[Qemu-devel] [PATCH v1 14/18] backup/pcache: pick up parts of the cache

[Pavel Butsykin]

Provided that the request size is less than the readahead size, a partial cache
hit can occur in the following three cases:
1. The request covers the bottom part of the node
2. The request covers the upper part of the node
3. The request is between two nodes and partially covers both of them

The function pickup_parts_of_cache() covers all three cases and for the
uncached part of the request it creates a new request.

Signed-off-by: Pavel Butsykin <address@hidden>
---
 block/pcache.c | 57 +++++++++++++++++++++++++++++++++++++++++++++++++++++++--
 1 file changed, 55 insertions(+), 2 deletions(-)

diff --git a/block/pcache.c b/block/pcache.c
index 55d1061..40fa3f3 100644
--- a/block/pcache.c
+++ b/block/pcache.c
@@ -100,6 +100,11 @@ typedef struct PCacheAIOCBRead {
     uint64_t offset;
     uint64_t bytes;
     QEMUIOVector *qiov;
+    struct {
+        QEMUIOVector qiov;
+        uint64_t offset;
+        uint64_t bytes;
+    } part;
     int ref;
     int ret;
 } PCacheAIOCBRead;
@@ -189,6 +194,10 @@ static void pcache_aio_read_cb(void *opaque, int ret)
 {
     PCacheAIOCBRead *acb = opaque;
 
+    if (acb->part.qiov.niov != 0) {
+        qemu_iovec_destroy(&acb->part.qiov);
+    }
+
     aio_read_complete(acb, ret);
 }
 
@@ -421,6 +430,44 @@ static void pcache_readahead_request(PCacheAIOCBRead *acb)
     qemu_coroutine_enter(co);
 }
 
+static void set_part_req(PCacheAIOCBRead *acb, uint64_t offset, uint64_t bytes)
+{
+    acb->part.offset = offset;
+    acb->part.bytes = bytes;
+
+    assert(acb->part.qiov.niov == 0);
+
+    qemu_iovec_init(&acb->part.qiov, acb->qiov->niov);
+    qemu_iovec_concat(&acb->part.qiov, acb->qiov, offset - acb->offset, bytes);
+}
+
+static void pickup_parts_of_cache(PCacheAIOCBRead *acb, PCacheNode *node,
+                                  uint64_t offset, uint64_t bytes)
+{
+    BDRVPCacheState *s = acb->bs->opaque;
+    uint64_t end_offs = offset + bytes;
+
+    do {
+        if (offset < node->common.offset) {
+            set_part_req(acb, offset,  node->common.offset - offset);
+        }
+
+        pcache_aio_read(acb, node);
+
+        offset = node->common.offset + node->common.bytes;
+        if (end_offs <= offset) {
+            break;
+        }
+
+        bytes = end_offs - offset;
+        node = rbcache_search(s->cache, offset, bytes);
+        if (node == NULL) {
+            set_part_req(acb, offset, bytes);
+            break;
+        }
+    } while (true);
+}
+
 enum {
     CACHE_MISS,
     CACHE_HIT,
@@ -444,7 +491,9 @@ static int pcache_lookup_data(PCacheAIOCBRead *acb)
         return CACHE_HIT;
     }
 
-    return PARTIAL_CACHE_HIT;
+    pickup_parts_of_cache(acb, node, acb->offset, acb->bytes);
+
+    return acb->part.qiov.niov == 0 ? CACHE_HIT : PARTIAL_CACHE_HIT;
 }
 
 static coroutine_fn int pcache_co_preadv(BlockDriverState *bs, uint64_t offset,
@@ -471,9 +520,13 @@ static coroutine_fn int pcache_co_preadv(BlockDriverState 
*bs, uint64_t offset,
     update_req_stats(s->req_stats, offset, bytes);
 
     status = pcache_lookup_data(&acb);
-    if (status == CACHE_MISS || status == PARTIAL_CACHE_HIT) {
+    if (status == CACHE_MISS) {
         bdrv_aio_preadv(bs->file, offset, qiov, bytes,
                         pcache_aio_read_cb, &acb);
+    } else if (status == PARTIAL_CACHE_HIT) {
+        assert(acb.part.qiov.niov != 0);
+        bdrv_aio_preadv(bs->file, acb.part.offset, &acb.part.qiov,
+                        acb.part.bytes, pcache_aio_read_cb, &acb);
     }
 
     pcache_readahead_request(&acb);
-- 
2.10.1