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Re: [Qemu-devel] [Qemu-block] [PATCH] block/raw-posix: Open file descrip
From: |
Jeff Cody |
Subject: |
Re: [Qemu-devel] [Qemu-block] [PATCH] block/raw-posix: Open file descriptor O_RDWR to work around glibc posix_fallocate emulation issue. |
Date: |
Wed, 30 Sep 2015 16:44:42 -0400 |
User-agent: |
Mutt/1.5.21 (2010-09-15) |
On Tue, Sep 29, 2015 at 04:54:10PM +0100, 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)).
>
> 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 <address@hidden>
> BZ: https://bugzilla.redhat.com/show_bug.cgi?id=1265196
> ---
> block/raw-posix.c | 2 +-
> 1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/block/raw-posix.c b/block/raw-posix.c
> index 30df8ad..86f8562 100644
> --- a/block/raw-posix.c
> +++ b/block/raw-posix.c
> @@ -1648,7 +1648,7 @@ static int raw_create(const char *filename, QemuOpts
> *opts, Error **errp)
> goto out;
> }
>
> - fd = qemu_open(filename, O_WRONLY | O_CREAT | O_TRUNC | O_BINARY,
> + fd = qemu_open(filename, O_RDWR | O_CREAT | O_TRUNC | O_BINARY,
> 0644);
> if (fd < 0) {
> result = -errno;
> --
> 2.5.0
>
>
Reviewed-by: Jeff Cody <address@hidden>