/filemap.c b/mm/filemap.c
index ec25ba1aef5f..ca04226bf1de 100644
--- 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;