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

[Mike Rapoport <rppt@kernel.org>]

Hello Hyeonggon,

On Tue, Apr 26, 2022 at 05:54:49PM +0900, Hyeonggon Yoo wrote:
> 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.

Maybe, I didn't got as far as to try merging split entries in the direct
map.  IIRC, Kirill sent a patch for collapsing huge pages in the direct map
some time ago, but there still was something that had to initiate the
collapse.

> 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.

For this we need to check permissions of all 512 pages to make sure we can
use a PMD entry to map them.
Not sure that doing the scan in each set_memory call won't cause an overall
slowdown.
 
> 	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

-- 
Sincerely yours,
Mike.