Re: [Qemu-devel] [PATCH v7 3/5] migration: API to clear bits of guest free pages from the dirty bitmap

[Peter Xu]

On Fri, Jun 01, 2018 at 03:36:01PM +0800, Wei Wang wrote:
> On 06/01/2018 12:00 PM, Peter Xu wrote:
> > On Tue, Apr 24, 2018 at 02:13:46PM +0800, Wei Wang wrote:
> > > This patch adds an API to clear bits corresponding to guest free pages
> > > from the dirty bitmap. Spilt the free page block if it crosses the QEMU
> > > RAMBlock boundary.
> > > 
> > > Signed-off-by: Wei Wang <address@hidden>
> > > CC: Dr. David Alan Gilbert <address@hidden>
> > > CC: Juan Quintela <address@hidden>
> > > CC: Michael S. Tsirkin <address@hidden>
> > > ---
> > >   include/migration/misc.h |  2 ++
> > >   migration/ram.c          | 44 
> > > ++++++++++++++++++++++++++++++++++++++++++++
> > >   2 files changed, 46 insertions(+)
> > > 
> > > diff --git a/include/migration/misc.h b/include/migration/misc.h
> > > index 4ebf24c..113320e 100644
> > > --- a/include/migration/misc.h
> > > +++ b/include/migration/misc.h
> > > @@ -14,11 +14,13 @@
> > >   #ifndef MIGRATION_MISC_H
> > >   #define MIGRATION_MISC_H
> > > +#include "exec/cpu-common.h"
> > >   #include "qemu/notify.h"
> > >   /* migration/ram.c */
> > >   void ram_mig_init(void);
> > > +void qemu_guest_free_page_hint(void *addr, size_t len);
> > >   /* migration/block.c */
> > > diff --git a/migration/ram.c b/migration/ram.c
> > > index 9a72b1a..0147548 100644
> > > --- a/migration/ram.c
> > > +++ b/migration/ram.c
> > > @@ -2198,6 +2198,50 @@ static int ram_init_all(RAMState **rsp)
> > >   }
> > >   /*
> > > + * This function clears bits of the free pages reported by the caller 
> > > from the
> > > + * migration dirty bitmap. @addr is the host address corresponding to the
> > > + * start of the continuous guest free pages, and @len is the total bytes 
> > > of
> > > + * those pages.
> > > + */
> > > +void qemu_guest_free_page_hint(void *addr, size_t len)
> > > +{
> > > +    RAMBlock *block;
> > > +    ram_addr_t offset;
> > > +    size_t used_len, start, npages;
> > Do we need to check here on whether a migration is in progress?  Since
> > if not I'm not sure whether this hint still makes any sense any more,
> > and more importantly it seems to me that block->bmap below at [1] is
> > only valid during a migration.  So I'm not sure whether QEMU will
> > crash if this function is called without a running migration.
> 
> OK. How about just adding comments above to have users noted that this
> function should be used during migration?
> 
> If we want to do a sanity check here, I think it would be easier to just
> check !block->bmap here.

I think the faster way might be that we check against the migration
state.

> 
> 
> > 
> > > +
> > > +    for (; len > 0; len -= used_len) {
> > > +        block = qemu_ram_block_from_host(addr, false, &offset);
> > > +        if (unlikely(!block)) {
> > > +            return;
> > We should never reach here, should we?  Assuming the callers of this
> > function should always pass in a correct host address. If we are very
> > sure that the host addr should be valid, could we just assert?
> 
> Probably not the case, because of the corner case that the memory would be
> hot unplugged after the free page is reported to QEMU.

Question: Do we allow to do hot plug/unplug for memory during
migration?

> 
> 
> 
> > 
> > > +        }
> > > +
> > > +        /*
> > > +         * This handles the case that the RAMBlock is resized after the 
> > > free
> > > +         * page hint is reported.
> > > +         */
> > > +        if (unlikely(offset > block->used_length)) {
> > > +            return;
> > > +        }
> > > +
> > > +        if (len <= block->used_length - offset) {
> > > +            used_len = len;
> > > +        } else {
> > > +            used_len = block->used_length - offset;
> > > +            addr += used_len;
> > > +        }
> > > +
> > > +        start = offset >> TARGET_PAGE_BITS;
> > > +        npages = used_len >> TARGET_PAGE_BITS;
> > > +
> > > +        qemu_mutex_lock(&ram_state->bitmap_mutex);
> > So now I think I understand the lock can still be meaningful since
> > this function now can be called outside the migration thread (e.g., in
> > vcpu thread).  But still it would be nice to mention it somewhere on

(Actually after read the next patch I think it's in iothread, so I'd
 better reply with all the series read over next time :)

> > the truth of the lock.
> > 
> 
> Yes. Thanks for the reminder. I will add some explanation to the patch 2
> commit log.

Thanks,

-- 
Peter Xu