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From 4e553943c8fe4924d194884b4719c5459210c686 Mon Sep 17 00:00:00 2001
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From: Kevin Wolf <kwolf@redhat.com>
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Date: Tue, 26 Jan 2021 17:21:03 -0500
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Subject: [PATCH 8/9] file-posix: Allow byte-aligned O_DIRECT with NFS
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RH-Author: Kevin Wolf <kwolf@redhat.com>
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Message-id: <20210126172103.136060-3-kwolf@redhat.com>
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Patchwork-id: 100785
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O-Subject: [RHEL-8.4.0 qemu-kvm PATCH 2/2] file-posix: Allow byte-aligned O_DIRECT with NFS
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Bugzilla: 1834281
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RH-Acked-by: Markus Armbruster <armbru@redhat.com>
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RH-Acked-by: Stefan Hajnoczi <stefanha@redhat.com>
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RH-Acked-by: Max Reitz <mreitz@redhat.com>
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Since commit a6b257a08e3 ('file-posix: Handle undetectable alignment'),
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we assume that if we open a file with O_DIRECT and alignment probing
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returns 1, we just couldn't find out the real alignment requirement
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because some filesystems make the requirement only for allocated blocks.
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In this case, a safe default of 4k is used.
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This is too strict for NFS, which does actually allow byte-aligned
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requests even with O_DIRECT. Because we can't distinguish both cases
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with generic code, let's just look at the file system magic and disable
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s->needs_alignment for NFS. This way, O_DIRECT can still be used on NFS
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for images that are not aligned to 4k.
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Signed-off-by: Kevin Wolf <kwolf@redhat.com>
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Reviewed-by: Eric Blake <eblake@redhat.com>
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Message-Id: <20200716142601.111237-3-kwolf@redhat.com>
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Reviewed-by: Max Reitz <mreitz@redhat.com>
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Signed-off-by: Kevin Wolf <kwolf@redhat.com>
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(cherry picked from commit 5edc85571e7b7269dce408735eba7507f18ac666)
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Signed-off-by: Kevin Wolf <kwolf@redhat.com>
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Signed-off-by: Jon Maloy <jmaloy.redhat.com>
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---
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block/file-posix.c | 26 +++++++++++++++++++++++++-
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1 file changed, 25 insertions(+), 1 deletion(-)
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diff --git a/block/file-posix.c b/block/file-posix.c
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index adafbfa1be..2d834fbdf6 100644
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--- a/block/file-posix.c
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+++ b/block/file-posix.c
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@@ -61,10 +61,12 @@
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#include <sys/ioctl.h>
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#include <sys/param.h>
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#include <sys/syscall.h>
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+#include <sys/vfs.h>
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#include <linux/cdrom.h>
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#include <linux/fd.h>
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#include <linux/fs.h>
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#include <linux/hdreg.h>
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+#include <linux/magic.h>
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#include <scsi/sg.h>
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#ifdef __s390__
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#include <asm/dasd.h>
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@@ -298,6 +300,28 @@ static int probe_physical_blocksize(int fd, unsigned int *blk_size)
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#endif
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}
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+/*
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+ * Returns true if no alignment restrictions are necessary even for files
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+ * opened with O_DIRECT.
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+ *
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+ * raw_probe_alignment() probes the required alignment and assume that 1 means
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+ * the probing failed, so it falls back to a safe default of 4k. This can be
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+ * avoided if we know that byte alignment is okay for the file.
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+ */
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+static bool dio_byte_aligned(int fd)
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+{
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+#ifdef __linux__
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+ struct statfs buf;
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+ int ret;
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+
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+ ret = fstatfs(fd, &buf;;
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+ if (ret == 0 && buf.f_type == NFS_SUPER_MAGIC) {
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+ return true;
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+ }
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+#endif
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+ return false;
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+}
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+
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/* Check if read is allowed with given memory buffer and length.
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*
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* This function is used to check O_DIRECT memory buffer and request alignment.
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@@ -602,7 +626,7 @@ static int raw_open_common(BlockDriverState *bs, QDict *options,
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s->has_discard = true;
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s->has_write_zeroes = true;
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- if ((bs->open_flags & BDRV_O_NOCACHE) != 0) {
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+ if ((bs->open_flags & BDRV_O_NOCACHE) != 0 && !dio_byte_aligned(s->fd)) {
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s->needs_alignment = true;
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}
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--
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2.18.2
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