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From: Kevin Wolf <kwolf@redhat.com>
To: qemu-block@nongnu.org
Cc: kwolf@redhat.com, nsoffer@redhat.com, qemu-devel@nongnu.org,
	mreitz@redhat.com
Subject: [PATCH for-5.1 2/2] file-posix: Allow byte-aligned O_DIRECT with NFS
Date: Fri, 10 Jul 2020 16:21:49 +0200	[thread overview]
Message-ID: <20200710142149.40962-3-kwolf@redhat.com> (raw)
In-Reply-To: <20200710142149.40962-1-kwolf@redhat.com>

Since commit a6b257a08e3 ('file-posix: Handle undetectable alignment'),
we assume that if we open a file with O_DIRECT and alignment probing
returns 1, we just couldn't find out the real alignment requirement
because some filesystems make the requirement only for allocated blocks.
In this case, a safe default of 4k is used.

This is too strict NFS, which does actually allow byte-aligned requests
even with O_DIRECT. Because we can't distinguish both cases with generic
code, let's just look at the file system magic and disable
s->needs_alignment for NFS. This way, O_DIRECT can still be used on NFS
for images that are not aligned to 4k.

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
---
 block/file-posix.c | 26 +++++++++++++++++++++++++-
 1 file changed, 25 insertions(+), 1 deletion(-)

diff --git a/block/file-posix.c b/block/file-posix.c
index 0c4e07c415..4e9dac461b 100644
--- a/block/file-posix.c
+++ b/block/file-posix.c
@@ -62,10 +62,12 @@
 #include <sys/ioctl.h>
 #include <sys/param.h>
 #include <sys/syscall.h>
+#include <sys/vfs.h>
 #include <linux/cdrom.h>
 #include <linux/fd.h>
 #include <linux/fs.h>
 #include <linux/hdreg.h>
+#include <linux/magic.h>
 #include <scsi/sg.h>
 #ifdef __s390__
 #include <asm/dasd.h>
@@ -300,6 +302,28 @@ static int probe_physical_blocksize(int fd, unsigned int *blk_size)
 #endif
 }
 
+/*
+ * Returns true if no alignment restrictions are necessary even for files
+ * opened with O_DIRECT.
+ *
+ * raw_probe_alignment() probes the required alignment and assume that 1 means
+ * the probing failed, so it falls back to a safe default of 4k. This can be
+ * avoided if we know that byte alignment is okay for the file.
+ */
+static bool dio_byte_aligned(int fd)
+{
+#ifdef __linux__
+    struct statfs buf;
+    int ret;
+
+    ret = fstatfs(fd, &buf);
+    if (ret == 0 && buf.f_type == NFS_SUPER_MAGIC) {
+        return true;
+    }
+#endif
+    return false;
+}
+
 /* Check if read is allowed with given memory buffer and length.
  *
  * This function is used to check O_DIRECT memory buffer and request alignment.
@@ -631,7 +655,7 @@ static int raw_open_common(BlockDriverState *bs, QDict *options,
 
     s->has_discard = true;
     s->has_write_zeroes = true;
-    if ((bs->open_flags & BDRV_O_NOCACHE) != 0) {
+    if ((bs->open_flags & BDRV_O_NOCACHE) != 0 && !dio_byte_aligned(s->fd)) {
         s->needs_alignment = true;
     }
 
-- 
2.25.4



  parent reply	other threads:[~2020-07-10 14:23 UTC|newest]

Thread overview: 20+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-07-10 14:21 [PATCH for-5.1 0/2] qemu-img convert: Fix abort with unaligned image size Kevin Wolf
2020-07-10 14:21 ` [PATCH for-5.1 1/2] block: Require aligned image size to avoid assertion failure Kevin Wolf
2020-07-10 14:37   ` Eric Blake
2020-07-13 11:19   ` Max Reitz
2020-07-13 11:52     ` Max Reitz
2020-07-13 14:29     ` Kevin Wolf
2020-07-14  9:56       ` Max Reitz
2020-07-14 11:08         ` Kevin Wolf
2020-07-14 16:22           ` Max Reitz
2020-07-15  9:20             ` Kevin Wolf
2020-07-13 16:33   ` Nir Soffer
2020-07-13 16:56     ` Kevin Wolf
2020-07-15 13:22       ` Nir Soffer
2020-07-15 13:42         ` Kevin Wolf
2020-07-15 14:03           ` Nir Soffer
2020-07-15 14:03         ` Daniel P. Berrangé
2020-07-10 14:21 ` Kevin Wolf [this message]
2020-07-10 14:39   ` [PATCH for-5.1 2/2] file-posix: Allow byte-aligned O_DIRECT with NFS Eric Blake
2020-07-13 16:29   ` Nir Soffer
2020-07-10 14:43 ` [PATCH for-5.1 0/2] qemu-img convert: Fix abort with unaligned image size no-reply

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