On 03.06.21 15:37, Paolo Bonzini wrote:
bs->sg is only true for character devices, but block devices can also
be used with scsi-block and scsi-generic. Unfortunately BLKSECTGET
returns bytes in an int for /dev/sgN devices, and sectors in a short
for block devices, so account for that in the code.
The maximum transfer also need not be a power of 2 (for example I have
seen disks with 1280 KiB maximum transfer) so there's no need to pass
the result through pow2floor.
This patch reintroduces the logic that was removed in commit 867eccfed8
("file-posix: Use max transfer length/segment count only for SCSI
passthrough", 2019-07-12). The removal was motivated by the performance
regression when using a block device's max_transfer with kernel I/O;
the new, separate max_hw_transfer limit avoids reintroducing the same
regression.
(And the fact that the result of hdev_get_max_segments() is no longer
used to cap max_transfer, but is just stored in max_iov instead.)
Signed-off-by: Paolo Bonzini <pbonz...@redhat.com>
---
block/file-posix.c | 40 ++++++++++++++++++++++------------------
1 file changed, 22 insertions(+), 18 deletions(-)
diff --git a/block/file-posix.c b/block/file-posix.c
index f55f92d0f5..1439293f63 100644
--- a/block/file-posix.c
+++ b/block/file-posix.c
@@ -1149,22 +1149,27 @@ static void raw_reopen_abort(BDRVReopenState *state)
s->reopen_state = NULL;
}
-static int sg_get_max_transfer_length(int fd)
+static int hdev_get_max_hw_transfer(int fd, struct stat *st)
{
#ifdef BLKSECTGET
- int max_bytes = 0;
-
- if (ioctl(fd, BLKSECTGET, &max_bytes) == 0) {
- return max_bytes;
+ if (S_ISBLK(st->st_mode)) {
+ unsigned short max_sectors = 0;
+ if (ioctl(fd, BLKSECTGET, &max_sectors) == 0) {
+ return max_sectors * 512;
+ }
} else {
- return -errno;
+ int max_bytes = 0;
+ if (ioctl(fd, BLKSECTGET, &max_bytes) == 0) {
+ return max_bytes;
+ }
}
+ return -errno;
#else
return -ENOSYS;
#endif
}
-static int sg_get_max_segments(int fd)
+static int hdev_get_max_segments(int fd, struct stat *st)
OK, now I see why in patch 1 you treated `st` as a pointer. Still, needs
to be `st.` in patch 1, and changed to `st->` in this patch only.
{
#ifdef CONFIG_LINUX
char buf[32];
@@ -1173,12 +1178,6 @@ static int sg_get_max_segments(int fd)
int ret;
int sysfd = -1;
long max_segments;
- struct stat st;
-
- if (fstat(fd, &st)) {
- ret = -errno;
- goto out;
- }
if (S_ISCHR(st->st_mode)) {
if (ioctl(fd, SG_GET_SG_TABLESIZE, &ret) == 0) {
@@ -1192,7 +1191,7 @@ static int sg_get_max_segments(int fd)
}
sysfspath = g_strdup_printf("/sys/dev/block/%u:%u/queue/max_segments",
- major(st.st_rdev), minor(st.st_rdev));
+ major(st->st_rdev), minor(st->st_rdev));
sysfd = open(sysfspath, O_RDONLY);
if (sysfd == -1) {
ret = -errno;
@@ -1229,15 +1228,20 @@ out:
static void raw_refresh_limits(BlockDriverState *bs, Error **errp)
{
BDRVRawState *s = bs->opaque;
+ struct stat st;
+
+ if (fstat(s->fd, &st)) {
+ return;
+ }
First, I think if we ignore an error, there should be a comment
explaining why that’s OK.
It is OK here because inquiring transfer limits is best-effort.
However, the alignment probes below the following block are completely
independent of `st`. Therefore, I don’t think we should skip them if
`fstat()` failed here; only the `if (bs->sg || ...)` block should be
skipped.
Max
- if (bs->sg) {
- int ret = sg_get_max_transfer_length(s->fd);
+ if (bs->sg || S_ISBLK(st.st_mode)) {
+ int ret = hdev_get_max_hw_transfer(s->fd, &st);
if (ret > 0 && ret <= BDRV_REQUEST_MAX_BYTES) {
- bs->bl.max_hw_transfer = pow2floor(ret);
+ bs->bl.max_hw_transfer = ret;
}
- ret = sg_get_max_segments(s->fd);
+ ret = hdev_get_max_segments(s->fd, &st);
if (ret > 0) {
bs->bl.max_iov = ret;
}