Re: [PATCH] mm: extend max struct page size for kmsan

[Matthew Wilcox <willy@infradead.org>]

On Mon, Jan 30, 2023 at 02:38:22PM +0100, Michal Hocko wrote:
> On Mon 30-01-23 14:07:26, Arnd Bergmann wrote:
> > From: Arnd Bergmann <arnd@arndb.de>
> > 
> > After x86 has enabled support for KMSAN, it has become possible
> > to have larger 'struct page' than was expected when commit
> > 5470dea49f53 ("mm: use mm_zero_struct_page from SPARC on all 64b
> > architectures") was merged:
> > 
> > include/linux/mm.h:156:10: warning: no case matching constant switch condition '96'
> >         switch (sizeof(struct page)) {
> > 
> > Extend the maximum accordingly.
> > 
> > Fixes: 5470dea49f53 ("mm: use mm_zero_struct_page from SPARC on all 64b architectures")
> > Fixes: 4ca8cc8d1bbe ("x86: kmsan: enable KMSAN builds for x86")
> > Signed-off-by: Arnd Bergmann <arnd@arndb.de>
> 
> Acked-by: Michal Hocko <mhocko@suse.com>
> 
> I haven't really followed KMSAN development but I would have expected
> that it would, like other debugging tools, add its metadata to page_ext
> rather than page directly.

Yes, that would have been preferable.  Also, I don't understand why we
need an entire page to store whether each "bit" of a page is initialised.
There are no CPUs which have bit-granularity stores; either you initialise
an entire byte or not.  So that metadata can shrink from 4096 bytes
to 512.