Re: [RFC PATCH 0/5] Prototype for direct map awareness in page allocator

[Mike Rapoport]

(adding Mel)

On Wed, Mar 08, 2023 at 11:41:01AM +0200, Mike Rapoport wrote:
> From: "Mike Rapoport (IBM)" <rppt@xxxxxxxxxx>
> 
> Hi,
> 
> This is a third attempt to make page allocator aware of the direct map
> layout and allow grouping of the pages that must be unmapped from
> the direct map.
> 
> This a new implementation of __GFP_UNMAPPED, kinda a follow up for this set:
> 
> https://lore.kernel.org/all/20220127085608.306306-1-rppt@xxxxxxxxxx
> 
> but instead of using a migrate type to cache the unmapped pages, the
> current implementation adds a dedicated cache to serve __GFP_UNMAPPED
> allocations.
> 
> The last two patches in the series demonstrate how __GFP_UNMAPPED can be
> used in two in-tree use cases.
> 
> First one is to switch secretmem to use the new mechanism, which is
> straight forward optimization.
> 
> The second use-case is to enable __GFP_UNMAPPED in x86::module_alloc()
> that is essentially used as a method to allocate code pages and thus
> requires permission changes for basic pages in the direct map.
> 
> This set is x86 specific at the moment because other architectures either
> do not support set_memory APIs that split the direct^w linear map (e.g.
> PowerPC) or only enable set_memory APIs when the linear map uses basic page
> size (like arm64).
> 
> The patches are only lightly tested.
> 
> == Motivation ==
> 
> There are use-cases that need to remove pages from the direct map or at
> least map them with 4K granularity. Whenever this is done e.g. with
> set_memory/set_direct_map APIs, the PUD and PMD sized mappings in the
> direct map are split into smaller pages.
> 
> To reduce the performance hit caused by the fragmentation of the direct map
> it makes sense to group and/or cache the pages removed from the direct
> map so that the split large pages won't be all over the place. 
> 
> There were RFCs for grouped page allocations for vmalloc permissions [1]
> and for using PKS to protect page tables [2] as well as an attempt to use a
> pool of large pages in secretmtm [3], but these suggestions address each
> use case separately, while having a common mechanism at the core mm level
> could be used by all use cases.
> 
> == Implementation overview ==
> 
> The pages that need to be removed from the direct map are grouped in a
> dedicated cache. When there is a page allocation request with
> __GFP_UNMAPPED set, it is redirected from __alloc_pages() to that cache
> using a new unmapped_alloc() function.
> 
> The cache is implemented as a buddy allocator and it can handle high order
> requests.
> 
> The cache starts empty and whenever it does not have enough pages to
> satisfy an allocation request the cache attempts to allocate PMD_SIZE page
> to replenish the cache. If PMD_SIZE page cannot be allocated, the cache is
> replenished with a page of the highest order available. That page is
> removed from the direct map and added to the local buddy allocator.
> 
> There is also a shrinker that releases pages from the unmapped cache when
> there us a memory pressure in the system. When shrinker releases a page it
> is mapped back into the direct map.
> 
> [1] https://lore.kernel.org/lkml/20210405203711.1095940-1-rick.p.edgecombe@xxxxxxxxx
> [2] https://lore.kernel.org/lkml/20210505003032.489164-1-rick.p.edgecombe@xxxxxxxxx
> [3] https://lore.kernel.org/lkml/20210121122723.3446-8-rppt@xxxxxxxxxx
> 
> Mike Rapoport (IBM) (5):
>   mm: intorduce __GFP_UNMAPPED and unmapped_alloc()
>   mm/unmapped_alloc: add debugfs file similar to /proc/pagetypeinfo
>   mm/unmapped_alloc: add shrinker
>   EXPERIMENTAL: x86: use __GFP_UNMAPPED for modele_alloc()
>   EXPERIMENTAL: mm/secretmem: use __GFP_UNMAPPED
> 
>  arch/x86/Kconfig                |   3 +
>  arch/x86/kernel/module.c        |   2 +-
>  include/linux/gfp_types.h       |  11 +-
>  include/linux/page-flags.h      |   6 +
>  include/linux/pageblock-flags.h |  28 +++
>  include/trace/events/mmflags.h  |  10 +-
>  mm/Kconfig                      |   4 +
>  mm/Makefile                     |   1 +
>  mm/internal.h                   |  24 +++
>  mm/page_alloc.c                 |  39 +++-
>  mm/secretmem.c                  |  26 +--
>  mm/unmapped-alloc.c             | 334 ++++++++++++++++++++++++++++++++
>  mm/vmalloc.c                    |   2 +-
>  13 files changed, 459 insertions(+), 31 deletions(-)
>  create mode 100644 mm/unmapped-alloc.c
> 
> 
> base-commit: fe15c26ee26efa11741a7b632e9f23b01aca4cc6
> -- 
> 2.35.1
> 

-- 
Sincerely yours,
Mike.