Re: [PATCH 5/7] mm/mm_init.c: Extend init unavailable range doc info

[Mike Rapoport <rppt@kernel.org>]

On Fri, Nov 24, 2023 at 02:18:44PM +0300, Serge Semin wrote:
> On Fri, Nov 24, 2023 at 10:19:00AM +0200, Mike Rapoport wrote:
> > On Thu, Nov 23, 2023 at 01:42:39PM +0300, Serge Semin wrote:
> > > On Thu, Nov 23, 2023 at 12:18:54PM +0200, Mike Rapoport wrote:
> > > > On Wed, Nov 22, 2023 at 09:24:03PM +0300, Serge Semin wrote:
> > > > > Besides of the already described reasons the pages backended memory holes
> > > > > might be persistent due to having memory mapped IO spaces behind those
> > > > > ranges in the framework of flatmem kernel config. Add such note to the
> > > > > init_unavailable_range() method kdoc in order to point out to one more
> > > > > reason of having the function executed for such regions.
> > > > > 
> > > > > Signed-off-by: Serge Semin <fancer.lancer@gmail.com>
> > > > > 
> > > > > ---
> > > > > 
> > > > > Please let me know if the IO-space pages must be initialized somehow
> > > > > differently rather relying on free_area_init() executing the
> > > > > init_unavailable_range() method.
> > > > 
> > > 
> > > > Maybe I'm missing something, but why do you need struct pages in the
> > > > IO space?
> > > 
> > > In my case at the very least that's due to having a SRAM device
> > > available in the middle of the MMIO-space. The region is getting
> > > mapped using the ioremap_wc() method (Uncached Write-Combine CA),
> > > which eventually is converted to calling get_vm_area() and
> > > ioremap_page_range() (see ioremap_prot() function on MIPS), which in
> > > its turn use the page structs for mapping. Another similar case is
> > > using ioremap_wc() in the PCIe outbound ATU space mapping of
> > > the graphic/video cards framebuffers.
> > 
> > ioremap_page_range() does not need struct pages, but rather physical
> > addresses.
> 
> Unless I miss something or MIPS32 is somehow special/wrong in that
> matter, but from my just got experience it actually does at least in
> the framework of the __update_cache() implementation which is called
> in the set_ptes() method (former set_pte_at()), which in its turn
> is eventually invoked by vmap_range_noflush() and finally by
> ioremap_page_range(). See the patch
> [PATCH 3/7] mips: Fix max_mapnr being uninitialized on early stages
> Link: https://lore.kernel.org/linux-mips/20231122182419.30633-4-fancer.lancer@gmail.com/
> of this series and the stack-trace of the bug fixed by that patch.
> 
> Is it wrong that on MIPS32 ioremap_page_range() eventually relies on
> the page structs? It has been like that for, I don't know, long time.
> If so then the sparse memory config might be broken on MIPS32..(
 
Do you mind posting your physical memory layout?

If I understand correctly, you have a hole in your RAM and there is MMIO
region somewhere in that hole. With FLATMEM the memory map exists for that
hole and hence pfn_valid() returns 1 for the MMIO range as well. That makes
__update_cache() to check folio state and that check would fail if the memory
map contained garbage. But since the hole in the memory map is initialized
with init_unavailable_range() you get a valid struct page/struct folio and
everything is fine.

With that, the init_unavailable_range() docs need not mention IO space at
all, they should mention holes within FLATMEM memory map.

As for SPARSEMEM, if the hole does not belong to any section, pfn_valid()
will be false for it and __update_cache() won't try to access memory map.
  
> > > In general having the pages array defined for the IO-memory is
> > > required for mapping the IO-space other than just uncached (my sram
> > > case for example) or, for instance, with special access attribute for
> > > the user-space (if I am not missing something in a way VM works in
> > > that case).
> > 
> 
> > No, struct pages are not required to map IO space. If you need to map MMIO
> > to userspace there's remap_pfn_range() for that.
> 
> Is this correct for both flat and sparse memory config? In anyway
> please see my comment above about the problem I recently got.
> 
> > 
> > My guess is that your system has a hole in the physical memory mappings and
> > with FLATMEM that hole will have essentially unused struct pages, which are
> > initialized by init_unavailable_range().  But from mm perspective this is
> > still a hole even though there's some MMIO ranges in that hole.
> 
> Absolutely right. Here is the physical memory layout in my system.
> 0     - 128MB: RAM
> 128MB - 512MB: Memory mapped IO
> 512MB - 768MB..8.256GB: RAM
> 
> > 
> > Now, if that hole is large you are wasting memory for unused memory map and
> > it maybe worth considering using SPARSEMEM.
> 
> Do you think it's worth to move to the sparse memory configuration in
> order to save the 384MB of mapping with the 16K page model? AFAIU flat
> memory config is more performant. Performance is critical on the most
> of the SoC applications especially when using the 10G ethernet or
> the high-speed PCIe devices.
> 
> -Serge(y)
> 
> >  
> > > -Serge(y)
> > > 
> > > > 
> > > > > ---
> > > > >  mm/mm_init.c | 1 +
> > > > >  1 file changed, 1 insertion(+)
> > > > > 
> > > > > diff --git a/mm/mm_init.c b/mm/mm_init.c
> > > > > index 077bfe393b5e..3fa33e2d32ba 100644
> > > > > --- a/mm/mm_init.c
> > > > > +++ b/mm/mm_init.c
> > > > > @@ -796,6 +796,7 @@ overlap_memmap_init(unsigned long zone, unsigned long *pfn)
> > > > >   * - physical memory bank size is not necessarily the exact multiple of the
> > > > >   *   arbitrary section size
> > > > >   * - early reserved memory may not be listed in memblock.memory
> > > > > + * - memory mapped IO space
> > > > >   * - memory layouts defined with memmap= kernel parameter may not align
> > > > >   *   nicely with memmap sections
> > > > >   *
> > > > > -- 
> > > > > 2.42.1
> > > > > 
> > > > 
> > > > -- 
> > > > Sincerely yours,
> > > > Mike.
> > > > 
> > 
> > -- 
> > Sincerely yours,
> > Mike.

-- 
Sincerely yours,
Mike.