Re: [Qemu-devel] [PATCH] block/gluster: glfs_lseek() workaround

[Jeff Cody <jcody@redhat.com>]

On Tue, May 23, 2017 at 01:27:50PM -0400, Jeff Cody wrote:
> On current released versions of glusterfs, glfs_lseek() will sometimes
> return invalid values for SEEK_DATA or SEEK_HOLE.  For SEEK_DATA and
> SEEK_HOLE, the returned value should be >= the passed offset, or < 0 in
> the case of error:
> 
> LSEEK(2):
> 
>     off_t lseek(int fd, off_t offset, int whence);
> 
>     [...]
> 
>     SEEK_HOLE
>               Adjust  the file offset to the next hole in the file greater
>               than or equal to offset.  If offset points into the middle of
>               a hole, then the file offset is set to offset.  If there is no
>               hole past offset, then the file offset is adjusted to the end
>               of the file (i.e., there is  an implicit hole at the end of
>               any file).
> 
>     [...]
> 
>     RETURN VALUE
>               Upon  successful  completion,  lseek()  returns  the resulting
>               offset location as measured in bytes from the beginning of the
>               file.  On error, the value (off_t) -1 is returned and errno is
>               set to indicate the error
> 
> However, occasionally glfs_lseek() for SEEK_HOLE/DATA will return a
> value less than the passed offset, yet greater than zero.
> 
> For instance, here are example values observed from this call:
> 
>     offs = glfs_lseek(s->fd, start, SEEK_HOLE);
>     if (offs < 0) {
>         return -errno;          /* D1 and (H3 or H4) */
>     }
> 
> start == 7608336384
> offs == 7607877632
> 
> This causes QEMU to abort on the assert test.  When this value is
> returned, errno is also 0.
> 
> This is a reported and known bug to glusterfs:
> https://bugzilla.redhat.com/show_bug.cgi?id=1425293
> 
> Although this is being fixed in gluster, we still should work around it
> in QEMU, given that multiple released versions of gluster behave this
> way.
> 
> This patch treats the return case of (offs < start) the same as if an
> error value other than ENXIO is returned; we will assume we learned
> nothing, and there are no holes in the file.
> 
> Signed-off-by: Jeff Cody <jcody@redhat.com>
> ---
>  block/gluster.c | 18 ++++++++++++++++--
>  1 file changed, 16 insertions(+), 2 deletions(-)
> 
> diff --git a/block/gluster.c b/block/gluster.c
> index 7c76cd0..c147909e 100644
> --- a/block/gluster.c
> +++ b/block/gluster.c
> @@ -1275,7 +1275,14 @@ static int find_allocation(BlockDriverState *bs, off_t start,
>      if (offs < 0) {
>          return -errno;          /* D3 or D4 */
>      }
> -    assert(offs >= start);
> +
> +    if (offs < start) {
> +        /* This is not a valid return by lseek().  We are safe to just return
> +         * -EIO in this case, and we'll treat it like D4. Unfortunately some
> +         *  versions of libgfapi will return offs < start, so an assert here
> +         *  will unneccesarily abort QEMU. */
> +        return -EIO;
> +    }
>  
>      if (offs > start) {
>          /* D2: in hole, next data at offs */
> @@ -1307,7 +1314,14 @@ static int find_allocation(BlockDriverState *bs, off_t start,
>      if (offs < 0) {
>          return -errno;          /* D1 and (H3 or H4) */
>      }
> -    assert(offs >= start);
> +
> +    if (offs < start) {
> +        /* This is not a valid return by lseek().  We are safe to just return
> +         * -EIO in this case, and we'll treat it like H4. Unfortunately some
> +         *  versions of libgfapi will return offs < start, so an assert here
> +         *  will unneccesarily abort QEMU. */
> +        return -EIO;
> +    }
>  
>      if (offs > start) {
>          /*
> -- 
> 2.9.3
> 

Thanks,

Applied (with comment fixes) to my block branch:

git://github.com/codyprime/qemu-kvm-jtc block

-Jeff