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dentry缓存

dentry缓存十一月 1st, 2015 filesystem

最近追查某线上服务发现一个问题：频繁的创建&删除目录或者文件，会消耗大量的物理内存。

原因是因为，目录或者文件删除后，dentry并没有立即释放，而是保存在了superblock上的lru链表上。通常只有内核认为内存紧张的时候才会回收此部分内存。

不过由于dentry结构非常小，这种case下内存增长是很慢。

// linux-2.6.32.68/fs/dcache.c /*  * dput - release a dentry  * @dentry: dentry to release   *  * Release a dentry. This will drop the usage count and if   * appropriate call the dentry unlink method as well as  * removing it from the queues and releasing its resources.   * If the parent dentries were scheduled for release  * they too may now get deleted.  *  * no dcache lock, please.  */  void dput(struct dentry *dentry) {  if (!dentry)   return;  repeat:  if (atomic_read(&dentry->d_count) == 1)   might_sleep();  if (!atomic_dec_and_lock(&dentry->d_count, &dcache_lock))   return;   spin_lock(&dentry->d_lock);  if (atomic_read(&dentry->d_count)) {   spin_unlock(&dentry->d_lock);   spin_unlock(&dcache_lock);   return;  }   /*   * AV: ->d_delete() is _NOT_ allowed to block now.   */  if (dentry->d_op && dentry->d_op->d_delete) {   if (dentry->d_op->d_delete(dentry))    goto unhash_it;  }  /* Unreachable? Get rid of it */   if (d_unhashed(dentry))   goto kill_it;    if (list_empty(&dentry->d_lru)) {     dentry->d_flags |= DCACHE_REFERENCED;                 // 这里把dentry放到sb的lru链表里   dentry_lru_add(dentry);    }   spin_unlock(&dentry->d_lock);  spin_unlock(&dcache_lock);  return;  unhash_it:  __d_drop(dentry); kill_it:  /* if dentry was on the d_lru list delete it from there */  dentry_lru_del(dentry);  dentry = d_kill(dentry);  if (dentry)   goto repeat; }

而内存回收有两种方式：

内核发现内存不足时主动处罚。例如分配不了内存时，达到某种指定的阈值时等等。
手动触发 sysctl vm.drop_caches。

这里只讨论第二种方法。内核在初始化的时候，注册了一个dcache_shrinker来释放dcache内存。shrink_dcache_memory的最主要的逻辑时prune_dcache()函数

static struct shrinker dcache_shrinker = {  .shrink = shrink_dcache_memory,  .seeks = DEFAULT_SEEKS, }; static void __init dcache_init(void) {  int loop;  /*    * A constructor could be added for stable state like the lists,   * but it is probably not worth it because of the cache nature   * of the dcache.    */  dentry_cache = KMEM_CACHE(dentry,   SLAB_RECLAIM_ACCOUNT|SLAB_PANIC|SLAB_MEM_SPREAD);  // 注册钩子函数，当用户手动触发内存回收时，调用dcache_shrinker  register_shrinker(&dcache_shrinker);  /* ... */ }

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