Re: [PATCH v2] KVM: selftests: Fix dirty_log_page_splitting_test as page migration

[Tao Su <tao1.su@linux.intel.com>]

On Fri, Jan 26, 2024 at 12:39:49PM -0800, Sean Christopherson wrote:
> +David
> 

[ ... ]

> >  
> >  	/*
> > @@ -192,7 +193,6 @@ static void run_test(enum vm_guest_mode mode, void *unused)
> >  	 * memory again, the page counts should be the same as they were
> >  	 * right after initial population of memory.
> >  	 */
> > -	TEST_ASSERT_EQ(stats_populated.pages_4k, stats_repopulated.pages_4k);
> >  	TEST_ASSERT_EQ(stats_populated.pages_2m, stats_repopulated.pages_2m);
> >  	TEST_ASSERT_EQ(stats_populated.pages_1g, stats_repopulated.pages_1g);
> 
> Isn't it possible that something other than guest data could be mapped by THP
> hugepage, and that that hugepage could get shattered between the initial run and
> the re-population run?

Good catch, I found that if the backing source is specified as THP, all hugepages
can also be migrated.

> 
> The test knows (or at least, darn well should know) exactly how much memory is
> being dirty logged.  Rather that rely *only* on before/after heuristics, can't
> we assert that the _delta_, i.e. the number of hugepages that are split, and then
> the number of hugepages that are reconstituted, is greater than or equal to the
> size of the memslots being dirty logged?

Due to page migration, the values of get_page_stats() are not available (including
pages_2m and pages_1g), and dirty logging can only count the pages that have been
split. It may be possible to use the existing guest_num_pages to construct the
following assert condition:

        guest_num_pages <= the number of dirty pages

Do you think this assert condition is correct and enough?

Thanks,
Tao

> 
> >  }
> > 
> > base-commit: 6613476e225e090cc9aad49be7fa504e290dd33d
> > -- 
> > 2.34.1
> >