Re: [Qemu-devel] [PATCH v1 2/2] migration: add bitmap for copied page

[Peter Xu <peterx@redhat.com>]

On Wed, Jun 14, 2017 at 09:39:53AM +0300, Alexey Perevalov wrote:
> On 06/14/2017 08:12 AM, Peter Xu wrote:
> >On Tue, Jun 13, 2017 at 12:36:33PM +0300, Alexey Perevalov wrote:
> >>This patch adds ability to track down already copied
> >>pages, it's necessary for calculation vCPU block time in
> >>postcopy migration feature, maybe for restore after
> >>postcopy migration failure.
> >>Also it's necessary to solve shared memory issue in
> >>postcopy livemigration. Information about copied pages
> >>will be transferred to the software virtual bridge
> >>(e.g. OVS-VSWITCHD), to avoid fallocate (unmap) for
> >>already copied pages. fallocate syscall is required for
> >>remmaped shared memory, due to remmaping itself blocks
> >>ioctl(UFFDIO_COPY, ioctl in this case will end with EEXIT
> >>error (struct page is exists after remmap).
> >>
> >>Bitmap is placed into RAMBlock as another postcopy/precopy
> >>related bitmaps.
> >>
> >>copied bitmap is not releasing due to ramblocks is not releasing
> >>too.
> >>
> >>Signed-off-by: Alexey Perevalov <a.perevalov@samsung.com>
> >>---
> >>  include/exec/ram_addr.h       |  3 +++
> >>  include/migration/migration.h |  3 +++
> >>  migration/postcopy-ram.c      |  7 ++++++
> >>  migration/ram.c               | 54 ++++++++++++++++++++++++++++++++++++++++++-
> >>  migration/ram.h               |  5 ++++
> >>  migration/savevm.c            |  1 +
> >>  6 files changed, 72 insertions(+), 1 deletion(-)
> >>
> >>diff --git a/include/exec/ram_addr.h b/include/exec/ram_addr.h
> >>index 140efa8..56cdf16 100644
> >>--- a/include/exec/ram_addr.h
> >>+++ b/include/exec/ram_addr.h
> >>@@ -47,6 +47,9 @@ struct RAMBlock {
> >>       * of the postcopy phase
> >>       */
> >>      unsigned long *unsentmap;
> >>+    /* bitmap of already copied pages in postcopy */
> >>+    unsigned long *copiedmap;
> >>+    size_t nr_copiedmap;
> >Do we really need this?
> This and question about
> 
> ram_discard_range Juan already asked.
> I just repeat nr_copiedmap - premature optimization )
> ram_discard_range - I forgot to clean up patch.
> 
> >
> >>  };
> >>  static inline bool offset_in_ramblock(RAMBlock *b, ram_addr_t offset)
> >>diff --git a/include/migration/migration.h b/include/migration/migration.h
> >>index 79b5484..8005c11 100644
> >>--- a/include/migration/migration.h
> >>+++ b/include/migration/migration.h
> >>@@ -226,4 +226,7 @@ void savevm_skip_configuration(void);
> >>  int global_state_store(void);
> >>  void global_state_store_running(void);
> >>+size_t get_copiedmap_size(RAMBlock *rb);
> >>+void movecopiedmap(void *dst, RAMBlock *rb, size_t len);
> >>+
> >>  #endif
> >>diff --git a/migration/postcopy-ram.c b/migration/postcopy-ram.c
> >>index f6244ee..e13b22e 100644
> >>--- a/migration/postcopy-ram.c
> >>+++ b/migration/postcopy-ram.c
> >>@@ -576,6 +576,13 @@ int postcopy_place_page(MigrationIncomingState *mis, void *host, void *from,
> >>      copy_struct.len = pagesize;
> >>      copy_struct.mode = 0;
> >>+
> >>+    /* copied page isn't feature of blocktime calculation,
> >>+     * it's more general entity, so keep it here,
> >>+     * but gup betwean two following operation could be high,
> >>+     * and in this case blocktime for such small interval will be lost */
> >>+    set_copiedmap_by_addr(host, rb);
> >>+
> >I guess this is not enough?
> yes, zero pages were missed
> >
> >For postcopy, you may have missed to trap in
> >postcopy_place_page_zero() when pagesize == getpagesize() (we used
> >UFFDIO_ZEROPAGE there)?
> >
> >(Btw, why not we trap all these in ram_load_postcopy()?)
> I tried to place
> 
> set_copiedmap_by_addr as closer as possible to ioctl. It could be important to reduce probability of
> race, also when I inform OVS-VSWITCH (patches not yet published), it's important to reduce time between
> mark page as copied and really coping.

If so, I would suggest we comment this in the codes.

Btw, could you elaborate a bit in detail on why you need to "reduce
the time" here? IMHO the ordering should matter and I can understand
(and actually we can also achieve the ordering requirement if we put
this handling in ram_load_postcopy()), but I cannot really understand
when you say "it's important to reduce time between A and B"...

Thanks,

-- 
Peter Xu