From: Mike Rapoport <firstname.lastname@example.org> To: email@example.com Cc: Andrew Morton <firstname.lastname@example.org>, Andy Lutomirski <email@example.com>, Dave Hansen <firstname.lastname@example.org>, Ira Weiny <email@example.com>, Kees Cook <firstname.lastname@example.org>, Mike Rapoport <email@example.com>, Mike Rapoport <firstname.lastname@example.org>, Peter Zijlstra <email@example.com>, Rick Edgecombe <firstname.lastname@example.org>, Vlastimil Babka <email@example.com>, firstname.lastname@example.org, email@example.com Subject: [RFC PATCH 0/4] mm/page_alloc: cache pte-mapped allocations Date: Mon, 23 Aug 2021 16:25:09 +0300 [thread overview] Message-ID: <firstname.lastname@example.org> (raw) From: Mike Rapoport <email@example.com> Hi, This is early prototype for addition of cache of pte-mapped pages to the page allocator. It survives boot and some cache shrinking, but it's still a long way to go for it to be ready for non-RFC posting. The example use-case for pte-mapped cache is protection of page tables and keeping them read-only except for the designated code that is allowed to modify the page tables. I'd like to get an early feedback for the approach to see what would be the best way to move forward with something like this. 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). == Motivation == There are usecases 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 make sense to group and/or cache the 4K 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  and for using PKS to protect page tables  as well as an attempt to use a pool of large pages in secretmtm . == Implementation overview == This set leverages ideas from the patches that added PKS protection to page tables, but instead of adding per-user grouped allocations it tries to move the cache of pte-mapped pages closer to the page allocator. The idea is to use a gfp flag that will instruct the page allocator to use the cache of pte-mapped pages because the caller needs to remove them from the direct map or change their attributes. When the cache is empty there is an attempt to refill it using PMD-sized allocation so that once the direct map is split we'll be able to use all 4K pages made available by the split. If the high order allocation fails, we fall back to order-0 and mark the entire pageblock as pte-mapped. When pages from that pageblock are freed to the page allocator they are put into the pte-mapped cache. There is also unimplemented provision to add free pages from such pageblock to the pte-mapped cache along with the page that was allocated and cause the split of the pageblock. For now only order-0 allocations of pte-mapped pages are supported, which prevents, for instance, allocation of PGD with PTI enabled. The free pages in the cache may be reclaimed using a shrinker, but for now they will remain mapped with PTEs in the direct map. == TODOs == Whenever pte-mapped cache is being shrunk, it is possible to add some kind of compaction to move all the free pages into PMD-sized chunks, free these chunks at once and restore large page in the direct map. There is also a possibility to add heuristics and knobs to control greediness of the cache vs memory pressure so that freed pte-mapped cache won't be necessarily put into the pte-mapped cache. Another thing that can be implemented is pre-populating the pte-cache at boot time to include the free pages that are anyway mapped by PTEs. == Alternatives == Current implementation uses a single global cache. Another option is to have per-user caches, e.g one for the page tables, another for vmalloc etc. This approach provides better control of the permissions of the pages allocated from these caches and allows the user to decide when (if at all) the pages can be accessed, e.g. for cache compaction. The down side of this approach is that it complicates the freeing path. A page allocated from a dedicated cache cannot be freed with put_page()/free_page() etc but it has to be freed with a dedicated API or there should be some back pointer in struct page so that page allocator will know what cache this page came from. Yet another possibility to make pte-mapped cache a migratetype of its own. Creating a new migratetype would allow higher order allocations of pte-mapped pages, but I don't have enough understanding of page allocator and reclaim internals to estimate the complexity associated with this approach.  https://firstname.lastname@example.org/  https://email@example.com  https://firstname.lastname@example.org/ Mike Rapoport (2): mm/page_alloc: introduce __GFP_PTE_MAPPED flag to allocate pte-mapped pages x86/mm: write protect (most) page tables Rick Edgecombe (2): list: Support getting most recent element in list_lru list: Support list head not in object for list_lru arch/Kconfig | 8 + arch/x86/Kconfig | 1 + arch/x86/boot/compressed/ident_map_64.c | 3 + arch/x86/include/asm/pgalloc.h | 2 + arch/x86/include/asm/pgtable.h | 21 +- arch/x86/include/asm/pgtable_64.h | 33 ++- arch/x86/mm/init.c | 2 +- arch/x86/mm/pgtable.c | 72 ++++++- include/asm-generic/pgalloc.h | 2 +- include/linux/gfp.h | 11 +- include/linux/list_lru.h | 26 +++ include/linux/mm.h | 2 + include/linux/pageblock-flags.h | 26 +++ init/main.c | 1 + mm/internal.h | 3 +- mm/list_lru.c | 38 +++- mm/page_alloc.c | 261 +++++++++++++++++++++++- 17 files changed, 496 insertions(+), 16 deletions(-) -- 2.28.0
next reply other threads:[~2021-08-23 13:25 UTC|newest] Thread overview: 27+ messages / expand[flat|nested] mbox.gz Atom feed top 2021-08-23 13:25 Mike Rapoport [this message] 2021-08-23 13:25 ` [RFC PATCH 1/4] list: Support getting most recent element in list_lru Mike Rapoport 2021-08-23 13:25 ` [RFC PATCH 2/4] list: Support list head not in object for list_lru Mike Rapoport 2021-08-23 13:25 ` [RFC PATCH 3/4] mm/page_alloc: introduce __GFP_PTE_MAPPED flag to allocate pte-mapped pages Mike Rapoport 2021-08-23 20:29 ` Edgecombe, Rick P 2021-08-24 13:02 ` Mike Rapoport 2021-08-24 16:38 ` Edgecombe, Rick P 2021-08-24 16:54 ` Mike Rapoport 2021-08-24 17:23 ` Edgecombe, Rick P 2021-08-24 17:37 ` Mike Rapoport 2021-08-24 16:12 ` Vlastimil Babka 2021-08-25 8:43 ` David Hildenbrand 2021-08-23 13:25 ` [RFC PATCH 4/4] x86/mm: write protect (most) page tables Mike Rapoport 2021-08-23 20:08 ` Edgecombe, Rick P 2021-08-23 23:50 ` Dave Hansen 2021-08-24 3:34 ` Andy Lutomirski 2021-08-25 14:59 ` Dave Hansen 2021-08-24 13:32 ` Mike Rapoport 2021-08-25 8:38 ` David Hildenbrand 2021-08-26 8:02 ` Mike Rapoport 2021-08-26 9:01 ` Vlastimil Babka [not found] ` <FB6C09CD-9CEA-4FE8-B179-98DB63EBDD68@gmail.com> 2021-08-24 5:34 ` Nadav Amit 2021-08-24 13:36 ` Mike Rapoport 2021-08-23 20:02 ` [RFC PATCH 0/4] mm/page_alloc: cache pte-mapped allocations Edgecombe, Rick P 2021-08-24 13:03 ` Mike Rapoport 2021-08-24 16:09 ` Vlastimil Babka 2021-08-29 7:06 ` Mike Rapoport
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