[GEDI] [PATCH] block: gluster: Probe alignment limits

[Nir Soffer]

Implement alignment probing similar to file-posix, by reading from the
first 4k of the image.

Before this change, provisioning a VM on storage with sector size of
4096 bytes would fail when the installer try to create filesystems. Here
is an example command that reproduces this issue:

    $ qemu-system-x86_64 -accel kvm -m 2048 -smp 2 \
        -drive file=gluster://gluster1/gv0/fedora29.raw,format=raw,cache=none \
        -cdrom Fedora-Server-dvd-x86_64-29-1.2.iso

The installer fails in few seconds when trying to create filesystem on
/dev/mapper/fedora-root. In error report we can see that it failed with
EINVAL (I could not extract the error from guest).

Copying disk fails with EINVAL:

    $ qemu-img convert -p -f raw -O raw -t none -T none \
        gluster://gluster1/gv0/fedora29.raw \
        gluster://gluster1/gv0/fedora29-clone.raw
    qemu-img: error while writing sector 4190208: Invalid argument

This is a fix to same issue fixed in commit a6b257a08e3d (file-posix:
Handle undetectable alignment) for gluster:// images.

This fix has the same limit, that the first block of the image should be
allocated, otherwise we cannot detect the alignment and fallback to a
safe value (4096) even when using storage with sector size of 512 bytes.

Signed-off-by: Nir Soffer <nsoffer at redhat.com>
---
 block/gluster.c | 47 +++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 47 insertions(+)

diff --git a/block/gluster.c b/block/gluster.c
index f64dc5b01e..d936240b72 100644
--- a/block/gluster.c
+++ b/block/gluster.c
@@ -52,6 +52,9 @@
 
 #define GERR_INDEX_HINT "hint: check in 'server' array index '%d'\n"
 
+/* The value is known only on the server side. */
+#define MAX_ALIGN 4096
+
 typedef struct GlusterAIOCB {
     int64_t size;
     int ret;
@@ -902,8 +905,52 @@ out:
     return ret;
 }
 
+/*
+ * Check if read is allowed with given memory buffer and length.
+ *
+ * This function is used to check O_DIRECT request alignment.
+ */
+static bool gluster_is_io_aligned(struct glfs_fd *fd, void *buf, size_t len)
+{
+    ssize_t ret = glfs_pread(fd, buf, len, 0, 0, NULL);
+    return ret >= 0 || errno != EINVAL;
+}
+
+static void gluster_probe_alignment(BlockDriverState *bs, struct glfs_fd *fd,
+                                    Error **errp)
+{
+    char *buf;
+    size_t alignments[] = {1, 512, 1024, 2048, 4096};
+    size_t align;
+    int i;
+
+    buf = qemu_memalign(MAX_ALIGN, MAX_ALIGN);
+
+    for (i = 0; i < ARRAY_SIZE(alignments); i++) {
+        align = alignments[i];
+        if (gluster_is_io_aligned(fd, buf, align)) {
+            /* Fallback to safe value. */
+            bs->bl.request_alignment = (align != 1) ? align : MAX_ALIGN;
+            break;
+        }
+    }
+
+    qemu_vfree(buf);
+
+    if (!bs->bl.request_alignment) {
+        error_setg(errp, "Could not find working O_DIRECT alignment");
+        error_append_hint(errp, "Try cache.direct=off\n");
+    }
+}
+
 static void qemu_gluster_refresh_limits(BlockDriverState *bs, Error **errp)
 {
+    BDRVGlusterState *s = bs->opaque;
+
+    gluster_probe_alignment(bs, s->fd, errp);
+
+    bs->bl.min_mem_alignment = bs->bl.request_alignment;
+    bs->bl.opt_mem_alignment = MAX(bs->bl.request_alignment, MAX_ALIGN);
     bs->bl.max_transfer = GLUSTER_MAX_TRANSFER;
 }
 
-- 
2.20.1