Re: [PATCH 0/9] migration/mapped-ram: Add direct-io support

[Peter Xu]

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?

-- 
Peter Xu