radix-tree: avoid atomic allocations for preloaded insertions

Most pagecache (and some other) radix tree insertions have the great
opportunity to preallocate a few nodes with relaxed gfp flags.  But the
preallocation is squandered when it comes time to allocate a node, we
default to first attempting a GFP_ATOMIC allocation -- that doesn't
normally fail, but it can eat into atomic memory reserves that we don't
need to be using.

Another upshot of this is that it removes the sometimes highly contended
zone->lock from underneath tree_lock.  Pagecache insertions are always
performed with a radix tree preload, and after this change, such a
situation will never fall back to kmem_cache_alloc within
radix_tree_node_alloc.

David Miller reports seeing this allocation fail on a highly threaded
sparc64 system:

[527319.459981] dd: page allocation failure. order:0, mode:0x20
[527319.460403] Call Trace:
[527319.460568]  [00000000004b71e0] __slab_alloc+0x1b0/0x6a8
[527319.460636]  [00000000004b7bbc] kmem_cache_alloc+0x4c/0xa8
[527319.460698]  [000000000055309c] radix_tree_node_alloc+0x20/0x90
[527319.460763]  [0000000000553238] radix_tree_insert+0x12c/0x260
[527319.460830]  [0000000000495cd0] add_to_page_cache+0x38/0xb0
[527319.460893]  [00000000004e4794] mpage_readpages+0x6c/0x134
[527319.460955]  [000000000049c7fc] __do_page_cache_readahead+0x170/0x280
[527319.461028]  [000000000049cc88] ondemand_readahead+0x208/0x214
[527319.461094]  [0000000000496018] do_generic_mapping_read+0xe8/0x428
[527319.461152]  [0000000000497948] generic_file_aio_read+0x108/0x170
[527319.461217]  [00000000004badac] do_sync_read+0x88/0xd0
[527319.461292]  [00000000004bb5cc] vfs_read+0x78/0x10c
[527319.461361]  [00000000004bb920] sys_read+0x34/0x60
[527319.461424]  [0000000000406294] linux_sparc_syscall32+0x3c/0x40

The calltrace is significant: __do_page_cache_readahead allocates a number
of pages with GFP_KERNEL, and hence it should have reclaimed sufficient
memory to satisfy GFP_ATOMIC allocations.  However after the list of pages
goes to mpage_readpages, there can be significant intervals (including disk
IO) before all the pages are inserted into the radix-tree.  So the reserves
can easily be depleted at that point.  The patch is confirmed to fix the
problem.

Signed-off-by: Nick Piggin <npiggin@suse.de>
Cc: "David S. Miller" <davem@davemloft.net>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

diff --git a/lib/radix-tree.c b/lib/radix-tree.c
index 48c250fe2233d82c6d8fd563e83c234cdf1109ee..65f0e758ec3819d57f1799e5563a9f90dc0e41eb 100644 (file)
--- a/lib/radix-tree.c
+++ b/lib/radix-tree.c
@@ -95,14 +95,17 @@ static inline gfp_t root_gfp_mask(struct radix_tree_root *root)
 static struct radix_tree_node *
 radix_tree_node_alloc(struct radix_tree_root *root)
 {
-       struct radix_tree_node *ret;
+       struct radix_tree_node *ret = NULL;
        gfp_t gfp_mask = root_gfp_mask(root);
 
-       ret = kmem_cache_alloc(radix_tree_node_cachep,
-                               set_migrateflags(gfp_mask, __GFP_RECLAIMABLE));
-       if (ret == NULL && !(gfp_mask & __GFP_WAIT)) {
+       if (!(gfp_mask & __GFP_WAIT)) {
                struct radix_tree_preload *rtp;
 
+               /*
+                * Provided the caller has preloaded here, we will always
+                * succeed in getting a node here (and never reach
+                * kmem_cache_alloc)
+                */
                rtp = &__get_cpu_var(radix_tree_preloads);
                if (rtp->nr) {
                        ret = rtp->nodes[rtp->nr - 1];
@@ -110,6 +113,10 @@ radix_tree_node_alloc(struct radix_tree_root *root)
                        rtp->nr--;
                }
        }
+       if (ret == NULL)
+               ret = kmem_cache_alloc(radix_tree_node_cachep,
+                               set_migrateflags(gfp_mask, __GFP_RECLAIMABLE));
+
        BUG_ON(radix_tree_is_indirect_ptr(ret));
        return ret;
 }

diff --git a/mm/filemap.c b/mm/filemap.c
index 76bea88cbebcb6201d87574890f476ff0051caee..96920f8405627fd98ae2347b4fda0c57b6aedee5 100644 (file)
--- a/mm/filemap.c
+++ b/mm/filemap.c
@@ -65,7 +65,6 @@ generic_file_direct_IO(int rw, struct kiocb *iocb, const struct iovec *iov,
  *    ->private_lock           (__free_pte->__set_page_dirty_buffers)
  *      ->swap_lock            (exclusive_swap_page, others)
  *        ->mapping->tree_lock
- *          ->zone.lock
  *
  *  ->i_mutex
  *    ->i_mmap_lock            (truncate->unmap_mapping_range)

diff --git a/mm/rmap.c b/mm/rmap.c
index dbc2ca2057a54ff2c4a709de4b6e9a01694241bb..0334c8f6b741072c4fb1fb4830223c1530e5919c 100644 (file)
--- a/mm/rmap.c
+++ b/mm/rmap.c
@@ -36,7 +36,6 @@
  *                 mapping->tree_lock (widely used, in set_page_dirty,
  *                           in arch-dependent flush_dcache_mmap_lock,
  *                           within inode_lock in __sync_single_inode)
- *                   zone->lock (within radix tree node alloc)
  */
 
 #include <linux/mm.h>