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

[Serge Semin]

On Wed, Nov 29, 2023 at 08:14:00AM +0200, Mike Rapoport wrote:
> 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.

Ok. Thank you very much for the comprehensive response. I'll give a
good thought towards moving our platform to the sparse memory config. Most
likely it will be done together with reducing SECTION_SIZE_BITS to
128MB in order to save a few more low-memory space. This will be
mostly useful it XPA is enabled and 8GB memory is available. Such case
requires a lot of low-memory for mapping, which is of just 128MB in
our device.

-Serge(y)

> 
> > > 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@xxxxxxxxx/
> > 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.