[PATCH] shmem_populate: avoid an useless check, and some comments

Either shmem_getpage returns a failure, or it found a page, or it was told
it couldn't do any I/O.  So it's useless to check nonblock in the else
branch.  We could add a BUG() there but I preferred to comment the
offending function.

This was taken out from one Ingo Molnar's old patch I'm resurrecting.

Signed-off-by: Paolo 'Blaisorblade' Giarrusso <blaisorblade@yahoo.it>
Cc: Ingo Molnar <mingo@elte.hu>
Cc: Hugh Dickins <hugh@veritas.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

2 files changed, 12 insertions, 1 deletions

diff --git a/mm/filemap.c b/mm/filemap.c
index edc54436fa94..88611928e71f 100644
--- a/mm/filemap.c
+++ b/mm/filemap.c
@@ -1504,8 +1504,12 @@ repeat:
                 return -EINVAL;
 
         page = filemap_getpage(file, pgoff, nonblock);
+
+        /* XXX: This is wrong, a filesystem I/O error may have happened. Fix that as
+         * done in shmem_populate calling shmem_getpage */
         if (!page && !nonblock)
                 return -ENOMEM;
+
         if (page) {
                 err = install_page(mm, vma, addr, page, prot);
                 if (err) {
@@ -1513,6 +1517,9 @@ repeat:
                         return err;
                 }
         } else {
+                /* No page was found just because we can't read it in now (being
+                 * here implies nonblock != 0), but the page may exist, so set
+                 * the PTE to fault it in later. */
                 err = install_file_pte(mm, vma, addr, pgoff, prot);
                 if (err)
                         return err;
diff --git a/mm/shmem.c b/mm/shmem.c
index 5a81b1ee4f7a..08a3bc2fba61 100644
--- a/mm/shmem.c
+++ b/mm/shmem.c
@@ -1195,6 +1195,7 @@ static int shmem_populate(struct vm_area_struct *vma,
                 err = shmem_getpage(inode, pgoff, &page, sgp, NULL);
                 if (err)
                         return err;
+                /* Page may still be null, but only if nonblock was set. */
                 if (page) {
                         mark_page_accessed(page);
                         err = install_page(mm, vma, addr, page, prot);
@@ -1202,7 +1203,10 @@ static int shmem_populate(struct vm_area_struct *vma,
                                 page_cache_release(page);
                                 return err;
                         }
-                } else if (nonblock) {
+                } else {
+                        /* No page was found just because we can't read it in
+                         * now (being here implies nonblock != 0), but the page
+                         * may exist, so set the PTE to fault it in later. */
                         err = install_file_pte(mm, vma, addr, pgoff, prot);
                         if (err)
                                 return err;