正常流程到flashcache_map的1623行或1625行，按顺序先看读流程：
1221static void
1222flashcache_read(struct cache_c *dmc, struct bio *bio)
1223{
1224 int index;
1225 int res;
1226 struct cacheblock *cacheblk;
1227 int queued;
1228
1229 DPRINTK("Got a %s for %llu %u bytes)",
1230 (bio_rw(bio) == READ ? "READ":"READA"),
1231 bio->bi_sector, bio->bi_size);
1232
1233 spin_lock_irq(&dmc->cache_spin_lock);
1234 res = flashcache_lookup(dmc, bio, &index);
1235 /*
1236 * Handle Cache Hit case first.
1237 * We need to handle 2 cases, BUSY and !BUSY. If BUSY, we enqueue the
1238 * bio for later.
1239 */
1240 if (res > 0) {
1241 cacheblk = &dmc->cache[index];
1242 if ((cacheblk->cache_state & VALID) &&
1243 (cacheblk->dbn == bio->bi_sector)) {
1244 flashcache_read_hit(dmc, bio, index);
1245 return;
1246 }
1247 }
1248 /*
1249 * In all cases except for a cache hit (and VALID), test for potential
1250 * invalidations that we need to do.
1251 */
1252 queued = flashcache_inval_blocks(dmc, bio);
1253 if (queued) {
1254 if (unlikely(queued < 0))
1255 flashcache_bio_endio(bio, -EIO);
1256 spin_unlock_irq(&dmc->cache_spin_lock);
1257 return;
1258 }
1259 if (res == -1 || flashcache_uncacheable(dmc)) {
1260 /* No room or non-cacheable */
1261 spin_unlock_irq(&dmc->cache_spin_lock);
1262 DPRINTK("Cache read: Block %llu(%lu):%s",
1263 bio->bi_sector, bio->bi_size, "CACHE MISS & NO ROOM");
1264 if (res == -1)
1265 flashcache_clean_set(dmc, hash_block(dmc, bio->bi_sector));
1266 /* Start uncached IO */
1267 flashcache_start_uncached_io(dmc, bio);
1268 return;
1269 }
1270 /*
1271 * (res == INVALID) Cache Miss
1272 * And we found cache blocks to replace
1273 * Claim the cache blocks before giving up the spinlock
1274 */
1275 if (dmc->cache[index].cache_state & VALID)
1276 dmc->replace++;
1277 else
1278 dmc->cached_blocks++;
1279 dmc->cache[index].cache_state = VALID | DISKREADINPROG;
1280 dmc->cache[index].dbn = bio->bi_sector;
1281 spin_unlock_irq(&dmc->cache_spin_lock);
1282
1283 DPRINTK("Cache read: Block %llu(%lu), index = %d:%s",
1284 bio->bi_sector, bio->bi_size, index, "CACHE MISS & REPLACE");
1285 flashcache_read_miss(dmc, bio, index);
1286}
我非常喜欢flashcache这种小资的感觉，每个函数都比较短，大部分都没有超过100行的。不像neil大哥写的md代码的函数动则几百行，上千行，总是望啊望啊望不到边。当然不是说我不喜欢neil的代码，实际上他的代码是非常非常好的，因为md代码已经有十多年的历史了，大的框架仍然没有太大的改变，仍能保持那么优雅已经是十分难得了。最新版本的md还加了许多新功能，像bad block和replacement机制都是非常实用的。所以对于一名优秀的软件工程师来说，并不在于写了多少行代码，而是编写的软件运行在多少台机器上，为用户创造了多少价值。
第1234行是查找bio是否命中，flashcache_lookup函数我们在之前的文章里已经分析过了。第1244行是命中的情况，我们跟进看看
1119static void
1120flashcache_read_hit(struct cache_c *dmc, struct bio* bio, int index)
1121{
1122 struct cacheblock *cacheblk;
1123 struct pending_job *pjob;
1124
1125 cacheblk = &dmc->cache[index];
1126 if (!(cacheblk->cache_state & BLOCK_IO_INPROG) && (cacheblk->head == NULL)) {
1127 struct kcached_job *job;
1128
1129 cacheblk->cache_state |= CACHEREADINPROG;
1130 dmc->read_hits++;
1131 spin_unlock_irq(&dmc->cache_spin_lock);
1132 DPRINTK("Cache read: Block %llu(%lu), index = %d:%s",
1133 bio->bi_sector, bio->bi_size, index, "CACHE HIT");
1134 job = new_kcached_job(dmc, bio, index);
1135 if (unlikely(sysctl_flashcache_error_inject & READ_HIT_JOB_ALLOC_FAIL)) {
1136 if (job)
1137 flashcache_free_cache_job(job);
1138 job = NULL;
1139 sysctl_flashcache_error_inject &= ~READ_HIT_JOB_ALLOC_FAIL;
1140 }
1141 if (unlikely(job == NULL)) {
1142 /*
1143 * We have a read hit, and can't allocate a job.
1144 * Since we dropped the spinlock, we have to drain any
1145 * pending jobs.
1146 */
1147 DMERR("flashcache: Read (hit) failed ! Can't allocate memory for cache IO, block %lu",
1148 cacheblk->dbn);
1149 flashcache_bio_endio(bio, -EIO);
1150 spin_lock_irq(&dmc->cache_spin_lock);
1151 flashcache_free_pending_jobs(dmc, cacheblk, -EIO);
1152 cacheblk->cache_state &= ~(BLOCK_IO_INPROG);
1153 spin_unlock_irq(&dmc->cache_spin_lock);
1154 } else {
1155 job->action = READCACHE; /* Fetch data from cache */
1156 atomic_inc(&dmc->nr_jobs);
1157 dmc->ssd_reads++;
1158 dm_io_async_bvec(1, &job->cache, READ,
1159 bio->bi_io_vec + bio->bi_idx,
1160 flashcache_io_callback, job);
1161 flashcache_unplug_device(dmc->cache_dev->bdev);
1162 }
1163 } else {
1164 pjob = flashcache_alloc_pending_job(dmc);
1165 if (unlikely(sysctl_flashcache_error_inject & READ_HIT_PENDING_JOB_ALLOC_FAIL)) {
1166 if (pjob) {
1167 flashcache_free_pending_job(pjob);
1168 pjob = NULL;
1169 }
1170 sysctl_flashcache_error_inject &= ~READ_HIT_PENDING_JOB_ALLOC_FAIL;
1171 }
1172 if (pjob == NULL)
1173 flashcache_bio_endio(bio, -EIO);
1174 else
1175 flashcache_enq_pending(dmc, bio, index, READCACHE, pjob);
1176 spin_unlock_irq(&dmc->cache_spin_lock);
1177 }
1178}
首先获取这个cache块管理结构，第1126行判断cache块不忙的情况，进入1129行设置状态为从cache读，第1134行创建一个kcached_job，在1141行申请kcached_job失败时就对bio返回失败。申请成功到1155行将kcached_job设置为READCACHE，再调用dm_io_async_bvec下发请求，当请求回来时就会调用这里设置的回调函数flashcache_io_callback。再继续看读SSD返回是怎么处理的？我们只看该函数是对READCACHE的处理：
151 case READCACHE:
152 DPRINTK("flashcache_io_callback: READCACHE %d",
153 index);
154 spin_lock_irqsave(&dmc->cache_spin_lock, flags);
155 if (unlikely(sysctl_flashcache_error_inject & READCACHE_ERROR)) {
156 job->error = error = -EIO;
157 sysctl_flashcache_error_inject &= ~READCACHE_ERROR;
158 }
159 VERIFY(cacheblk->cache_state & CACHEREADINPROG);
160 spin_unlock_irqrestore(&dmc->cache_spin_lock, flags);
161 if (unlikely(error))
162 dmc->ssd_read_errors++;
163#ifdef FLASHCACHE_DO_CHECKSUMS
164 if (likely(error == 0)) {
165 if (flashcache_validate_checksum(job)) {
166 DMERR("flashcache_io_callback: Checksum mismatch at disk offset %lu",
167 job->disk.sector);
168 error = -EIO;
169 }
170 }
171#endif
172 flashcache_bio_endio(bio, error);
173 break;
实际上真正有意义的就是第172行，将请求done回去了。这样我们就完成的一次读命中的处理。
读命中处理还有一种情况就是第1163行cache块忙的情况，这个时候就申请一个pending_job，挂到cache块上，等cache块上一个请求回来的时候调度。
继续回到flashcache_read，看不命中的情况。
到第1259行flashcache_uncacheable函数是管理命令相关的，加了黑名单之后会跳过cache，直接下发到磁盘。
到1264行res是flashcache_lookup返回的，为-1就表示获取不到可用的cache块，这时就调用flashcache_clean_set清除一下脏块。但获取不到cache块并不是说请求就结束了，还得下发到磁盘，1267行flashcache_start_uncached_io将请求直接下发到磁盘。
第1275行到1278行是统计信息，根据这些信息可以知道flashcache的运行状况，用于flashcache的性能优化。
接着1279行设置cache块的状态，1280行设置cache块对应磁盘上的扇区，最后调用flashcache_read_miss下发请求：
1180static void
1181flashcache_read_miss(struct cache_c *dmc, struct bio* bio,
1182 int index)
1183{
1184 struct kcached_job *job;
1185 struct cacheblock *cacheblk = &dmc->cache[index];
1186
1187 job = new_kcached_job(dmc, bio, index);
1188 if (unlikely(sysctl_flashcache_error_inject & READ_MISS_JOB_ALLOC_FAIL)) {
1189 if (job)
1190 flashcache_free_cache_job(job);
1191 job = NULL;
1192 sysctl_flashcache_error_inject &= ~READ_MISS_JOB_ALLOC_FAIL;
1193 }
1194 if (unlikely(job == NULL)) {
1195 /*
1196 * We have a read miss, and can't allocate a job.
1197 * Since we dropped the spinlock, we have to drain any
1198 * pending jobs.
1199 */
1200 DMERR("flashcache: Read (miss) failed ! Can't allocate memory for cache IO, block %lu",
1201 cacheblk->dbn);
1202 flashcache_bio_endio(bio, -EIO);
1203 spin_lock_irq(&dmc->cache_spin_lock);
1204 dmc->cached_blocks--;
1205 cacheblk->cache_state &= ~VALID;
1206 cacheblk->cache_state |= INVALID;
1207 flashcache_free_pending_jobs(dmc, cacheblk, -EIO);
1208 cacheblk->cache_state &= ~(BLOCK_IO_INPROG);
1209 spin_unlock_irq(&dmc->cache_spin_lock);
1210 } else {
1211 job->action = READDISK; /* Fetch data from the source device */
1212 atomic_inc(&dmc->nr_jobs);
1213 dmc->disk_reads++;
1214 dm_io_async_bvec(1, &job->disk, READ,
1215 bio->bi_io_vec + bio->bi_idx,
1216 flashcache_io_callback, job);
1217 flashcache_clean_set(dmc, index / dmc->assoc);
1218 }
1219}
在第1187行申请了一个kcached_job，申请成功就到1211行，设置job->action=READDISK，调用dm_io_async_bvec直接从磁盘读取数据。接着调用flashcache_clean_set检查一下水位线。再看这里读磁盘的回调函数flashcache_io_callback，按理说读完磁盘就可以直接向上层返回数据，但这里还要把数据缓存起来之后再返回。
113void
114flashcache_io_callback(unsigned long error, void *context)
115{
116 struct kcached_job *job = (struct kcached_job *) context;
117 struct cache_c *dmc = job->dmc;
118 struct bio *bio;
119 unsigned long flags;
120 int index = job->index;
121 struct cacheblock *cacheblk = &dmc->cache[index];
122
123 VERIFY(index != -1);
124 bio = job->bio;
125 VERIFY(bio != NULL);
126 if (error)
127 DMERR("flashcache_io_callback: io error %ld block %lu action %d",
128 error, job->disk.sector, job->action);
129 job->error = error;
130 switch (job->action) {
131 case READDISK:
132 DPRINTK("flashcache_io_callback: READDISK %d",
133 index);
134 spin_lock_irqsave(&dmc->cache_spin_lock, flags);
135 if (unlikely(sysctl_flashcache_error_inject & READDISK_ERROR)) {
136 job->error = error = -EIO;
137 sysctl_flashcache_error_inject &= ~READDISK_ERROR;
138 }
139 VERIFY(cacheblk->cache_state & DISKREADINPROG);
140 spin_unlock_irqrestore(&dmc->cache_spin_lock, flags);
141 if (likely(error == 0)) {
142 /* Kick off the write to the cache */
143 job->action = READFILL;
144 flashcache_enqueue_readfill(dmc, job);
145 return;
146 } else {
147 dmc->disk_read_errors++;
148 flashcache_bio_endio(bio, error);
149 }
150 break; 174 case READFILL:
175 DPRINTK("flashcache_io_callback: READFILL %d",
176 index);
177 spin_lock_irqsave(&dmc->cache_spin_lock, flags);
178 if (unlikely(sysctl_flashcache_error_inject & READFILL_ERROR)) {
179 job->error = error = -EIO;
180 sysctl_flashcache_error_inject &= ~READFILL_ERROR;
181 }
182 if (unlikely(error))
183 dmc->ssd_write_errors++;
184 VERIFY(cacheblk->cache_state & DISKREADINPROG);
185 spin_unlock_irqrestore(&dmc->cache_spin_lock, flags);
186 flashcache_bio_endio(bio, error);
187 break;
归纳一下读不命中的流程：
1）创建一个kcached_job，直接下发到磁盘
2）读磁盘返回到flashcache_io_callback，到131行下发READFILL，将读出来的数据写到缓存中
3）写缓存成功并返回到flashcache_io_callback，到174行将数据返回给上层
到这里已经将读流程简单过了一遍，下一个小节介绍写流程。