Re: [PATCH 8/9] migration: Add support for fdset with multifd + file

[Fabiano Rosas]

Daniel P. Berrangé <berrange@redhat.com> writes:

> On Wed, May 08, 2024 at 05:39:53PM -0300, Fabiano Rosas wrote:
>> Peter Xu <peterx@redhat.com> writes:
>> 
>> > On Wed, May 08, 2024 at 09:53:48AM +0100, Daniel P. Berrangé wrote:
>> >> On Fri, Apr 26, 2024 at 11:20:41AM -0300, Fabiano Rosas wrote:
>> >> > Allow multifd to use an fdset when migrating to a file. This is useful
>> >> > for the scenario where the management layer wants to have control over
>> >> > the migration file.
>> >> > 
>> >> > By receiving the file descriptors directly, QEMU can delegate some
>> >> > high level operating system operations to the management layer (such
>> >> > as mandatory access control). The management layer might also want to
>> >> > add its own headers before the migration stream.
>> >> > 
>> >> > Enable the "file:/dev/fdset/#" syntax for the multifd migration with
>> >> > mapped-ram. The requirements for the fdset mechanism are:
>> >> > 
>> >> > On the migration source side:
>> >> > 
>> >> > - the fdset must contain two fds that are not duplicates between
>> >> >   themselves;
>> >> > - if direct-io is to be used, exactly one of the fds must have the
>> >> >   O_DIRECT flag set;
>> >> > - the file must be opened with WRONLY both times.
>> >> > 
>> >> > On the migration destination side:
>> >> > 
>> >> > - the fdset must contain one fd;
>> >> > - the file must be opened with RDONLY.
>> >> > 
>> >> > Signed-off-by: Fabiano Rosas <farosas@suse.de>
>> >> > ---
>> >> >  docs/devel/migration/main.rst       | 18 ++++++++++++++
>> >> >  docs/devel/migration/mapped-ram.rst |  6 ++++-
>> >> >  migration/file.c                    | 38 ++++++++++++++++++++++++++++-
>> >> >  3 files changed, 60 insertions(+), 2 deletions(-)
>> >> > 
>> >> > diff --git a/docs/devel/migration/main.rst 
>> >> > b/docs/devel/migration/main.rst
>> >> > index 54385a23e5..50f6096470 100644
>> >> > --- a/docs/devel/migration/main.rst
>> >> > +++ b/docs/devel/migration/main.rst
>> >> > @@ -47,6 +47,24 @@ over any transport.
>> >> >    QEMU interference. Note that QEMU does not flush cached file
>> >> >    data/metadata at the end of migration.
>> >> >  
>> >> > +  The file migration also supports using a file that has already been
>> >> > +  opened. A set of file descriptors is passed to QEMU via an "fdset"
>> >> > +  (see add-fd QMP command documentation). This method allows a
>> >> > +  management application to have control over the migration file
>> >> > +  opening operation. There are, however, strict requirements to this
>> >> > +  interface:
>> >> > +
>> >> > +  On the migration source side:
>> >> > +    - if the multifd capability is to be used, the fdset must contain
>> >> > +      two file descriptors that are not duplicates between themselves;
>> >> > +    - if the direct-io capability is to be used, exactly one of the
>> >> > +      file descriptors must have the O_DIRECT flag set;
>> >> > +    - the file must be opened with WRONLY.
>> >> > +
>> >> > +  On the migration destination side:
>> >> > +    - the fdset must contain one file descriptor;
>> >> > +    - the file must be opened with RDONLY.
>> >> > +
>> >> >  In addition, support is included for migration using RDMA, which
>> >> >  transports the page data using ``RDMA``, where the hardware takes care 
>> >> > of
>> >> >  transporting the pages, and the load on the CPU is much lower.  While 
>> >> > the
>> >> > diff --git a/docs/devel/migration/mapped-ram.rst 
>> >> > b/docs/devel/migration/mapped-ram.rst
>> >> > index fa4cefd9fc..e6505511f0 100644
>> >> > --- a/docs/devel/migration/mapped-ram.rst
>> >> > +++ b/docs/devel/migration/mapped-ram.rst
>> >> > @@ -16,7 +16,7 @@ location in the file, rather than constantly being 
>> >> > added to a
>> >> >  sequential stream. Having the pages at fixed offsets also allows the
>> >> >  usage of O_DIRECT for save/restore of the migration stream as the
>> >> >  pages are ensured to be written respecting O_DIRECT alignment
>> >> > -restrictions (direct-io support not yet implemented).
>> >> > +restrictions.
>> >> >  
>> >> >  Usage
>> >> >  -----
>> >> > @@ -35,6 +35,10 @@ Use a ``file:`` URL for migration:
>> >> >  Mapped-ram migration is best done non-live, i.e. by stopping the VM on
>> >> >  the source side before migrating.
>> >> >  
>> >> > +For best performance enable the ``direct-io`` capability as well:
>> >> > +
>> >> > +    ``migrate_set_capability direct-io on``
>> >> > +
>> >> >  Use-cases
>> >> >  ---------
>> >> >  
>> >> > diff --git a/migration/file.c b/migration/file.c
>> >> > index b9265b14dd..3bc8bc7463 100644
>> >> > --- a/migration/file.c
>> >> > +++ b/migration/file.c
>> >> > @@ -17,6 +17,7 @@
>> >> >  #include "io/channel-file.h"
>> >> >  #include "io/channel-socket.h"
>> >> >  #include "io/channel-util.h"
>> >> > +#include "monitor/monitor.h"
>> >> >  #include "options.h"
>> >> >  #include "trace.h"
>> >> >  
>> >> > @@ -54,10 +55,18 @@ static void file_remove_fdset(void)
>> >> >      }
>> >> >  }
>> >> >  
>> >> > +/*
>> >> > + * With multifd, due to the behavior of the dup() system call, we need
>> >> > + * the fdset to have two non-duplicate fds so we can enable direct IO
>> >> > + * in the secondary channels without affecting the main channel.
>> >> > + */
>> >> >  static bool file_parse_fdset(const char *filename, int64_t *fdset_id,
>> >> >                               Error **errp)
>> >> >  {
>> >> > +    FdsetInfoList *fds_info;
>> >> > +    FdsetFdInfoList *fd_info;
>> >> >      const char *fdset_id_str;
>> >> > +    int nfds = 0;
>> >> >  
>> >> >      *fdset_id = -1;
>> >> >  
>> >> > @@ -71,6 +80,32 @@ static bool file_parse_fdset(const char *filename, 
>> >> > int64_t *fdset_id,
>> >> >          return false;
>> >> >      }
>> >> >  
>> >> > +    if (!migrate_multifd() || !migrate_direct_io()) {
>> >> > +        return true;
>> >> > +    }
>> >> > +
>> >> > +    for (fds_info = qmp_query_fdsets(NULL); fds_info;
>> >> > +         fds_info = fds_info->next) {
>> >> > +
>> >> > +        if (*fdset_id != fds_info->value->fdset_id) {
>> >> > +            continue;
>> >> > +        }
>> >> > +
>> >> > +        for (fd_info = fds_info->value->fds; fd_info; fd_info = 
>> >> > fd_info->next) {
>> >> > +            if (nfds++ > 2) {
>> >> > +                break;
>> >> > +            }
>> >> > +        }
>> >> > +    }
>> >> > +
>> >> > +    if (nfds != 2) {
>> >> > +        error_setg(errp, "Outgoing migration needs two fds in the 
>> >> > fdset, "
>> >> > +                   "got %d", nfds);
>> >> > +        qmp_remove_fd(*fdset_id, false, -1, NULL);
>> >> > +        *fdset_id = -1;
>> >> > +        return false;
>> >> > +    }
>> >> > +
>> >> >      return true;
>> >> >  }
>> >> 
>> >> Related to my thoughts in an earlier patch, where I say that use of fdsets
>> >> ought to be transparent to QEMU code, I'm not a fan of having this logic
>> >> in migration code.
>> >> 
>> >> IIUC, the migration code will call  qio_channel_file_new_path twice,
>> >> once with O_DIRECT and once without. This should trigger two calls
>> >> into monitor_fdset_dup_fd_add with different flags. If we're matching
>> >> flags in that monitor_fdset_dup_fd_add(), then if only 1 FD was
>> >> provided, are we not able to report an error there ?
>> >
>> > Right, this sounds working.
>> 
>> It works, but due to how low-level fdset is, it's difficult to match the
>> low level error to anything meaningful we can report to the user. I'll
>> have to add an errp to monitor_fdset_dup_fd_add(). Its returns are not
>> very useful.
>> 
>> -1 with no errno
>> -1 with EACCES (should actually be EBADF)
>> -1 with ENOENT
>> 
>> There has been some discusstion around this before actually:
>> 
>> https://lists.gnu.org/archive/html/qemu-devel/2021-08/msg02544.html
>
> The only caller of monitor_fdset_dup_fd_add is qemu_open_internal
> and that has a "Error **errp" parameter.  We should rewrite
> monitor_fdset_dup_fd_add to also have an "Error **errp" at which
> point we can actually report useful, actionable error messages
> from it. Errnos be gone !

I can do that, but qemu_open_old() does not pass the error in. Please
see if this works for you:

-->8--
>From 16e333cc5aeca1fab3f75f79048c0ab0d62d5b08 Mon Sep 17 00:00:00 2001
From: Fabiano Rosas <farosas@suse.de>
Date: Fri, 17 May 2024 19:30:39 -0300
Subject: [PATCH] io: Stop using qemu_open_old in channel-file

We want to make use of the Error object to report fdset errors from
qemu_open_internal() and passing the error pointer to qemu_open_old()
would require changing all callers. Move the file channel to the new
API instead.

Signed-off-by: Fabiano Rosas <farosas@suse.de>
---
 io/channel-file.c | 8 +++++---
 1 file changed, 5 insertions(+), 3 deletions(-)

diff --git a/io/channel-file.c b/io/channel-file.c
index 6436cfb6ae..2ea8d08360 100644
--- a/io/channel-file.c
+++ b/io/channel-file.c
@@ -68,11 +68,13 @@ qio_channel_file_new_path(const char *path,
 
     ioc = QIO_CHANNEL_FILE(object_new(TYPE_QIO_CHANNEL_FILE));
 
-    ioc->fd = qemu_open_old(path, flags, mode);
+    if (flags & O_CREAT) {
+        ioc->fd = qemu_create(path, flags & ~O_CREAT, mode, errp);
+    } else {
+        ioc->fd = qemu_open(path, flags, errp);
+    }
     if (ioc->fd < 0) {
         object_unref(OBJECT(ioc));
-        error_setg_errno(errp, errno,
-                         "Unable to open %s", path);
         return NULL;
     }
 
-- 
2.35.3