| Message ID | 20240426142042.14573-1-farosas@suse.de (mailing list archive) |
|---|---|
| Headers | show |
| Series | migration/mapped-ram: Add direct-io support | expand |
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 <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.