Re: [RFC PATCH 0/3] Prototype for direct map awareness in page allocator

[Hyeonggon Yoo <42.hyeyoo@gmail.com>]

On Thu, Jan 27, 2022 at 10:56:05AM +0200, Mike Rapoport wrote:
> From: Mike Rapoport <rppt@linux.ibm.com>
> 
> Hi,
> 
> This is a second attempt to make page allocator aware of the direct map
> layout and allow grouping of the pages that must be mapped at PTE level in
> the direct map.
>

Hello mike, It may be a silly question...

Looking at implementation of set_memory*(), they only split
PMD/PUD-sized entries. But why not _merge_ them when all entries
have same permissions after changing permission of an entry?

I think grouping __GFP_UNMAPPED allocations would help reducing
direct map fragmentation, but IMHO merging split entries seems better
to be done in those helpers than in page allocator.

For example:
	1) set_memory_ro() splits 1 RW PMD entry into 511 RW PTE
	entries and 1 RO PTE entry.

	2) before freeing the pages, we call set_memory_rw() and we have
	512 RW PTE entries. Then we can merge it to 1 RW PMD entry.

	3) after 2) we can do same thing about PMD-sized entries
	and merge them into 1 PUD entry if 512 PMD entries have
	same permissions.

[...]

> Mike Rapoport (3):
>   mm/page_alloc: introduce __GFP_UNMAPPED and MIGRATE_UNMAPPED
>   mm/secretmem: use __GFP_UNMAPPED to allocate pages
>   EXPERIMENTAL: x86/module: use __GFP_UNMAPPED in module_alloc
> 
>  arch/Kconfig                   |   7 ++
>  arch/x86/Kconfig               |   1 +
>  arch/x86/kernel/module.c       |   2 +-
>  include/linux/gfp.h            |  13 +++-
>  include/linux/mmzone.h         |  11 +++
>  include/trace/events/mmflags.h |   3 +-
>  mm/internal.h                  |   2 +-
>  mm/page_alloc.c                | 129 ++++++++++++++++++++++++++++++++-
>  mm/secretmem.c                 |   8 +-
>  9 files changed, 162 insertions(+), 14 deletions(-)
> 
> 
> base-commit: e783362eb54cd99b2cac8b3a9aeac942e6f6ac07
> -- 
> 2.34.1
> 

-- 
Thanks,
Hyeonggon