| Message ID | 20200710142149.40962-3-kwolf@redhat.com |
|---|---|
| State | New |
| Headers | show |
| Series | qemu-img convert: Fix abort with unaligned image size | expand |
On 7/10/20 9:21 AM, Kevin Wolf wrote: > 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 strict for > 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(-) > Reviewed-by: Eric Blake <eblake@redhat.com>
On Fri, Jul 10, 2020 at 5:22 PM Kevin Wolf <kwolf@redhat.com> wrote: > > 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; I did not know we have needs_alignment. Isn't this the same as using request_alignment = 1? For example we can check if we are on NFS and avoid the fallback to max_align: if (!bs->bl.request_alignment) { int i; size_t align; buf = qemu_memalign(max_align, max_align); for (i = 0; i < ARRAY_SIZE(alignments); i++) { align = alignments[i]; if (raw_is_io_aligned(fd, buf, align)) { /* Fallback to safe value. */ bs->bl.request_alignment = (align != 1) ? align : max_align; break; } } qemu_vfree(buf); } After this we will have correct bl.request_alignment and buf_align. Hopefully this will not break code expecting request_alignment >= 512. Assuming that needs_alignment is well tested, this patch may be safer. Nir > } > > -- > 2.25.4 >
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; }
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(-)