Re: [PATCH RFC v3 00/21] mm: change behavior of pXdp_get()/pXd_page() in compile-time folded pgtable

[Yeoreum Yun]

Hi all,

Are there any further comments on this series?
If not, I’ll post the series without the RFC tag sometime next week :)

Thanks!

> Using ptep_get() and its counterparts in common code is suboptimal on
> kernel configurations with generic compile-time folded page tables.
> By default, ptep_get() and its friends expands to READ_ONCE(),
> forcing the compiler to emit a load even when the value is not used afterwards.
> 
> This issue was recently reported by Christophe Leroy [1] for ppc32
> preventing futher code conversion to ptep_get()/pmdp_get()/... helper
> and the same behavior can also be observed on arm64 when built with
> 2- or 3-level page tables
> 
> e.g) perf_get_page_size() in arm64 with CONFIG_PGTABLE_LEVEL=3:
> 
> 00000000000052a0 <perf_get_page_size>:
>     ...
>     52dc: d53b4234     	mrs	x20, DAIF
>     52e0: d50343df     	msr	DAIFSet, #0x3
>     ...
>     52fc: d35e9a69     	ubfx	x9, x19, #30, #9        /* pud_offset_lockless() */
>     5300: f9403508     	ldr	x8, [x8, #0x68]
>     5304: f869790a     	ldr	x10, [x8, x9, lsl #3]   /* pudp_get() */
>     5308: f90007ea     	str	x10, [sp, #0x8]
>     530c: f8697908     	ldr	x8, [x8, x9, lsl #3]    /* pudp_get() */
>     ...
>     5360: 90000009     	adrp	x9, 0x5000 <perf_prepare_sample+0x548>
>     5364: 92746908     	and	x8, x8, #0x7ffffff000
>     5368: d3557675     	ubfx	x21, x19, #21, #9       /* pmd_offset_lockless() */
>     ...
>     5394: f8757ac8     	ldr	x8, [x22, x21, lsl #3]  /* pmdp_get() */
> 
> Though PGTABLE_LEVEL=3, since the pudp_get() still remain with
> READ_ONCE(), there's redundant load for the pud which is folded.
> 
> To prevent generating suboptimal code, make pXdp_get() return a dummy
> entry for compile-time folded page tables, make the helpers such as
> pXd_offset()/pXd_offset_lockless(), set_pXd() validate dummy entries
> at compile time to catch the wrong usage and prohibit calls to
> pXd_page() in pgtable-nopXd.h.
> 
> This series does not change the behaviour of existing code that directly
> manipulates folded page-table levels using set_pgd(), pgd_page_vaddr(), and
> related helpers. Those helpers continue to behave as before.
> 
> The new restrictions only apply to code that adopts the pXdp_get()-based
> access model for compile-time folded page tables.
> 
> As the pXdp_get() can return *dummy* entry, some of code using
> the stack value where saves the pXdp_get() could be a problematic:
> 
>   1. Passing address of stack value where saves the pXdp_get() result
>      to pXd_offset() for example:
> 
>        pud_t *pudp, pud;
>        pmd_t *pmdp;
> 
>        pud = pudp_get(pudp, address);
>        pmdp = pmd_offset(&pud, pud, address);
> 
>      (e.g. host_pfn_mapping_level() in loongarch).
> 
>   2. Using the pXdp_get() result to use as argument of pXd_val() and
>      to check prot without checking pgtable is folded.
>      for example, x86's effective_prot().
> 
>   3. Using set_pXd() with pXdp_get() will set problematic dummy entry
>      in folded page table like:
> 
>        set_pXd(pxdp, pXdp_get(pxdp_k));
> 
>   4. Using pgd_page_vaddr() to get the first-level pgtable.
>      passing dummy pxdp_get() for pgd_page_vaddr() will return wrong
>      address. Therefore, make pgd_page_vaddr() and pXd_pgtable() to
>      trigger the error for improper usage with folded dummy entry in the
>      generic compile-time folded pgtable.
> 
> Thanksfully, above cases are rare since (1) most of usage using
> pXd_offset() with result of upper pXd_offset(), (2) it's extreamely
> rare to use pXd_val() for non-leaf entry in the kernel,
> (3) is to handle the vmalloc_fault or set the first level of page table
> and (4) to setup early page table and etc.
> 
> Therefore, properly handle this uncommon and problematic pattern, and
> document the current design of compile-time folded page tables.
> 
> This patch is based on mm-unstable.
> 
> Future work
> ===========
>  - print_bad_page_map() and show_pte() still prints dummy values
>    instead of printing the same content for all generic compile-time
>    folded page tables. We might want to skip printing dummy values later.
> 
>  - We currently catch abuse of dummy values on the stack at compile-time by
>    relying on constant propagation by the compiler. Usama's work [3] on using
>    distinct types for sw vs. hw PTEs could help here as well."
> 
>  - Clean up vmalloc fault handling by synchronizing the vmalloc entry on
>    32-bit architectures. This code is almost identical across architectures.
> 
>  - Unfortunately, the current design of compile-time folded page tables appears
>    to be internally consistent but confusing. For example,
>    when CONFIG_PGTABLE_LEVELS is 2, p4d, pud, and pmd are expected to
>    be folded into pgd. However, the architecture code uses set_pmd()
>    to update the top-level page-table entry, even though it includes pgtable-nopmd.h.
> 
>    In the future, it would be good to eliminate this source of confusion,
>    possibly by treating all folded upper levels consistently as dummy wrappers
>    around the highest real page-table level:
> 
>          NOPGD
>    --> +------+           P4D
>        | ptr0 |-------> +------+           PUD
>        +------+         | ptr0 |-------> +-----+
>                         | ptr1 |-        | ptr | -------> ...
>                         | ptr2 | \       | ptr |
>                         | ptr3 |  \        ...
>                           ...      \
>                                     \        PUD
>                                      +----> +-----+
>                                             | ptr | -------> ...
>                                             | ptr |
>                                               ...
> 
> Patch History
> =============
> from v2 to v3:
>   - repasre commit message
>   - move ptdump_pgtable_first_level() into pgalloc.h
>   - skip the huge pud operation when CONFIG_X86_DIRECT_GBPAGES is disabled
>   - docuemtns compile-time folded page table
>   - drop the applied patch.
>   - https://lore.kernel.org/all/20260722-dummy_ptxp3-v2-0-d9e4bad31e0a@xxxxxxx/
> 
> from v1 to v2:
>   - Restore slient fallback to next pXd in set_pXd() and pXd_pgtable()
>     and add check whether they're called with dummy entry.
>   - Add some comment for returning first entry of pgd in arm with
>     2 pgtable-level
>   - https://lore.kernel.org/all/20260713135614.1618183-1-yeoreum.yun@xxxxxxx/
> 
> Link: [1] https://lore.kernel.org/all/0019d675-ce3d-4a5c-89ed-f126c45145c9@xxxxxxxxxx/
> Link: [2] https://lore.kernel.org/all/20251113014656.2605447-1-samuel.holland@xxxxxxxxxx/
> Link: [3] https://lore.kernel.org/r/74182e50-b54f-4d2d-a27f-3a59a538d6bc@xxxxxxx
> ---
> David Hildenbrand (Arm) (13):
>       ARM: mm: make nommu pgd_t a scalar
>       ARM: mm: make 2-level pgd_t a scalar
>       ARM: mm: remove custom pgdp_get()
>       LoongArch: mm: define pud_leaf() only when PUD exists
>       MIPS: mm: define pud_leaf() only when PUD exists
>       mm/pgtable: define (pgd|p4d|pud)_leaf() for folded page tables
>       mm/pgtable: define (pgd|p4d|pud)_offset_lockless() for folded page tables
>       openrisc/pgtable: drop __pmd_offset()
>       mm/pgtable: optimize pmdp_get() and friends for folded pagetable levels
>       mm/pgtable: catch abuse of folded dummy pgd_t/p4d_t/pud_t
>       mm/pgtable: disallow calling (pgd|p4d|pud)_page, pgd_page_vaddr() and (p4d|pud)_pgtable with dummy
>       mm/pgtable: disallow calling folded set_pgd/set_p4d/set_pud with dummy
>       Documentation: mm: clarify behaviour of compile-time folded page tables
> 
> Yeoreum Yun (8):
>       mm: vmscan: remove stack copy address of pud/pmd pass in walk_pud/pmd_range()
>       loongarch: kvm: remove stack copy address of pXd in pXd_offset()
>       riscv: kvm: remove stack copy address of pXd in pXd_offset()
>       riscv: mm: use proper set_pXd() for generic compile-time folded patable in vmalloc_fault()
>       mm/pgtable: redefine PGTABLE_LEVEL enum with ascend order from PGD
>       x86: mm: use pgtable_level enum in effective_prot_pXd()
>       x86: mm: carve out the generic compile-time folded pgtable case in effective_prot()
>       x86: mm: skip collapse_pud_page() when CONFIG_X86_DIRECT_GBPAGES disabled
> 
>  Documentation/mm/page_tables.rst            | 84 +++++++++++++++++++++++++----
>  arch/arm/include/asm/page-nommu.h           |  4 +-
>  arch/arm/include/asm/pgtable-2level-types.h | 20 +++++--
>  arch/arm/include/asm/pgtable.h              |  2 -
>  arch/arm64/include/asm/pgtable.h            | 23 --------
>  arch/loongarch/include/asm/pgtable.h        |  2 +
>  arch/loongarch/kvm/mmu.c                    | 20 ++++---
>  arch/mips/include/asm/pgtable.h             |  2 +
>  arch/openrisc/include/asm/pgtable.h         |  3 --
>  arch/riscv/kvm/mmu.c                        | 20 ++++---
>  arch/riscv/mm/fault.c                       | 52 +++++++++++-------
>  arch/x86/mm/dump_pagetables.c               | 18 ++++---
>  arch/x86/mm/pat/set_memory.c                |  2 +-
>  include/asm-generic/pgtable-nop4d.h         | 54 +++++++++++++++----
>  include/asm-generic/pgtable-nopmd.h         | 54 +++++++++++++++----
>  include/asm-generic/pgtable-nopud.h         | 56 +++++++++++++++----
>  include/linux/pgtable.h                     | 69 +++++++++++++++++-------
>  mm/vmscan.c                                 |  4 +-
>  18 files changed, 352 insertions(+), 137 deletions(-)
> ---
> base-commit: e3b5239afe1b8f0194db7436b17c33e94c1988c4
> change-id: 20260722-dummy_ptxp3-3741d78cc70f
> 
> Best regards,
> -- 
> Sincerely,
> Yeoreum Yun
> 

-- 
Sincerely,
Yeoreum Yun