[PATCH 0/2] block: discard alignment fixes

[PATCH 0/2] block: discard alignment fixes

[Stefan Hajnoczi]

Two discard alignment issues were identified in
https://issues.redhat.com/browse/RHEL-86032:
1. pdiscard_alignment is not populated for host_device in file-posix.c.
2. Misaligned head/tail discard requests are not skipped when file-posix.c
   returns -EINVAL. This causes an undesired pause when guests are configured
   with werror=stop.

Stefan Hajnoczi (2):
  file-posix: probe discard alignment on Linux block devices
  block/io: skip head/tail requests on EINVAL

 block/file-posix.c | 56 +++++++++++++++++++++++++++++++++++++++++++++-
 block/io.c         |  6 ++++-
 2 files changed, 60 insertions(+), 2 deletions(-)

-- 
2.49.0

[PATCH 1/2] file-posix: probe discard alignment on Linux block devices

[Stefan Hajnoczi]

Populate the pdiscard_alignment block limit so the block layer is able
align discard requests correctly.

Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
---
 block/file-posix.c | 56 +++++++++++++++++++++++++++++++++++++++++++++-
 1 file changed, 55 insertions(+), 1 deletion(-)

diff --git a/block/file-posix.c b/block/file-posix.c
index 56d1972d15..2a1e1f48c0 100644
--- a/block/file-posix.c
+++ b/block/file-posix.c
@@ -1276,10 +1276,10 @@ static int get_sysfs_zoned_model(struct stat *st, BlockZoneModel *zoned)
 }
 #endif /* defined(CONFIG_BLKZONED) */
 
+#ifdef CONFIG_LINUX
 /*
  * Get a sysfs attribute value as a long integer.
  */
-#ifdef CONFIG_LINUX
 static long get_sysfs_long_val(struct stat *st, const char *attribute)
 {
     g_autofree char *str = NULL;
@@ -1299,6 +1299,30 @@ static long get_sysfs_long_val(struct stat *st, const char *attribute)
     }
     return ret;
 }
+
+/*
+ * Get a sysfs attribute value as a uint32_t.
+ */
+static int get_sysfs_u32_val(struct stat *st, const char *attribute,
+                             uint32_t *u32)
+{
+    g_autofree char *str = NULL;
+    const char *end;
+    unsigned int val;
+    int ret;
+
+    ret = get_sysfs_str_val(st, attribute, &str);
+    if (ret < 0) {
+        return ret;
+    }
+
+    /* The file is ended with '\n', pass 'end' to accept that. */
+    ret = qemu_strtoui(str, &end, 10, &val);
+    if (ret == 0 && end && *end == '\0') {
+        *u32 = val;
+    }
+    return ret;
+}
 #endif
 
 static int hdev_get_max_segments(int fd, struct stat *st)
@@ -1318,6 +1342,23 @@ static int hdev_get_max_segments(int fd, struct stat *st)
 #endif
 }
 
+/*
+ * Fills in *dalign with the discard alignment and returns 0 on success,
+ * -errno otherwise.
+ */
+static int hdev_get_pdiscard_alignment(struct stat *st, uint32_t *dalign)
+{
+#ifdef CONFIG_LINUX
+    /*
+     * Note that Linux "discard_granularity" is QEMU "discard_alignment". Linux
+     * "discard_alignment" is something else.
+     */
+    return get_sysfs_u32_val(st, "discard_granularity", dalign);
+#else
+    return -ENOTSUP;
+#endif
+}
+
 #if defined(CONFIG_BLKZONED)
 /*
  * If the reset_all flag is true, then the wps of zone whose state is
@@ -1527,6 +1568,19 @@ static void raw_refresh_limits(BlockDriverState *bs, Error **errp)
         }
     }
 
+    if (S_ISBLK(st.st_mode)) {
+        uint32_t dalign = 0;
+        int ret;
+
+        ret = hdev_get_pdiscard_alignment(&st, &dalign);
+        if (ret == 0) {
+            /* Must be a multiple of request_alignment */
+            assert(dalign % bs->bl.request_alignment == 0);
+
+            bs->bl.pdiscard_alignment = dalign;
+        }
+    }
+
     raw_refresh_zoned_limits(bs, &st, errp);
 }
 
-- 
2.49.0

[PATCH 2/2] block/io: skip head/tail requests on EINVAL

[Stefan Hajnoczi]

When guests send misaligned discard requests, the block layer breaks
them up into a misaligned head, an aligned main body, and a misaligned
tail.

The file-posix block driver on Linux returns -EINVAL on misaligned
discard requests. This causes bdrv_co_pdiscard() to fail and guests
configured with werror=stop will pause.

Add a special case for misaligned head/tail requests. Simply continue
when EINVAL is encountered so that the aligned main body of the request
can be completed and the guest is not paused. This is the best we can do
when guest discard limits do not match the host discard limits.

Fixes: https://issues.redhat.com/browse/RHEL-86032
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
---
 block/io.c | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/block/io.c b/block/io.c
index 1ba8d1aeea..5975f4e9a3 100644
--- a/block/io.c
+++ b/block/io.c
@@ -3180,7 +3180,11 @@ int coroutine_fn bdrv_co_pdiscard(BdrvChild *child, int64_t offset,
             }
         }
         if (ret && ret != -ENOTSUP) {
-            goto out;
+            if (ret == -EINVAL && (offset % align == 0 || num % align == 0)) {
+                /* Silently skip rejected unaligned head/tail requests */
+            } else {
+                goto out; /* bail out */
+            }
         }
 
         offset += num;
-- 
2.49.0

Re: [PATCH 1/2] file-posix: probe discard alignment on Linux block devices

[Eric Blake]

On Wed, Apr 09, 2025 at 02:29:13PM -0400, Stefan Hajnoczi wrote:
> Populate the pdiscard_alignment block limit so the block layer is able
> align discard requests correctly.
> 
> Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
> ---
>  block/file-posix.c | 56 +++++++++++++++++++++++++++++++++++++++++++++-
>  1 file changed, 55 insertions(+), 1 deletion(-)
> 
> diff --git a/block/file-posix.c b/block/file-posix.c
> index 56d1972d15..2a1e1f48c0 100644
> --- a/block/file-posix.c
> +++ b/block/file-posix.c
> @@ -1276,10 +1276,10 @@ static int get_sysfs_zoned_model(struct stat *st, BlockZoneModel *zoned)
>  }
>  #endif /* defined(CONFIG_BLKZONED) */
>  
> +#ifdef CONFIG_LINUX
>  /*
>   * Get a sysfs attribute value as a long integer.
>   */
> -#ifdef CONFIG_LINUX
>  static long get_sysfs_long_val(struct stat *st, const char *attribute)

Random change to pull the comment inside of the #if, but tangentially
related so I don't mind keeping it here.

>  {
>      g_autofree char *str = NULL;
> @@ -1299,6 +1299,30 @@ static long get_sysfs_long_val(struct stat *st, const char *attribute)
>      }
>      return ret;
>  }
> +
> +/*
> + * Get a sysfs attribute value as a uint32_t.
> + */
> +static int get_sysfs_u32_val(struct stat *st, const char *attribute,
> +                             uint32_t *u32)
> +{
> +    g_autofree char *str = NULL;
> +    const char *end;
> +    unsigned int val;
> +    int ret;
> +
> +    ret = get_sysfs_str_val(st, attribute, &str);
> +    if (ret < 0) {
> +        return ret;
> +    }
> +
> +    /* The file is ended with '\n', pass 'end' to accept that. */
> +    ret = qemu_strtoui(str, &end, 10, &val);
> +    if (ret == 0 && end && *end == '\0') {
> +        *u32 = val;
> +    }
> +    return ret;
> +}
>  #endif

May be worth amending this line to "#endif /* CONFIG_LINUX */" now
that it is further from the opening #if.

Reviewed-by: Eric Blake <eblake@redhat.com>

-- 
Eric Blake, Principal Software Engineer
Red Hat, Inc.
Virtualization:  qemu.org | libguestfs.org