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Re: [PATCH RFC] memory: Don't allow to resize RAM while migrating
From: |
Peter Xu |
Subject: |
Re: [PATCH RFC] memory: Don't allow to resize RAM while migrating |
Date: |
Fri, 14 Feb 2020 13:11:46 -0500 |
On Fri, Feb 14, 2020 at 06:32:35PM +0100, David Hildenbrand wrote:
> On 14.02.20 18:29, Peter Xu wrote:
> > On Fri, Feb 14, 2020 at 01:02:46PM +0100, David Hildenbrand wrote:
> >> From c0049ac2e95d6756037db918852c507fb88297d9 Mon Sep 17 00:00:00 2001
> >> From: David Hildenbrand <address@hidden>
> >> Date: Fri, 14 Feb 2020 13:01:03 +0100
> >> Subject: [PATCH v1] tmp
> >>
> >> Signed-off-by: David Hildenbrand <address@hidden>
> >> ---
> >> migration/migration.c | 9 +++++++--
> >> migration/migration.h | 1 +
> >> migration/ram.c | 42 ++++++++++++++++++++++++++++++++++++++++++
> >> 3 files changed, 50 insertions(+), 2 deletions(-)
> >>
> >> diff --git a/migration/migration.c b/migration/migration.c
> >> index 3a21a4686c..0e7efe2920 100644
> >> --- a/migration/migration.c
> >> +++ b/migration/migration.c
> >> @@ -175,13 +175,18 @@ void migration_object_init(void)
> >> }
> >> }
> >>
> >> +void migration_cancel(void)
> >> +{
> >> + migrate_fd_cancel(current_migration);
> >> +}
> >> +
> >> void migration_shutdown(void)
> >> {
> >> /*
> >> * Cancel the current migration - that will (eventually)
> >> * stop the migration using this structure
> >> */
> >> - migrate_fd_cancel(current_migration);
> >> + migration_cancel();
> >> object_unref(OBJECT(current_migration));
> >> }
> >>
> >> @@ -2019,7 +2024,7 @@ void qmp_migrate(const char *uri, bool has_blk, bool
> >> blk,
> >>
> >> void qmp_migrate_cancel(Error **errp)
> >> {
> >> - migrate_fd_cancel(migrate_get_current());
> >> + migration_cancel();
> >> }
> >>
> >> void qmp_migrate_continue(MigrationStatus state, Error **errp)
> >> diff --git a/migration/migration.h b/migration/migration.h
> >> index 8473ddfc88..79fd74afa5 100644
> >> --- a/migration/migration.h
> >> +++ b/migration/migration.h
> >> @@ -343,5 +343,6 @@ int foreach_not_ignored_block(RAMBlockIterFunc func,
> >> void *opaque);
> >> void migration_make_urgent_request(void);
> >> void migration_consume_urgent_request(void);
> >> bool migration_rate_limit(void);
> >> +void migration_cancel(void);
> >>
> >> #endif
> >> diff --git a/migration/ram.c b/migration/ram.c
> >> index ed23ed1c7c..f86f32b453 100644
> >> --- a/migration/ram.c
> >> +++ b/migration/ram.c
> >> @@ -52,6 +52,7 @@
> >> #include "migration/colo.h"
> >> #include "block.h"
> >> #include "sysemu/sysemu.h"
> >> +#include "sysemu/runstate.h"
> >> #include "savevm.h"
> >> #include "qemu/iov.h"
> >> #include "multifd.h"
> >> @@ -3710,8 +3711,49 @@ static SaveVMHandlers savevm_ram_handlers = {
> >> .resume_prepare = ram_resume_prepare,
> >> };
> >>
> >> +static void ram_mig_ram_block_resized(RAMBlockNotifier *n, void *host,
> >> + size_t old_size, size_t new_size)
> >> +{
> >> + /*
> >> + * We don't care about resizes triggered on incoming migration (when
> >> + * syncing ram blocks), or of course, when no migration is going on.
> >> + */
> >> + if (migration_is_idle() || !runstate_is_running()) {
> >> + return;
> >> + }
> >
> > I feel like migration_is_idle() check is enough. Firstly, I feel like
> > we allow migration even with VM stopped. At the meantime, if VM is
> > not running, I see no reason that this resizing will happen after all? :)
>
> Migration code resizes ram blocks when synchronizing the ram state. See
> qemu_ram_resize() in ram_load_precopy()
>
> That happens on incoming migration while the vm is stopped.
Ah so your comment is for that which I misread. I'm surprised even
the incoming migration will set MigrationState and migration_is_idle()
will return false for it, since we've got MigrationIncomingState after
all. But yeh that seems to be correct.
Then it seems fine. It's just a bit unclear even with the comment.
Another alternative is we only register this resize() hook when
migration starts, and unregister it when stopped. Then we can even
drop the migration_is_idle().
>
> >
> >> +
> >> + if (!postcopy_is_running()) {
> >> + Error *err = NULL;
> >> +
> >> + /*
> >> + * Precopy code cannot deal with the size of ram blocks changing
> >> at
> >> + * random points in time. We're still running on the source, abort
> >> + * the migration and continue running here. Make sure to wait
> >> until
> >> + * migration was canceled.
> >> + */
> >> + error_setg(&err, "RAM resized during precopy.");
> >> + migrate_set_error(migrate_get_current(), err);
> >> + error_free(err);
> >> + migration_cancel();
> >> + } else {
> >> + /*
> >> + * Postcopy code cannot deal with the size of ram blocks changing
> >> at
> >> + * random points in time. We're running on the target. Fail hard.
> >> + *
> >> + * TODO: How to handle this in a better way?
> >> + */
> >> + error_report("RAM resized during postcopy.");
> >> + exit(-1);
> >
> > Now I'm rethinking the postcopy case....
> >
> > ram_dirty_bitmap_reload() should only happen during the postcopy
> > recovery, and when that happens the VM should be stopped on both
> > sides. Which means, ram resizing should not trigger there...
>
> But that guest got the chance to run for a bit and eventually reboot
> AFAIK. Also, there are other data races possible when used_length
> suddenly changes, this is just the most obvious one where things will;
> get screwed up.
Right, the major one could be in ram_load_postcopy() where we call
host_from_ram_block_offset(). However if FW extension is the major
use case then it seems to still work (still better than crashing,
isn't it? :). Or could you let me know where else did I overlook?
Thanks,
--
Peter Xu
- Re: [PATCH RFC] memory: Don't allow to resize RAM while migrating, (continued)
- Re: [PATCH RFC] memory: Don't allow to resize RAM while migrating, Dr. David Alan Gilbert, 2020/02/14
- Re: [PATCH RFC] memory: Don't allow to resize RAM while migrating, David Hildenbrand, 2020/02/14
- Re: [PATCH RFC] memory: Don't allow to resize RAM while migrating, Dr. David Alan Gilbert, 2020/02/14
- Re: [PATCH RFC] memory: Don't allow to resize RAM while migrating, David Hildenbrand, 2020/02/14
- Re: [PATCH RFC] memory: Don't allow to resize RAM while migrating, David Hildenbrand, 2020/02/14
- Re: [PATCH RFC] memory: Don't allow to resize RAM while migrating, Juan Quintela, 2020/02/14
- Re: [PATCH RFC] memory: Don't allow to resize RAM while migrating, David Hildenbrand, 2020/02/14
- Re: [PATCH RFC] memory: Don't allow to resize RAM while migrating, Dr. David Alan Gilbert, 2020/02/14
- Re: [PATCH RFC] memory: Don't allow to resize RAM while migrating, Peter Xu, 2020/02/14
- Re: [PATCH RFC] memory: Don't allow to resize RAM while migrating, David Hildenbrand, 2020/02/14
- Re: [PATCH RFC] memory: Don't allow to resize RAM while migrating,
Peter Xu <=
- Re: [PATCH RFC] memory: Don't allow to resize RAM while migrating, David Hildenbrand, 2020/02/14
- Re: [PATCH RFC] memory: Don't allow to resize RAM while migrating, Peter Xu, 2020/02/14
- Re: [PATCH RFC] memory: Don't allow to resize RAM while migrating, David Hildenbrand, 2020/02/14
- Re: [PATCH RFC] memory: Don't allow to resize RAM while migrating, Peter Xu, 2020/02/14