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file-posix: Allow byte-aligned O_DIRECT with NFS

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Since commit a6b257a08e ('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 for 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>
Reviewed-by: Eric Blake <eblake@redhat.com>
Message-Id: <20200716142601.111237-3-kwolf@redhat.com>
Reviewed-by: Max Reitz <mreitz@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
This commit is contained in:
Kevin Wolf 2020-07-16 16:26:01 +02:00
parent 9c60a5d197
commit 5edc85571e
1 changed files with 25 additions and 1 deletions
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26
block/file-posix.c
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@ -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.
@ -629,7 +653,7 @@ 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;
}