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

[Alexander Potapenko <glider@google.com>]

On Tue, Jan 31, 2023 at 4:14 PM Michal Hocko <mhocko@suse.com> wrote:
>
> On Mon 30-01-23 18:59:45, Alexander Potapenko wrote:
> > On Mon, Jan 30, 2023 at 2:38 PM Michal Hocko <mhocko@suse.com> 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.
> >
> > Thanks for the comment!
> > I was considering page_ext at some point, but managed to convince
> > myself it didn't suit the purpose well enough.
> >
> > Right now KMSAN allocates its metadata at boot time, when tearing down memblock.
> > At that point only a handful of memory ranges exist, and it is pretty
> > easy to carve out some unused pages for the metadata for those ranges,
> > then divide the rest evenly and return 1/3 to the system, spending 2/3
> > to keep the metadata for the returned pages.
> > I tried allocating the memory lazily (at page_alloc(), for example),
> > and it turned out to be very tricky because of fragmentation: for an
> > allocation of a given order, one needs shadow and origin allocations
> > of the same order [1], and alloc_pages() simply started with ripping
> > apart the biggest chunk of memory available.
>
> page_ext allocation happens quite early as well. There shouldn't be any
> real fragmentation that early during the boot.
>
> > IIRC if we choose to allocate metadata via page_ext, the memory will
> > be already too fragmented to easily handle it, because it will only
> > happen once alloc_pages() is available.
> > We also can't get rid of the shadow/origin pointers in struct page_ext
> > (storing two 4K-sized arrays in that struct would defeat all the
> > possible alignments), so we won't save any memory by switching to
> > page_ext.
>
> With page_ext you would allow to compile the feature in disabled by
> default and allow to boot time enable it.

This makes little sense to do, because KMSAN requires heavy
compile-time instrumentation to work. One cannot simply enable/disable
it at boot time anyway.