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[Qemu-block] [PATCH] block/raw-posix: Open file descriptor O_RDWR to wor
From: |
Richard W.M. Jones |
Subject: |
[Qemu-block] [PATCH] block/raw-posix: Open file descriptor O_RDWR to work around glibc posix_fallocate emulation issue. |
Date: |
Tue, 29 Sep 2015 16:54:10 +0100 |
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
- [Qemu-block] [PATCH] block/raw-posix: Open file descriptor O_RDWR to work around glibc posix_fallocate emulation issue.,
Richard W.M. Jones <=