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migration: Pre-fault memory before starting background snasphot

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This commit solves the issue with userfault_fd WP feature that
background snapshot is based on. For any never poluated or discarded
memory page, the UFFDIO_WRITEPROTECT ioctl() would skip updating
PTE for that page, thereby loosing WP setting for it.

So we need to pre-fault pages for each RAM block to be protected
before making a userfault_fd wr-protect ioctl().

Fixes: 278e2f551a (migration: support
  UFFD write fault processing in ram_save_iterate())
Signed-off-by: Andrey Gruzdev <andrey.gruzdev@virtuozzo.com>
Reported-by: David Hildenbrand <david@redhat.com>
Reviewed-by: David Hildenbrand <david@redhat.com>
Message-Id: <20210401092226.102804-4-andrey.gruzdev@virtuozzo.com>
Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
dgilbert:
  Bodged ifdef __linux__ on ram_write_tracking_prepare, should really
      go in a stub
This commit is contained in:
Andrey Gruzdev 2021-04-01 12:22:25 +03:00 • committed by Dr. David Alan Gilbert
parent 1a8e44a89f
commit eeccb99c9d
3 changed files with 58 additions and 0 deletions
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49
migration/ram.c
View file

@ -1560,6 +1560,55 @@ out:
return ret;
}
/*
* ram_block_populate_pages: populate memory in the RAM block by reading
* an integer from the beginning of each page.
*
* Since it's solely used for userfault_fd WP feature, here we just
* hardcode page size to qemu_real_host_page_size.
*
* @bs: RAM block to populate
*/
static void ram_block_populate_pages(RAMBlock *bs)
{
char *ptr = (char *) bs->host;
for (ram_addr_t offset = 0; offset < bs->used_length;
offset += qemu_real_host_page_size) {
char tmp = *(ptr + offset);
/* Don't optimize the read out */
asm volatile("" : "+r" (tmp));
}
}
/*
* ram_write_tracking_prepare: prepare for UFFD-WP memory tracking
*/
void ram_write_tracking_prepare(void)
{
RAMBlock *bs;
RCU_READ_LOCK_GUARD();
RAMBLOCK_FOREACH_NOT_IGNORED(bs) {
/* Nothing to do with read-only and MMIO-writable regions */
if (bs->mr->readonly || bs->mr->rom_device) {
continue;
}
/*
* Populate pages of the RAM block before enabling userfault_fd
* write protection.
*
* This stage is required since ioctl(UFFDIO_WRITEPROTECT) with
* UFFDIO_WRITEPROTECT_MODE_WP mode setting would silently skip
* pages with pte_none() entries in page table.
*/
ram_block_populate_pages(bs);
}
}
/*
* ram_write_tracking_start: start UFFD-WP memory tracking
*