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

[Mike Rapoport <rppt@kernel.org>]

On Tue, Nov 28, 2023 at 01:51:32PM +0300, Serge Semin wrote:
> On Tue, Nov 28, 2023 at 09:13:39AM +0200, Mike Rapoport wrote:
> > On Fri, Nov 24, 2023 at 02:18:44PM +0300, Serge Semin wrote:
> 
> > Do you mind posting your physical memory layout?
> 
> I actually already did in response to the last part of your previous
> message. You must have missed it. Here is the copy of the message:
 
Sorry, for some reason I didn't scroll down your previous mail :)

> > 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:
> > > ...
> > > > 
> > > > 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.
> 
> Could you also answer to my question above regarding using the
> sparsemem instead on my hw memory layout?
 
Currently MIPS defines section size to 256MB, so with your memory layout
with SPARSMEM there will be two sections of 256MB, at 0 and at 512MB, so
you'll save memory map for 256M which is roughly 1M with 16k pages.

It's possible 

With SPARSEMEM the pfn_to_page() and page_to_pfn() are a bit longer in
terms of assembly instructions, but I really doubt you'll notice any
performance difference in real world applications.

> > 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.
> 
> Right. That's what currently happens on MIPS32 and that's what I had
> to fix in the framework of this series by the next patch:
> Link: https://lore.kernel.org/linux-mips/20231122182419.30633-4-fancer.lancer@gmail.com/
> flatmem version of the pfn_valid() method has been broken due to
> max_mapnr being uninitialized before mem_init() is called. So
> init_unavailable_range() didn't initialize the pages on the early
> bootup stage. Thus afterwards, when max_mapnr has finally got a valid
> value any attempts to call the __update_cache() method on the MMIO
> memory hole caused the unaligned access crash.

The fix for max_mapnr makes pfn_valid()==1 for the entire memory map and
this fixes up the struct pages in the hole.
 
> > 
> > With that, the init_unavailable_range() docs need not mention IO space at
> > all, they should mention holes within FLATMEM memory map.
> 
> Ok. I'll resend the patch with mentioning flatmem holes instead of
> mentioning the IO-spaces.
> 
> > 
> > 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.
> 
> Ah, I see. In case of the SPARSEMEM config an another version of
> pfn_valid() will be called. It's defined in the include/linux/mmzone.h
> header file. Right? If so then no problem there indeed.
 
Yes, SPARSMEM uses pfn_valid() defined in include/linux/mmzone.h

> -Serge(y)

-- 
Sincerely yours,
Mike.