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Re: [PATCH v10 6/9] copy-on-read: skip non-guest reads if no copy needed


From: Vladimir Sementsov-Ogievskiy
Subject: Re: [PATCH v10 6/9] copy-on-read: skip non-guest reads if no copy needed
Date: Wed, 7 Oct 2020 13:06:27 +0300
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101 Thunderbird/78.3.1

29.09.2020 15:38, Andrey Shinkevich wrote:
If the flag BDRV_REQ_PREFETCH was set, pass it further to the
COR-driver to skip unneeded reading. It can be taken into account for
the COR-algorithms optimization. That check is being made during the
block stream job by the moment.

Signed-off-by: Andrey Shinkevich <andrey.shinkevich@virtuozzo.com>
---
  block/copy-on-read.c | 14 ++++++++++----
  block/io.c           |  2 +-
  2 files changed, 11 insertions(+), 5 deletions(-)

diff --git a/block/copy-on-read.c b/block/copy-on-read.c
index f53f7e0..5389dca 100644
--- a/block/copy-on-read.c
+++ b/block/copy-on-read.c
@@ -145,10 +145,16 @@ static int coroutine_fn 
cor_co_preadv_part(BlockDriverState *bs,
              }
          }
- ret = bdrv_co_preadv_part(bs->file, offset, n, qiov, qiov_offset,
-                                  local_flags);
-        if (ret < 0) {
-            return ret;
+        if ((flags & BDRV_REQ_PREFETCH) &

BDRV_REQ_PREFETCH is documented to be only used with BDRV_REQ_COPY_ON_READ. But 
here
BDRV_REQ_COPY_ON_READ appears intermediately. We should change documentation in 
block.h
in a separate patch (and probably code in bdrv_aligned_preadv())

+            !(local_flags & BDRV_REQ_COPY_ON_READ)) {
+            /* Skip non-guest reads if no copy needed */
+        } else {
+

extra new-line ?

+            ret = bdrv_co_preadv_part(bs->file, offset, n, qiov, qiov_offset,
+                                      local_flags);
+            if (ret < 0) {
+                return ret;
+            }
          }
offset += n;
diff --git a/block/io.c b/block/io.c
index 11df188..62b75a5 100644
--- a/block/io.c
+++ b/block/io.c
@@ -1388,7 +1388,7 @@ static int coroutine_fn jk(BdrvChild *child,
              qemu_iovec_init_buf(&local_qiov, bounce_buffer, pnum);
ret = bdrv_driver_preadv(bs, cluster_offset, pnum,
-                                     &local_qiov, 0, 0);
+                                     &local_qiov, 0, flags & 
BDRV_REQ_PREFETCH);

Why? In this place we want to read. We'll write back the data a few lines 
below. What will we write,
if underlying driver decide to do nothing because of BDRV_REQ_PREFETCH?

              if (ret < 0) {
                  goto err;
              }



--
Best regards,
Vladimir



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