From: Fabiano Rosas <farosas@suse.de>
To: Peter Xu <peterx@redhat.com>
Cc: qemu-devel@nongnu.org, berrange@redhat.com, armbru@redhat.com,
Claudio Fontana <cfontana@suse.de>, Jim Fehlig <jfehlig@suse.com>
Subject: Re: [PATCH 0/9] migration/mapped-ram: Add direct-io support
Date: Thu, 02 May 2024 17:34:22 -0300 [thread overview]
Message-ID: <87ikzwneb5.fsf@suse.de> (raw)
In-Reply-To: <ZjPxLLwtJL3ATUML@x1n>
Peter Xu <peterx@redhat.com> writes:
> On Fri, Apr 26, 2024 at 11:20:33AM -0300, Fabiano Rosas wrote:
>> If the user is not passing in a file name which QEMU can open at will,
>> we must then require that the user pass the two file descriptors with
>> the flags already properly set. We'll use the already existing fdset +
>> QMP add-fd infrastructure for this.
>
> Yes I remember such requirement that one extra fd is needed for direct-io,
> however today when I looked closer at the man page it looks like F_SETFL
> works with O_DIRECT too?
>
> F_SETFL (int)
> Set the file status flags to the value specified by arg.
> File access mode (O_RDONLY, O_WRONLY, O_RDWR) and file
> creation flags (i.e., O_CREAT, O_EXCL, O_NOCTTY, O_TRUNC) in
> arg are ignored. On Linux, this command can change only the
> O_APPEND, O_ASYNC, O_DIRECT, O_NOATIME, and O_NONBLOCK flags.
> It is not possible to change the O_DSYNC and O_SYNC flags;
> see BUGS, below.
>
> ====8<====
> $ cat fcntl.c
> #define _GNU_SOURCE
> #include <stdio.h>
> #include <fcntl.h>
> #include <assert.h>
> #include <unistd.h>
>
> int main(void)
> {
> int fd, newfd, ret, flags;
>
> fd = open("test.txt", O_RDWR | O_CREAT, 0660);
> assert(fd != -1);
>
> flags = fcntl(fd, F_GETFL);
> printf("old fd flags: 0x%x\n", flags);
>
> newfd = dup(fd);
> assert(newfd != -1);
>
> flags = fcntl(newfd, F_GETFL);
> printf("new fd flags: 0x%x\n", flags);
>
> flags |= O_DIRECT;
> ret = fcntl(newfd, F_SETFL, flags);
>
> flags = fcntl(fd, F_GETFL);
> printf("updated new flags: 0x%x\n", flags);
>
> return 0;
> }
> $ make fcntl
> cc fcntl.c -o fcntl
> $ ./fcntl
> old fd flags: 0x8002
> new fd flags: 0x8002
> updated new flags: 0xc002
> ====8<====
>
> Perhaps I missed something important?
The dup()'ed file descriptor shares file status flags with the original
fd. Your code example proves just that. In the last two blocks you're
doing F_SETFL on the 'newfd' and then seeing the change take effect on
'fd'. That's what we don't want to happen.
prev parent reply other threads:[~2024-05-02 20:35 UTC|newest]
Thread overview: 57+ messages / expand[flat|nested] mbox.gz Atom feed top
2024-04-26 14:20 [PATCH 0/9] migration/mapped-ram: Add direct-io support Fabiano Rosas
2024-04-26 14:20 ` [PATCH 1/9] monitor: Honor QMP request for fd removal immediately Fabiano Rosas
2024-05-03 16:02 ` Peter Xu
2024-05-16 21:46 ` Fabiano Rosas
2024-05-08 7:17 ` Daniel P. Berrangé
2024-05-16 22:00 ` Fabiano Rosas
2024-05-17 7:33 ` Daniel P. Berrangé
2024-04-26 14:20 ` [PATCH 2/9] migration: Fix file migration with fdset Fabiano Rosas
2024-05-03 16:23 ` Peter Xu
2024-05-03 19:56 ` Fabiano Rosas
2024-05-03 21:04 ` Peter Xu
2024-05-03 21:31 ` Fabiano Rosas
2024-05-03 21:56 ` Peter Xu
2024-05-08 8:02 ` Daniel P. Berrangé
2024-05-08 12:49 ` Peter Xu
2024-05-08 8:00 ` Daniel P. Berrangé
2024-05-08 20:45 ` Fabiano Rosas
2024-04-26 14:20 ` [PATCH 3/9] tests/qtest/migration: Fix file migration offset check Fabiano Rosas
2024-05-03 16:47 ` Peter Xu
2024-05-03 20:36 ` Fabiano Rosas
2024-05-03 21:08 ` Peter Xu
2024-05-08 8:10 ` Daniel P. Berrangé
2024-04-26 14:20 ` [PATCH 4/9] migration: Add direct-io parameter Fabiano Rosas
2024-04-26 14:33 ` Markus Armbruster
2024-05-03 18:05 ` Peter Xu
2024-05-03 20:49 ` Fabiano Rosas
2024-05-03 21:16 ` Peter Xu
2024-05-14 14:10 ` Markus Armbruster
2024-05-14 17:57 ` Fabiano Rosas
2024-05-15 7:17 ` Markus Armbruster
2024-05-15 12:51 ` Fabiano Rosas
2024-05-08 8:25 ` Daniel P. Berrangé
2024-04-26 14:20 ` [PATCH 5/9] migration/multifd: Add direct-io support Fabiano Rosas
2024-05-03 18:29 ` Peter Xu
2024-05-03 20:54 ` Fabiano Rosas
2024-05-03 21:18 ` Peter Xu
2024-05-08 8:27 ` Daniel P. Berrangé
2024-04-26 14:20 ` [PATCH 6/9] tests/qtest/migration: Add tests for file migration with direct-io Fabiano Rosas
2024-05-03 18:38 ` Peter Xu
2024-05-03 21:05 ` Fabiano Rosas
2024-05-03 21:25 ` Peter Xu
2024-05-08 8:34 ` Daniel P. Berrangé
2024-04-26 14:20 ` [PATCH 7/9] monitor: fdset: Match against O_DIRECT Fabiano Rosas
2024-05-03 18:53 ` Peter Xu
2024-05-03 21:19 ` Fabiano Rosas
2024-05-03 22:16 ` Peter Xu
2024-04-26 14:20 ` [PATCH 8/9] migration: Add support for fdset with multifd + file Fabiano Rosas
2024-05-08 8:53 ` Daniel P. Berrangé
2024-05-08 18:23 ` Peter Xu
2024-05-08 20:39 ` Fabiano Rosas
2024-05-09 8:08 ` Daniel P. Berrangé
2024-05-17 22:43 ` Fabiano Rosas
2024-05-18 8:36 ` Daniel P. Berrangé
2024-04-26 14:20 ` [PATCH 9/9] tests/qtest/migration: Add a test for mapped-ram with passing of fds Fabiano Rosas
2024-05-08 8:56 ` Daniel P. Berrangé
2024-05-02 20:01 ` [PATCH 0/9] migration/mapped-ram: Add direct-io support Peter Xu
2024-05-02 20:34 ` Fabiano Rosas [this message]
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