linux内核源码阅读之facebook硬盘加速flashcache之八

前面我们的分析中重点关注正常的数据流程，这一小节关注如果有异常，那么流程是怎么走完的呢？

1）创建新任务时kcached_job申请不到

2）读写命中时cache块为忙

3）系统关机时处理，系统开机时处理，系统异常掉电后的处理

首先来看第1种情况，申请kcached_job是在函数flashcache_lookup中，

543/* 544 * dbn is the starting sector, io_size is the number of sectors.545 */546static int 547flashcache_lookup(struct cache_c *dmc, struct bio *bio, int *index)548{549     sector_t dbn = bio->bi_sector;550#if DMC_DEBUG551     int io_size = to_sector(bio->bi_size);552#endif553     unsigned long set_number = hash_block(dmc, dbn);554     int invalid, oldest_clean = -1;555     int start_index;556557     start_index = dmc->assoc * set_number;558     DPRINTK("Cache lookup : dbn %llu(%lu), set = %d",559          dbn, io_size, set_number);560     find_valid_dbn(dmc, dbn, start_index, index);561     if (*index > 0) {562          DPRINTK("Cache lookup HIT: Block %llu(%lu): VALID index %d",563                    dbn, io_size, *index);564          /* We found the exact range of blocks we are looking for */565          return VALID;566     }567     invalid = find_invalid_dbn(dmc, start_index);568     if (invalid == -1) {569          /* We didn't find an invalid entry, search for oldest valid entry */570          find_reclaim_dbn(dmc, start_index, &oldest_clean);571     }572     /* 573     * Cache miss :574     * We can't choose an entry marked INPROG, but choose the oldest575     * INVALID or the oldest VALID entry.576     */577     *index = start_index + dmc->assoc;578     if (invalid != -1) {579          DPRINTK("Cache lookup MISS (INVALID): dbn %llu(%lu), set = %d, index = %d, start_index = %d",580                    dbn, io_size, set_number, invalid, start_index);581          *index = invalid;582     } else if (oldest_clean != -1) {583          DPRINTK("Cache lookup MISS (VALID): dbn %llu(%lu), set = %d, index = %d, start_index = %d",584                    dbn, io_size, set_number, oldest_clean, start_index);585          *index = oldest_clean;586     } else {587          DPRINTK_LITE("Cache read lookup MISS (NOROOM): dbn %llu(%lu), set = %d",588               dbn, io_size, set_number);589     }590     if (*index < (start_index + dmc->assoc))591          return INVALID;592     else {593          dmc->noroom++;594          return -1;595     }596}

直接看返回值有三种情况：valid, invalid, -1

valid 命中，invalid 找到空闲块，-1 没有可用cache块，为什么会没有可用cache块呢？逐一来看代码。

553行，hash_block返回当前dbn所在的集合下标。

557行，start_index为当前集合第1个cache块下标。

560行，在当前集合里查找dbn是否命中，如果命中index返回cache块下标，否则置index=-1。

561行，命中，返回cache块下标。

567行，在当前集合里查找一个可用的cache块，找到返回cache块下标，否则返回-1.

568行，找不到可用的cache块。继续查看可否回收一个cache块。

578行，找到可用cache块。

582行，回收了一个cache块。

590行，不管是找到还是回收的，反正已经有cache块了，返回invalid

594行，没有cache块可用。

回到flashcache_read函数，1234行返回的值为-1，接着到1252行将相交的cache块的置为无效。如果说设置无效也失败的话，那这个请求就不能下发了，因为下发到磁盘之后，后面缓存中cache块往磁盘回写，这块数据就被覆盖了。所以就来到了1255行直接返回-EIO错误。

将相交cache块设置无效之后，来到1264行，尝试先刷一些脏cache块。最后到1267行将数据直接下发到磁盘。

直接下发到磁盘的回调函数是flashcache_uncached_io_callback

1864static void 1865flashcache_uncached_io_callback(unsigned long error, void *context)1866{1867     struct kcached_job *job = (struct kcached_job *) context;18681869     VERIFY(job->index == -1);1870     push_uncached_io_complete(job);1871     schedule_work(&_kcached_wq);1872}

_kcached_wq调用到函数flashcache_uncached_io_complete，

1805/*1806 * We handle uncached IOs ourselves to deal with the problem of out of ordered1807 * IOs corrupting the cache. Consider the case where we get 2 concurent IOs1808 * for the same block Write-Read (or a Write-Write). Consider the case where1809 * the first Write is uncacheable and the second IO is cacheable. If the 1810 * 2 IOs are out-of-ordered below flashcache, then we will cache inconsistent1811 * data in flashcache (persistently).1812 * 1813 * We do invalidations before launching uncacheable IOs to disk. But in case1814 * of out of ordering the invalidations before launching the IOs does not help.1815 * We need to invalidate after the IO completes.1816 * 1817 * Doing invalidations after the completion of an uncacheable IO will cause 1818 * any overlapping dirty blocks in the cache to be written out and the IO 1819 * relaunched. If the overlapping blocks are busy, the IO is relaunched to 1820 * disk also (post invalidation). In these 2 cases, we will end up sending1821 * 2 disk IOs for the block. But this is a rare case.1822 * 1823 * When 2 IOs for the same block are sent down (by un co-operating processes)1824 * the storage stack is allowed to re-order the IOs at will. So the applications1825 * cannot expect any ordering at all.1826 * 1827 * What we try to avoid here is inconsistencies between disk and the ssd cache.1828 */

首先看注释。uncached的IO由于其返回顺序无法预测，可能引起cache数据错误。例如，有2个对同一个块并发IO，一个是写，另一个是读。写IO直接下发到磁盘，读IO找到可用的cache块，读先回来而写后回来，读回来时将cache块设置为读出数据，而实际上这个时候该数据已经不是最新的了。

在下发uncached IO时已经将相关cache块设置为invalid。但是如果出现了前面讲了乱序IO下发时invalid是没有用的。因此还需要在IO结束的时候再次invalid cache块。

做invalid cache块将可能触发脏块写回磁盘，然后然发这个IO。如果这个要写的脏块忙，需要等到空闲再发起，然后再次启动这个uncached IO。在这两种情况下，都要发起两次IO，一次是写脏次，一次是重新发uncached IO。

两个IO同时操作同一块数据时（由不相关进程下发），是有可能按任意次序完成的，所以上层应用不能预测其次序。

我们所能做的只是保持缓存和磁盘的一致。

其实上面说了半天，就是用来解释为什么在uncached IO结束的时候也要调用一下1844行的flashcache_inval_blocks。

1829void 1830flashcache_uncached_io_complete(struct kcached_job *job)1831{1832     struct cache_c *dmc = job->dmc;1833     unsigned long flags;1834     int queued;1835     int error = job->error;18361837     if (unlikely(error)) {1838          if (bio_data_dir(job->bio) == WRITE)1839               dmc->disk_write_errors++;1840          else1841               dmc->disk_read_errors++;1842     }1843     spin_lock_irqsave(&dmc->cache_spin_lock, flags);1844     queued = flashcache_inval_blocks(dmc, job->bio);1845     spin_unlock_irqrestore(&dmc->cache_spin_lock, flags);1846     if (queued) {1847          if (unlikely(queued < 0))1848               flashcache_bio_endio(job->bio, -EIO);1849          /* 1850          * The IO will be re-executed.1851          * The do_pending logic will re-launch the 1852          * disk IO post-invalidation calling start_uncached_io.1853          * This should be a rare occurrence though.1854          * XXX - We should track this.1855          */1856     } else {1857          flashcache_bio_endio(job->bio, error);1858     }1859     flashcache_free_cache_job(job);1860     if (atomic_dec_and_test(&dmc->nr_jobs))1861          wake_up(&dmc->destroyq);1862}

1837行，uncached IO失败，做下统计。

1846行，看简简单单就一个queued判断，其背后的哲学可真不小。从1847行我们看出queued不仅有可能是小于0的，还有可能是大于0的，等于0的情况下最简单：到1857行返回IO。

先看queued小于0的情况，小于0表示申请pending_job失败，到1848行，返回失败，但这时候悲剧就出现了，就像注释里描述的一样，磁盘的数据是uncached IO写回的数据，但缓存里却是另一份数据。

如果queued大于0，实际上在这个函数里就什么都没做，到1859行释放kcached_job。
但这只是表面现象，就好像看到别人成功很容易，却不曾知道别人在背后下了多少苦功。queued大于0在这里没有做什么事情。但在背后默默努力工作着。所以现实中你看到的都是片面的，你听到的都是不可靠的，就连孔老夫子也曾经感慨说，自己亲眼看到的事情都不一定是事情的真相。所以有一句话叫做谣言止于智者，缺少思考的人只会成为他人利用的对象。所以下一次再看到一篇没有证实的微博、一段评论、一则小道消息时，如果对他人会产生伤害就不要再随意转发了。

为了追踪到queued何时返回大于0，我们跟到flashcache_inval_blocks，再继续跟到flashcache_inval_block_set：

1288/*1289 * Invalidate any colliding blocks if they are !BUSY and !DIRTY. If the colliding1290 * block is DIRTY, we need to kick off a write. In both cases, we need to wait 1291 * until the underlying IO is finished, and then proceed with the invalidation.1292 */1293static int1294flashcache_inval_block_set(struct cache_c *dmc, int set, struct bio *bio, int rw,1295                  struct pending_job *pjob)1296{1297     sector_t io_start = bio->bi_sector;1298     sector_t io_end = bio->bi_sector + (to_sector(bio->bi_size) - 1);1299     int start_index, end_index, i;1300     struct cacheblock *cacheblk;1301     1302     start_index = dmc->assoc * set;1303     end_index = start_index + dmc->assoc;1304     for (i = start_index ; i < end_index ; i++) {1305          sector_t start_dbn = dmc->cache[i].dbn;1306          sector_t end_dbn = start_dbn + dmc->block_size;1307          1308          cacheblk = &dmc->cache[i];1309          if (cacheblk->cache_state & INVALID)1310               continue;1311          if ((io_start >= start_dbn && io_start < end_dbn) ||1312              (io_end >= start_dbn && io_end < end_dbn)) {1313               /* We have a match */1314               if (rw == WRITE)1315                    dmc->wr_invalidates++;1316               else1317                    dmc->rd_invalidates++;1318               if (!(cacheblk->cache_state & (BLOCK_IO_INPROG | DIRTY)) &&1319                   (cacheblk->head == NULL)) {1320                    dmc->cached_blocks--;               1321                    DPRINTK("Cache invalidate (!BUSY): Block %llu %lx",1322                         start_dbn, cacheblk->cache_state);1323                    cacheblk->cache_state = INVALID;1324                    continue;1325               }1326               /*1327               * The conflicting block has either IO in progress or is 1328               * Dirty. In all cases, we need to add ourselves to the 1329               * pending queue. Then if the block is dirty, we kick off1330               * an IO to clean the block. 1331               * Note that if the block is dirty and IO is in progress1332               * on it, the do_pending handler will clean the block1333               * and then process the pending queue.1334               */1335               flashcache_enq_pending(dmc, bio, i, INVALIDATE, pjob);1336               if ((cacheblk->cache_state & (DIRTY | BLOCK_IO_INPROG)) == DIRTY) {1337                    /* 1338                    * Kick off block write.1339                    * We can't kick off the write under the spinlock.1340                    * Instead, we mark the slot DISKWRITEINPROG, drop 1341                    * the spinlock and kick off the write. A block marked1342                    * DISKWRITEINPROG cannot change underneath us. 1343                    * to enqueue ourselves onto it's pending queue.1344                    *1345                    * XXX - The dropping of the lock here can be avoided if1346                    * we punt the cleaning of the block to the worker thread,1347                    * at the cost of a context switch.1348                    */1349                    cacheblk->cache_state |= DISKWRITEINPROG;1350                    spin_unlock_irq(&dmc->cache_spin_lock);1351                    flashcache_dirty_writeback(dmc, i); /* Must inc nr_jobs */1352                    spin_lock_irq(&dmc->cache_spin_lock);1353               }1354               return 1;1355          }1356     }1357     return 0;1358}

我们直接找返回大于0的地方就在1354行，再继续往回找是1311行if里面，这个if语句就表示bio跟cache块有交集。

1318行，如果cache块不为脏且不忙的话直接设置invalid，并continue。

接着看1327行注释，冲突块可能是忙或者脏，在这两种情况下，都需要加入pending队列。如果只是脏，立即触发一次写回磁盘。如果同时是脏和忙，那么do_pending处理函数会先将脏块写回然后再继续处理。作者真是费了苦心来写这一大堆注释，但如果没有这些注释，后面在do_pending的处理也确实不大好看懂。

到这里故事还没有结束，因为在1335行插入了一个pending_job，那么这个任务什么时候执行呢？

在flashcache_md_write_done里会看到调用到flashcache_do_pending，

359void360flashcache_do_pending(struct kcached_job *job)361{362     if (job->error)363          flashcache_do_pending_error(job);364     else365          flashcache_do_pending_noerror(job);366}

362行，IO返回错误，跟进去看看错误处理

262/* 263 * Common error handling for everything.264 * 1) If the block isn't dirty, invalidate it.265 * 2) Error all pending IOs that totally or partly overlap this block.266 * 3) Free the job.267 */268static void269flashcache_do_pending_error(struct kcached_job *job)270{271     struct cache_c *dmc = job->dmc;272     unsigned long flags;273     struct cacheblock *cacheblk = &dmc->cache[job->index];274275     DMERR("flashcache_do_pending_error: error %d block %lu action %d", 276           -job->error, job->disk.sector, job->action);277     spin_lock_irqsave(&dmc->cache_spin_lock, flags);278     VERIFY(cacheblk->cache_state & VALID);279     /* Invalidate block if possible */280     if ((cacheblk->cache_state & DIRTY) == 0) {281          dmc->cached_blocks--;282          dmc->pending_inval++;283          cacheblk->cache_state &= ~VALID;284          cacheblk->cache_state |= INVALID;285     }286     flashcache_free_pending_jobs(dmc, cacheblk, job->error);287     cacheblk->cache_state &= ~(BLOCK_IO_INPROG);288     spin_unlock_irqrestore(&dmc->cache_spin_lock, flags);289     flashcache_free_cache_job(job);290     if (atomic_dec_and_test(&dmc->nr_jobs))291          wake_up(&dmc->destroyq);292}

如果cache块不为脏，则直接设置为invalid。将pending IO都返回错误，释放kcached_job。

但我们更关心flashcache_do_pending_noerror

294static void295flashcache_do_pending_noerror(struct kcached_job *job)296{297     struct cache_c *dmc = job->dmc;298     int index = job->index;299     unsigned long flags;300     struct pending_job *pending_job;301     int queued;302     struct cacheblock *cacheblk = &dmc->cache[index];303304     spin_lock_irqsave(&dmc->cache_spin_lock, flags);305     if (cacheblk->cache_state & DIRTY) {306          cacheblk->cache_state &= ~(BLOCK_IO_INPROG);307          cacheblk->cache_state |= DISKWRITEINPROG;308          spin_unlock_irqrestore(&dmc->cache_spin_lock, flags);309          flashcache_dirty_writeback(dmc, index);310          goto out;311     }312     DPRINTK("flashcache_do_pending: Index %d %lx",313          index, cacheblk->cache_state);314     VERIFY(cacheblk->cache_state & VALID);315     dmc->cached_blocks--;316     dmc->pending_inval++;317     cacheblk->cache_state &= ~VALID;318     cacheblk->cache_state |= INVALID;319     while (cacheblk->head) {320          VERIFY(!(cacheblk->cache_state & DIRTY));321          pending_job = cacheblk->head;322          cacheblk->head = pending_job->next;323          VERIFY(cacheblk->nr_queued > 0);324          cacheblk->nr_queued--;325          if (pending_job->action == INVALIDATE) {326               DPRINTK("flashcache_do_pending: INVALIDATE  %llu",327                    next_job->bio->bi_sector);328               VERIFY(pending_job->bio != NULL);329               queued = flashcache_inval_blocks(dmc, pending_job->bio);330               if (queued) {331                    if (unlikely(queued < 0)) {332                         /*333                         * Memory allocation failure inside inval_blocks.334                         * Fail this io.335                         */336                         flashcache_bio_endio(pending_job->bio, -EIO);337                    }338                    flashcache_free_pending_job(pending_job);339                    continue;340               }341          }342          spin_unlock_irqrestore(&dmc->cache_spin_lock, flags);343          DPRINTK("flashcache_do_pending: Sending down IO %llu",344               pending_job->bio->bi_sector);345          /* Start uncached IO */346          flashcache_start_uncached_io(dmc, pending_job->bio);347          flashcache_free_pending_job(pending_job);348          spin_lock_irqsave(&dmc->cache_spin_lock, flags);349     }350     VERIFY(cacheblk->nr_queued == 0);351     cacheblk->cache_state &= ~(BLOCK_IO_INPROG);352     spin_unlock_irqrestore(&dmc->cache_spin_lock, flags);353out:354     flashcache_free_cache_job(job);355     if (atomic_dec_and_test(&dmc->nr_jobs))356          wake_up(&dmc->destroyq);357}

这个函数分为两部分，第一部分是如果cache块为脏，则下发后立即返回，等待第二次调用，第二次调用才真正到处理pending_job，记得我们在上文中插入的pending_job是INVALIDATE的，那么这里也正如前面注释里所说下发了两次IO，一次是写回脏块，后一次是下发uncached IO。

319行，取出invalid的pending_job

320行，确认非dirty，因为第一次调用的时候已经写回了

325行，if语句成立，

329行，因为cache块已写回，就不脏不忙了，flashcache_inval_blocks只要设置invalid就可以返回成功

346行，下发uncached IO

至此uncached IO之旅告一个段落了。

接下来讲第2种情况读写命中但cache块忙的情况下是怎么处理的。

读写IO在cache块忙的情况下做出的表现是惊人的一致，那就是创建pending_job并挂入cache块队列中。这对我们来说已经是轻车熟路，不过我们这一次要跟踪的是读写IO的情况。经过前面的分析我们知道，pending_job是在flashcache_do_pending_noerror函数中处理的。同样如果为脏块要刷一次脏块，第二次进入到319行循环，由于Action为READCACHE或者WRITECACHE，直接到346行下发uncached IO。第2种情况的处理也就宣告结束了。似乎显得仓促，现实就是这样的，永远别想像电影里那样大起大落，只要你内心够从容，平平淡淡才是真。

第3种情况就留给大家自己分析，如果对这几个小节都已经熟悉，那就已经是小case了。

至此，flashcache源码的分析也就结束了，我也非常高兴能够坚持写完，因为这确实是一个非常耗时间的过程。如果你阅读之后能有所收获，那将是我最大的欢喜了。