Re: [PATCH v2] mm/memblock: Correct totalram_pages accounting with KMSAN

[Mike Rapoport <rppt@kernel.org>]

On Thu, Sep 25, 2025 at 05:37:59AM -0700, SeongJae Park wrote:
> Hello,
> 
> On Wed, 24 Sep 2025 12:03:01 +0200 Alexander Potapenko <glider@google.com> wrote:
> 
> > When KMSAN is enabled, `kmsan_memblock_free_pages()` can hold back pages
> > for metadata instead of returning them to the early allocator. The callers,
> > however, would unconditionally increment `totalram_pages`, assuming the
> > pages were always freed. This resulted in an incorrect calculation of the
> > total available RAM, causing the kernel to believe it had more memory than
> > it actually did.
> > 
> > This patch refactors `memblock_free_pages()` to return the number of pages
> > it successfully frees. If KMSAN stashes the pages, the function now
> > returns 0; otherwise, it returns the number of pages in the block.
> > 
> > The callers in `memblock.c` have been updated to use this return value,
> > ensuring that `totalram_pages` is incremented only by the number of pages
> > actually returned to the allocator. This corrects the total RAM accounting
> > when KMSAN is active.
> > 
> > Cc: Aleksandr Nogikh <nogikh@google.com>
> > Fixes: 3c2065098260 ("init: kmsan: call KMSAN initialization routines")
> > Signed-off-by: Alexander Potapenko <glider@google.com>
> > Reviewed-by: David Hildenbrand <david@redhat.com>
> [...]
> > --- a/mm/mm_init.c
> > +++ b/mm/mm_init.c
> > @@ -2548,24 +2548,25 @@ void *__init alloc_large_system_hash(const char *tablename,
> >  	return table;
> >  }
> >  
> > -void __init memblock_free_pages(struct page *page, unsigned long pfn,
> > -							unsigned int order)
> > +unsigned long __init memblock_free_pages(struct page *page, unsigned long pfn,
> > +					 unsigned int order)
> >  {
> >  	if (IS_ENABLED(CONFIG_DEFERRED_STRUCT_PAGE_INIT)) {
> >  		int nid = early_pfn_to_nid(pfn);
> >  
> >  		if (!early_page_initialised(pfn, nid))
> > -			return;
> > +			return 0;
> >  	}
> 
> I found this patch on mm-new tree is making my test machine (QEMU) reports much
> less MemTotal even though KMSAN is disabled.  And modifying the above part to
> be considered as free success (returning '1UL << order') fixed my issue.
> Because the commit message says the purpose of this change is only for
> KMSAN-stashed memory, maybe the above behavior change is not really intended?
> 
> I'm not familiar with this code so I'm unsure if the workaround is the right
> fix.  But since I have no time to look this in deep for now, reporting first

With DEFERRED_STRUCT_PAGE_INIT we count totalram_pages in
memblock_free_all() but actually free them in deferred_init_memmap() and
deferred_grow_zone().

So returning '1UL << order' is a correct workaround, but the proper fix
should update totalram_pages in the deferred path IMHO.

-- 
Sincerely yours,
Mike.