Re: [Qemu-devel] [PATCH V6 07/10] migration: add bitmap for copied page

[Peter Xu]

On Thu, May 25, 2017 at 09:28:57AM +0300, Alexey Perevalov wrote:
> On 05/25/2017 02:30 AM, Peter Xu wrote:
> >On Wed, May 24, 2017 at 03:16:23PM +0300, Alexey Perevalov wrote:
> >>On 05/24/2017 03:01 PM, Peter Xu wrote:
> >>>On Wed, May 24, 2017 at 10:56:37AM +0300, Alexey wrote:
> >>>>On Wed, May 24, 2017 at 02:57:36PM +0800, Peter Xu wrote:
> >>>>>On Tue, May 23, 2017 at 02:31:08PM +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 and maybe for restore after
> >>>>>>postcopy migration failure.
> >>>>>>
> >>>>>>Functions which work with RAMBlock are placed into ram.c,
> >>>>>>due to TARGET_WORDS_BIGENDIAN is poisoned int postcopy-ram.c -
> >>>>>>hardware independed code.
> >>>>>>
> >>>>>>Signed-off-by: Alexey Perevalov <address@hidden>
> >>>>>>---
> >>>>>>  include/migration/migration.h | 16 +++++++++++
> >>>>>>  migration/postcopy-ram.c      | 22 ++++++++++++---
> >>>>>>  migration/ram.c               | 63 
> >>>>>> +++++++++++++++++++++++++++++++++++++++++++
> >>>>>>  3 files changed, 97 insertions(+), 4 deletions(-)
> >>>>>>
> >>>>>>diff --git a/include/migration/migration.h 
> >>>>>>b/include/migration/migration.h
> >>>>>>index 449cb07..4e05c83 100644
> >>>>>>--- a/include/migration/migration.h
> >>>>>>+++ b/include/migration/migration.h
> >>>>>>@@ -101,6 +101,20 @@ struct MigrationIncomingState {
> >>>>>>      LoadStateEntry_Head loadvm_handlers;
> >>>>>>      /*
> >>>>>>+     * bitmap indicates whether page copied,
> >>>>>>+     * based on ramblock offset
> >>>>>>+     * now it is using only for blocktime calculation in
> >>>>>>+     * postcopy migration, so livetime of this entry:
> >>>>>>+     * since user requested blocktime calculation,
> >>>>>>+     * till the end of postcopy migration
> >>>>>>+     * as an example it could represend following memory map
> >>>>>>+     * ___________________________________
> >>>>>>+     * |4k pages | hugepages | 4k pages
> >>>>>Can we really do this?
> >>>>>The problem is AFAIU migration stream is sending pages only in target
> >>>>>page size, even for huge pages... so even for huge pages we should
> >>>>>still need per TARGET_PAGE_SIZE bitmap?
> >>>>sending maybe, but copying to userfault fd is doing in hugepage size
> >>>Yes you are right. :)
> >>>
> >>>>in case of hugetlbfs memory backend.
> >>>>>(Please refer to ram_state_init() on init of RAMBlock.bmap)
> >>>>I thought to use bitmap per ramblock, but I decided to not do it,
> >>>>because of following reasons:
> >>>>  1. There is only 4k ramblocks, for such ramblock it's not
> >>>>  optimal
> >>>Could you explain what do you mean by "there is only 4k ramblocks"?
> >>sorry, could be ramblocks with 4k size,
> >>as example, your's qemu hmp info ramblock shows
> >>  Block Name    PSize              Offset Used              Total
> >>             /objects/mem    2 MiB  0x0000000000000000 0x0000000020000000
> >>0x0000000020000000
> >>                 vga.vram    4 KiB  0x0000000020060000 0x0000000001000000
> >>0x0000000001000000
> >>     /address@hidden/acpi/tables    4 KiB  0x0000000021130000 
> >> 0x0000000000020000
> >>0x0000000000200000
> >>                  pc.bios    4 KiB  0x0000000020000000 0x0000000000040000
> >>0x0000000000040000
> >>   0000:00:03.0/e1000.rom    4 KiB  0x0000000021070000 0x0000000000040000
> >>0x0000000000040000
> >>   0000:00:04.0/e1000.rom    4 KiB  0x00000000210b0000 0x0000000000040000
> >>0x0000000000040000
> >>   0000:00:05.0/e1000.rom    4 KiB  0x00000000210f0000 0x0000000000040000
> >>0x0000000000040000
> >>                   pc.rom    4 KiB  0x0000000020040000 0x0000000000020000
> >>0x0000000000020000
> >>     0000:00:02.0/vga.rom    4 KiB  0x0000000021060000 0x0000000000010000
> >>0x0000000000010000
> >>    /address@hidden/table-loader    4 KiB  0x0000000021330000 
> >> 0x0000000000001000
> >>0x0000000000001000
> >>       /address@hidden/acpi/rsdp    4 KiB  0x0000000021331000 
> >> 0x0000000000001000
> >>0x0000000000001000
> >>
> >>/address@hidden/table-loader and /address@hidden/acpi/rsdp has only one 4K 
> >>page.
> >I see. Thanks for explaining this.
> >
> >A 4k sized ramblock means the bitmap would be only one unsigned long
> >width (via bitmap_new(1)). I still don't see why it's not good... :-)
> >
> Ok, I'll take into account your comment, and I'll place copied
> 
> bitmap into RAMBlock.

Or we can wait for further comments in case you'll do extra work. :-)

Just to mention that Dave won't be here until next week, so you may
need to wait for some days until he can provide some further input.

Thanks,

-- 
Peter Xu