tmpfs: radix_tree_preloading

Nick has observed that shmem.c still uses GFP_ATOMIC when adding to page cache
or swap cache, without any radix tree preload: so tending to deplete emergency
reserves of memory.

GFP_ATOMIC remains appropriate in shmem_writepage's add_to_swap_cache: it's
being called under memory pressure, so must not wait for more memory to become
available.  But shmem_unuse_inode now has a window in which it can and should
preload with GFP_KERNEL, and say GFP_NOWAIT instead of GFP_ATOMIC in its
add_to_page_cache.

shmem_getpage is not so straightforward: its filepage/swappage integrity
relies upon exchanging between caches under spinlock, and it would need a lot
of restructuring to place the preloads correctly.  Instead, follow its pattern
of retrying on races: use GFP_NOWAIT instead of GFP_ATOMIC in
add_to_page_cache, and begin each circuit of the repeat loop with a sleeping
radix_tree_preload, followed immediately by radix_tree_preload_end - that
won't guarantee success in the next add_to_page_cache, but doesn't need to.

And we can then remove that bothersome congestion_wait: when needed, it'll
automatically get done in the course of the radix_tree_preload.

Signed-off-by: Hugh Dickins <hugh@veritas.com>
Looks-good-to: 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, 18 insertions, 7 deletions

diff --git a/mm/shmem.c b/mm/shmem.c
index a0126c437105..530c5033d028 100644
--- a/mm/shmem.c
+++ b/mm/shmem.c
@@ -901,12 +901,16 @@ found:
         error = 1;
         if (!inode)
                 goto out;
+        error = radix_tree_preload(GFP_KERNEL);
+        if (error)
+                goto out;
+        error = 1;
 
         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);
+                                                idx, GFP_NOWAIT);
         if (error == -EEXIST) {
                 struct page *filepage = find_get_page(inode->i_mapping, idx);
                 error = 1;
@@ -931,6 +935,7 @@ found:
         if (ptr)
                 shmem_swp_unmap(ptr);
         spin_unlock(&info->lock);
+        radix_tree_preload_end();
 out:
         unlock_page(page);
         page_cache_release(page);
@@ -1185,6 +1190,16 @@ repeat:
                 goto done;
         error = 0;
         gfp = mapping_gfp_mask(mapping);
+        if (!filepage) {
+                /*
+                 * Try to preload while we can wait, to not make a habit of
+                 * draining atomic reserves; but don't latch on to this cpu.
+                 */
+                error = radix_tree_preload(gfp & ~__GFP_HIGHMEM);
+                if (error)
+                        goto failed;
+                radix_tree_preload_end();
+        }
 
         spin_lock(&info->lock);
         shmem_recalc_inode(inode);
@@ -1266,7 +1281,7 @@ repeat:
                         set_page_dirty(filepage);
                         swap_free(swap);
                 } else if (!(error = add_to_page_cache(
-                                swappage, mapping, idx, GFP_ATOMIC))) {
+                                swappage, mapping, idx, GFP_NOWAIT))) {
                         info->flags |= SHMEM_PAGEIN;
                         shmem_swp_set(info, entry, 0);
                         shmem_swp_unmap(entry);
@@ -1280,10 +1295,6 @@ repeat:
                         spin_unlock(&info->lock);
                         unlock_page(swappage);
                         page_cache_release(swappage);
-                        if (error == -ENOMEM) {
-                                /* let kswapd refresh zone for GFP_ATOMICs */
-                                congestion_wait(WRITE, HZ/50);
-                        }
                         goto repeat;
                 }
         } else if (sgp == SGP_READ && !filepage) {
@@ -1338,7 +1349,7 @@ repeat:
                                 shmem_swp_unmap(entry);
                         }
                         if (error || swap.val || 0 != add_to_page_cache_lru(
-                                        filepage, mapping, idx, GFP_ATOMIC)) {
+                                        filepage, mapping, idx, GFP_NOWAIT)) {
                                 spin_unlock(&info->lock);
                                 page_cache_release(filepage);
                                 shmem_unacct_blocks(info->flags, 1);