Re: [PATCH 1/4] arch: consolidate existing CONFIG_PAGE_SIZE_*KB definitions

Christophe Leroy
2024-02-27T15:44:58Z

Le 27/02/2024 à 16:40, Arnd Bergmann a écrit :
> On Mon, Feb 26, 2024, at 17:55, Samuel Holland wrote:
>> On 2024-02-26 10:14 AM, Arnd Bergmann wrote:
>>>
>>> +config HAVE_PAGE_SIZE_4KB
>>> + bool
>>> +
>>> +config HAVE_PAGE_SIZE_8KB
>>> + bool
>>> +
>>> +config HAVE_PAGE_SIZE_16KB
>>> + bool
>>> +
>>> +config HAVE_PAGE_SIZE_32KB
>>> + bool
>>> +
>>> +config HAVE_PAGE_SIZE_64KB
>>> + bool
>>> +
>>> +config HAVE_PAGE_SIZE_256KB
>>> + bool
>>> +
>>> +choice
>>> + prompt "MMU page size"
>>
>> Should this have some generic help text (at least a warning about
>> compatibility)?
>
> Good point. I've added some of this now, based on the mips
> text with some generalizations for other architectures:
>
> config PAGE_SIZE_4KB
> bool "4KiB pages"
> depends on HAVE_PAGE_SIZE_4KB
> help
> This option select the standard 4KiB Linux page size and the only
> available option on many architectures. Using 4KiB page size will
> minimize memory consumption and is therefore recommended for low
> memory systems.
> Some software that is written for x86 systems makes incorrect
> assumptions about the page size and only runs on 4KiB pages.
>
> config PAGE_SIZE_8KB
> bool "8KiB pages"
> depends on HAVE_PAGE_SIZE_8KB
> help
> This option is the only supported page size on a few older
> processors, and can be slightly faster than 4KiB pages.
>
> config PAGE_SIZE_16KB
> bool "16KiB pages"
> depends on HAVE_PAGE_SIZE_16KB
> help
> This option is usually a good compromise between memory
> consumption and performance for typical desktop and server
> workloads, often saving a level of page table lookups compared
> to 4KB pages as well as reducing TLB pressure and overhead of
> per-page operations in the kernel at the expense of a larger
> page cache.
>
> config PAGE_SIZE_32KB
> bool "32KiB pages"
> depends on HAVE_PAGE_SIZE_32KB
> Using 32KiB page size will result in slightly higher performance
> kernel at the price of higher memory consumption compared to
> 16KiB pages. This option is available only on cnMIPS cores.
> Note that you will need a suitable Linux distribution to
> support this.
>
> config PAGE_SIZE_64KB
> bool "64KiB pages"
> depends on HAVE_PAGE_SIZE_64KB
> Using 64KiB page size will result in slightly higher performance
> kernel at the price of much higher memory consumption compared to
> 4KiB or 16KiB pages.
> This is not suitable for general-purpose workloads but the
> better performance may be worth the cost for certain types of
> supercomputing or database applications that work mostly with
> large in-memory data rather than small files.
>
> config PAGE_SIZE_256KB
> bool "256KiB pages"
> depends on HAVE_PAGE_SIZE_256KB
> help
> 256KB pages have little practical value due to their extreme
> memory usage.

For 256K pages, powerpc has the following help. I think you should have
it too:

The kernel will only be able to run applications that have been
compiled with '-zmax-page-size' set to 256K (the default is 64K) using
binutils later than 2.17.50.0.3, or by patching the ELF_MAXPAGESIZE
definition from 0x10000 to 0x40000 in older versions.