tmpfs: open a window in shmem_unuse_inode

There are a couple of reasons (patches follow) why it would be good to open a
window for sleep in shmem_unuse_inode, between its search for a matching swap
entry, and its handling of the entry found.

shmem_unuse_inode must then use igrab to hold the inode against deletion in
that window, and its corresponding iput might result in deletion: so it had
better unlock_page before the iput, and might as well release the page too.

Nor is there any need to hold on to shmem_swaplist_mutex once we know we'll
leave the loop.  So this unwinding moves from try_to_unuse and shmem_unuse
into shmem_unuse_inode, in the case when it finds a match.

Let try_to_unuse break on error in the shmem_unuse case, as it does in the
unuse_mm case: though at this point in the series, no error to break on.

Signed-off-by: Hugh Dickins <hugh@veritas.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

2 files changed, 45 insertions, 35 deletions

diff --git a/mm/shmem.c b/mm/shmem.c
index 2e03d6031c24..a0126c437105 100644
--- a/mm/shmem.c
+++ b/mm/shmem.c
@@ -838,10 +838,8 @@ static int shmem_unuse_inode(struct shmem_inode_info *info, swp_entry_t entry, s
         if (size > SHMEM_NR_DIRECT)
                 size = SHMEM_NR_DIRECT;
         offset = shmem_find_swp(entry, ptr, ptr+size);
-        if (offset >= 0) {
-                shmem_swp_balance_unmap();
+        if (offset >= 0)
                 goto found;
-        }
         if (!info->i_indirect)
                 goto lost2;
 
@@ -879,11 +877,11 @@ static int shmem_unuse_inode(struct shmem_inode_info *info, swp_entry_t entry, s
                         if (size > ENTRIES_PER_PAGE)
                                 size = ENTRIES_PER_PAGE;
                         offset = shmem_find_swp(entry, ptr, ptr+size);
+                        shmem_swp_unmap(ptr);
                         if (offset >= 0) {
                                 shmem_dir_unmap(dir);
                                 goto found;
                         }
-                        shmem_swp_unmap(ptr);
                 }
         }
 lost1:
@@ -893,10 +891,25 @@ lost2:
         return 0;
 found:
         idx += offset;
-        inode = &info->vfs_inode;
-        error = add_to_page_cache(page, inode->i_mapping, idx, GFP_ATOMIC);
+        inode = igrab(&info->vfs_inode);
+        spin_unlock(&info->lock);
+
+        /* move head to start search for next from here */
+        list_move_tail(&shmem_swaplist, &info->swaplist);
+        mutex_unlock(&shmem_swaplist_mutex);
+
+        error = 1;
+        if (!inode)
+                goto out;
+
+        spin_lock(&info->lock);
+        ptr = shmem_swp_entry(info, idx, NULL);
+        if (ptr && ptr->val == entry.val)
+                error = add_to_page_cache(page, inode->i_mapping,
+                                                idx, GFP_ATOMIC);
         if (error == -EEXIST) {
                 struct page *filepage = find_get_page(inode->i_mapping, idx);
+                error = 1;
                 if (filepage) {
                         /*
                          * There might be a more uptodate page coming down
@@ -911,16 +924,18 @@ found:
                 delete_from_swap_cache(page);
                 set_page_dirty(page);
                 info->flags |= SHMEM_PAGEIN;
-                shmem_swp_set(info, ptr + offset, 0);
+                shmem_swp_set(info, ptr, 0);
+                swap_free(entry);
+                error = 1;      /* not an error, but entry was found */
         }
-        shmem_swp_unmap(ptr);
+        if (ptr)
+                shmem_swp_unmap(ptr);
         spin_unlock(&info->lock);
-        /*
-         * Decrement swap count even when the entry is left behind:
-         * try_to_unuse will skip over mms, then reincrement count.
-         */
-        swap_free(entry);
-        return 1;
+out:
+        unlock_page(page);
+        page_cache_release(page);
+        iput(inode);            /* allows for NULL */
+        return error;
 }
 
 /*
@@ -935,18 +950,16 @@ int shmem_unuse(swp_entry_t entry, struct page *page)
         mutex_lock(&shmem_swaplist_mutex);
         list_for_each_safe(p, next, &shmem_swaplist) {
                 info = list_entry(p, struct shmem_inode_info, swaplist);
-                if (!info->swapped)
+                if (info->swapped)
+                        found = shmem_unuse_inode(info, entry, page);
+                else
                         list_del_init(&info->swaplist);
-                else if (shmem_unuse_inode(info, entry, page)) {
-                        /* move head to start search for next from here */
-                        list_move_tail(&shmem_swaplist, &info->swaplist);
-                        found = 1;
-                        break;
-                }
                 cond_resched();
+                if (found)
+                        goto out;
         }
         mutex_unlock(&shmem_swaplist_mutex);
-        return found;
+out:    return found;   /* 0 or 1 or -ENOMEM */
 }
 
 /*
diff --git a/mm/swapfile.c b/mm/swapfile.c
index 14bc4f28a8cc..eade24da9310 100644
--- a/mm/swapfile.c
+++ b/mm/swapfile.c
@@ -814,7 +814,7 @@ static int try_to_unuse(unsigned int type)
                         atomic_inc(&new_start_mm->mm_users);
                         atomic_inc(&prev_mm->mm_users);
                         spin_lock(&mmlist_lock);
-                        while (*swap_map > 1 && !retval &&
+                        while (*swap_map > 1 && !retval && !shmem &&
                                         (p = p->next) != &start_mm->mmlist) {
                                 mm = list_entry(p, struct mm_struct, mmlist);
                                 if (!atomic_inc_not_zero(&mm->mm_users))
@@ -846,6 +846,13 @@ static int try_to_unuse(unsigned int type)
                         mmput(start_mm);
                         start_mm = new_start_mm;
                 }
+                if (shmem) {
+                        /* page has already been unlocked and released */
+                        if (shmem > 0)
+                                continue;
+                        retval = shmem;
+                        break;
+                }
                 if (retval) {
                         unlock_page(page);
                         page_cache_release(page);
@@ -884,12 +891,6 @@ static int try_to_unuse(unsigned int type)
                  * read from disk into another page.  Splitting into two
                  * pages would be incorrect if swap supported "shared
                  * private" pages, but they are handled by tmpfs files.
-                 *
-                 * Note shmem_unuse already deleted a swappage from
-                 * the swap cache, unless the move to filepage failed:
-                 * in which case it left swappage in cache, lowered its
-                 * swap count to pass quickly through the loops above,
-                 * and now we must reincrement count to try again later.
                  */
                 if ((*swap_map > 1) && PageDirty(page) && PageSwapCache(page)) {
                         struct writeback_control wbc = {
@@ -900,12 +901,8 @@ static int try_to_unuse(unsigned int type)
                         lock_page(page);
                         wait_on_page_writeback(page);
                 }
-                if (PageSwapCache(page)) {
-                        if (shmem)
-                                swap_duplicate(entry);
-                        else
-                                delete_from_swap_cache(page);
-                }
+                if (PageSwapCache(page))
+                        delete_from_swap_cache(page);
 
                 /*
                  * So we could skip searching mms once swap count went