Re: [Qemu-devel] [PATCH] block/raw-posix: Open file descriptor O_RDWR to work around glibc posix_fallocate emulation issue.

[Eric Blake <eblake@redhat.com>]

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On 09/29/2015 09:54 AM, Richard W.M. Jones wrote:
>   https://bugzilla.redhat.com/show_bug.cgi?id=1265196
> 
> The following command fails on an NFS mountpoint:
> 
>   $ qemu-img create -f qcow2 -o preallocation=falloc disk.img 262144
>   Formatting 'disk.img', fmt=qcow2 size=262144 encryption=off cluster_size=65536 preallocation='falloc' lazy_refcounts=off
>   qemu-img: disk.img: Could not preallocate data for the new file: Bad file descriptor
> 
> The reason turns out to be because NFS doesn't support the
> posix_fallocate call.  glibc emulates it instead.  However glibc's
> emulation involves using the pread(2) syscall.  The pread syscall
> fails with EBADF if the file descriptor is opened without the read
> open-flag (ie. open (..., O_WRONLY)).
> 

Failing with EBADF feels fishy (POSIX requests EINVAL in that case) -
but that's an upstream glibc discussion that has forked from this thread
and doesn't hold up the qemu fix.

> I contacted glibc upstream about this, and their response is here:
> 
>   https://bugzilla.redhat.com/show_bug.cgi?id=1265196#c9
> 
> There are two possible fixes: Use Linux fallocate directly, or (this
> fix) work around the problem in qemu by opening the file with O_RDWR
> instead of O_WRONLY.
> 
> Signed-off-by: Richard W.M. Jones <rjones@redhat.com>
> BZ: https://bugzilla.redhat.com/show_bug.cgi?id=1265196
> ---
>  block/raw-posix.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)

Reviewed-by: Eric Blake <eblake@redhat.com>

-- 
Eric Blake   eblake redhat com    +1-919-301-3266
Libvirt virtualization library http://libvirt.org


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