percpu: make sure chunk->map array has available space

An allocation attempt may require extending chunk->map array which
requires GFP_KERNEL context which isn't available for atomic
allocations.  This patch ensures that chunk->map array usually keeps
some amount of available space by directly allocating buffer space
during GFP_KERNEL allocations and scheduling async extension during
atomic ones.  This should make atomic allocation failures from map
space exhaustion rare.

Signed-off-by: Tejun Heo <tj@kernel.org>

1 files changed, 45 insertions, 8 deletions

diff --git a/mm/percpu.c b/mm/percpu.c
index c52b93117dc2..546ced05cf33 100644
--- a/mm/percpu.c
+++ b/mm/percpu.c
@@ -76,6 +76,8 @@
 
 #define PCPU_SLOT_BASE_SHIFT            5       /* 1-31 shares the same slot */
 #define PCPU_DFL_MAP_ALLOC              16      /* start a map with 16 ents */
+#define PCPU_ATOMIC_MAP_MARGIN_LOW      32
+#define PCPU_ATOMIC_MAP_MARGIN_HIGH     64
 
 #ifdef CONFIG_SMP
 /* default addr <-> pcpu_ptr mapping, override in asm/percpu.h if necessary */
@@ -102,9 +104,12 @@ struct pcpu_chunk {
         int                     free_size;      /* free bytes in the chunk */
         int                     contig_hint;    /* max contiguous size hint */
         void                    *base_addr;     /* base address of this chunk */
+
         int                     map_used;       /* # of map entries used before the sentry */
         int                     map_alloc;      /* # of map entries allocated */
         int                     *map;           /* allocation map */
+        struct work_struct      map_extend_work;/* async ->map[] extension */
+
         void                    *data;          /* chunk data */
         int                     first_free;     /* no free below this */
         bool                    immutable;      /* no [de]population allowed */
@@ -318,9 +323,14 @@ static void pcpu_chunk_relocate(struct pcpu_chunk *chunk, int oslot)
 /**
  * pcpu_need_to_extend - determine whether chunk area map needs to be extended
  * @chunk: chunk of interest
+ * @is_atomic: the allocation context
  *
- * Determine whether area map of @chunk needs to be extended to
- * accommodate a new allocation.
+ * Determine whether area map of @chunk needs to be extended.  If
+ * @is_atomic, only the amount necessary for a new allocation is
+ * considered; however, async extension is scheduled if the left amount is
+ * low.  If !@is_atomic, it aims for more empty space.  Combined, this
+ * ensures that the map is likely to have enough available space to
+ * accomodate atomic allocations which can't extend maps directly.
  *
  * CONTEXT:
  * pcpu_lock.
@@ -329,15 +339,25 @@ static void pcpu_chunk_relocate(struct pcpu_chunk *chunk, int oslot)
  * New target map allocation length if extension is necessary, 0
  * otherwise.
  */
-static int pcpu_need_to_extend(struct pcpu_chunk *chunk)
+static int pcpu_need_to_extend(struct pcpu_chunk *chunk, bool is_atomic)
 {
-        int new_alloc;
+        int margin, new_alloc;
+
+        if (is_atomic) {
+                margin = 3;
 
-        if (chunk->map_alloc >= chunk->map_used + 3)
+                if (chunk->map_alloc <
+                    chunk->map_used + PCPU_ATOMIC_MAP_MARGIN_LOW)
+                        schedule_work(&chunk->map_extend_work);
+        } else {
+                margin = PCPU_ATOMIC_MAP_MARGIN_HIGH;
+        }
+
+        if (chunk->map_alloc >= chunk->map_used + margin)
                 return 0;
 
         new_alloc = PCPU_DFL_MAP_ALLOC;
-        while (new_alloc < chunk->map_used + 3)
+        while (new_alloc < chunk->map_used + margin)
                 new_alloc *= 2;
 
         return new_alloc;
@@ -394,6 +414,20 @@ out_unlock:
         return 0;
 }
 
+static void pcpu_map_extend_workfn(struct work_struct *work)
+{
+        struct pcpu_chunk *chunk = container_of(work, struct pcpu_chunk,
+                                                map_extend_work);
+        int new_alloc;
+
+        spin_lock_irq(&pcpu_lock);
+        new_alloc = pcpu_need_to_extend(chunk, false);
+        spin_unlock_irq(&pcpu_lock);
+
+        if (new_alloc)
+                pcpu_extend_area_map(chunk, new_alloc);
+}
+
 /**
  * pcpu_fit_in_area - try to fit the requested allocation in a candidate area
  * @chunk: chunk the candidate area belongs to
@@ -647,6 +681,7 @@ static struct pcpu_chunk *pcpu_alloc_chunk(void)
         chunk->map_used = 1;
 
         INIT_LIST_HEAD(&chunk->list);
+        INIT_WORK(&chunk->map_extend_work, pcpu_map_extend_workfn);
         chunk->free_size = pcpu_unit_size;
         chunk->contig_hint = pcpu_unit_size;
 
@@ -767,7 +802,7 @@ static void __percpu *pcpu_alloc(size_t size, size_t align, bool reserved,
                         goto fail_unlock;
                 }
 
-                while ((new_alloc = pcpu_need_to_extend(chunk))) {
+                while ((new_alloc = pcpu_need_to_extend(chunk, is_atomic))) {
                         spin_unlock_irqrestore(&pcpu_lock, flags);
                         if (is_atomic ||
                             pcpu_extend_area_map(chunk, new_alloc) < 0) {
@@ -792,7 +827,7 @@ restart:
                         if (size > chunk->contig_hint)
                                 continue;
 
-                        new_alloc = pcpu_need_to_extend(chunk);
+                        new_alloc = pcpu_need_to_extend(chunk, is_atomic);
                         if (new_alloc) {
                                 if (is_atomic)
                                         continue;
@@ -1418,6 +1453,7 @@ int __init pcpu_setup_first_chunk(const struct pcpu_alloc_info *ai,
          */
         schunk = memblock_virt_alloc(pcpu_chunk_struct_size, 0);
         INIT_LIST_HEAD(&schunk->list);
+        INIT_WORK(&schunk->map_extend_work, pcpu_map_extend_workfn);
         schunk->base_addr = base_addr;
         schunk->map = smap;
         schunk->map_alloc = ARRAY_SIZE(smap);
@@ -1446,6 +1482,7 @@ int __init pcpu_setup_first_chunk(const struct pcpu_alloc_info *ai,
         if (dyn_size) {
                 dchunk = memblock_virt_alloc(pcpu_chunk_struct_size, 0);
                 INIT_LIST_HEAD(&dchunk->list);
+                INIT_WORK(&dchunk->map_extend_work, pcpu_map_extend_workfn);
                 dchunk->base_addr = base_addr;
                 dchunk->map = dmap;
                 dchunk->map_alloc = ARRAY_SIZE(dmap);