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>

diff --git a/mm/percpu.c b/mm/percpu.c
index c52b931..546ced0 100644 (file)
--- 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);