tmpfs: simplify unuse and writepage

shmem_unuse_inode() and shmem_writepage() contain a little code to cope
with pages inserted independently into the filecache, probably by a
filesystem stacked on top of tmpfs, then fed to its ->readpage() or
->writepage().

Unionfs was indeed experimenting with working in that way three years ago,
but I find no current examples: nowadays the stacking filesystems use vfs
interfaces to the lower filesystem.

It's now illegal: remove most of that code, adding some WARN_ON_ONCEs.

Signed-off-by: Hugh Dickins <hughd@google.com>
Cc: Erez Zadok <ezk@fsl.cs.sunysb.edu>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

diff --git a/mm/shmem.c b/mm/shmem.c
index bf6e9c11d85995328f2c6ea65c1fd56a69ed2d59..7533574109dae9e754a7cd0667b0e55782da44c8 100644 (file)
--- a/mm/shmem.c
+++ b/mm/shmem.c
@@ -972,20 +972,7 @@ found:
        error = add_to_page_cache_locked(page, mapping, idx, GFP_NOWAIT);
        /* which does mem_cgroup_uncharge_cache_page on error */
 
-       if (error == -EEXIST) {
-               struct page *filepage = find_get_page(mapping, idx);
-               error = 1;
-               if (filepage) {
-                       /*
-                        * There might be a more uptodate page coming down
-                        * from a stacked writepage: forget our swappage if so.
-                        */
-                       if (PageUptodate(filepage))
-                               error = 0;
-                       page_cache_release(filepage);
-               }
-       }
-       if (!error) {
+       if (error != -ENOMEM) {
                delete_from_swap_cache(page);
                set_page_dirty(page);
                info->flags |= SHMEM_PAGEIN;
@@ -1072,16 +1059,17 @@ static int shmem_writepage(struct page *page, struct writeback_control *wbc)
        /*
         * shmem_backing_dev_info's capabilities prevent regular writeback or
         * sync from ever calling shmem_writepage; but a stacking filesystem
-        * may use the ->writepage of its underlying filesystem, in which case
+        * might use ->writepage of its underlying filesystem, in which case
         * tmpfs should write out to swap only in response to memory pressure,
-        * and not for the writeback threads or sync.  However, in those cases,
-        * we do still want to check if there's a redundant swappage to be
-        * discarded.
+        * and not for the writeback threads or sync.
         */
-       if (wbc->for_reclaim)
-               swap = get_swap_page();
-       else
-               swap.val = 0;
+       if (!wbc->for_reclaim) {
+               WARN_ON_ONCE(1);        /* Still happens? Tell us about it! */
+               goto redirty;
+       }
+       swap = get_swap_page();
+       if (!swap.val)
+               goto redirty;
 
        /*
         * Add inode to shmem_unuse()'s list of swapped-out inodes,
@@ -1092,15 +1080,12 @@ static int shmem_writepage(struct page *page, struct writeback_control *wbc)
         * we've taken the spinlock, because shmem_unuse_inode() will
         * prune a !swapped inode from the swaplist under both locks.
         */
-       if (swap.val) {
-               mutex_lock(&shmem_swaplist_mutex);
-               if (list_empty(&info->swaplist))
-                       list_add_tail(&info->swaplist, &shmem_swaplist);
-       }
+       mutex_lock(&shmem_swaplist_mutex);
+       if (list_empty(&info->swaplist))
+               list_add_tail(&info->swaplist, &shmem_swaplist);
 
        spin_lock(&info->lock);
-       if (swap.val)
-               mutex_unlock(&shmem_swaplist_mutex);
+       mutex_unlock(&shmem_swaplist_mutex);
 
        if (index >= info->next_index) {
                BUG_ON(!(info->flags & SHMEM_TRUNCATE));
@@ -1108,16 +1093,13 @@ static int shmem_writepage(struct page *page, struct writeback_control *wbc)
        }
        entry = shmem_swp_entry(info, index, NULL);
        if (entry->val) {
-               /*
-                * The more uptodate page coming down from a stacked
-                * writepage should replace our old swappage.
-                */
+               WARN_ON_ONCE(1);        /* Still happens? Tell us about it! */
                free_swap_and_cache(*entry);
                shmem_swp_set(info, entry, 0);
        }
        shmem_recalc_inode(inode);
 
-       if (swap.val && add_to_swap_cache(page, swap, GFP_ATOMIC) == 0) {
+       if (add_to_swap_cache(page, swap, GFP_ATOMIC) == 0) {
                delete_from_page_cache(page);
                shmem_swp_set(info, entry, swap.val);
                shmem_swp_unmap(entry);