Commit 5edc8557 authored by Kevin Wolf's avatar Kevin Wolf
Browse files

file-posix: Allow byte-aligned O_DIRECT with NFS 



Since commit a6b257a0 ('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: default avatarKevin Wolf <kwolf@redhat.com>
Reviewed-by: default avatarEric Blake <eblake@redhat.com>
Message-Id: <20200716142601.111237-3-kwolf@redhat.com>
Reviewed-by: default avatarMax Reitz <mreitz@redhat.com>
Signed-off-by: default avatarKevin Wolf <kwolf@redhat.com>
parent 9c60a5d1

block/file-posix.c

+25 −1
Original line number Diff line number Diff line
@@ -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;

    }