Re: [PATCH v1 0/5] arm64/mm: uffd write-protect and soft-dirty tracking

[Ryan Roberts <ryan.roberts@arm.com>]

On 19/04/2024 08:43, Ryan Roberts wrote:
> Hi All,
> 
> This series adds uffd write-protect and soft-dirty tracking support for arm64. I
> consider the soft-dirty support (patches 3 and 4) as RFC - see rationale below.
> 
> Previous attempts to add these features have failed because of a perceived lack
> of available PTE SW bits. However it actually turns out that there are 2
> available but they are hidden. PTE_PROT_NONE was previously occupying a SW bit,
> but it only applies when PTE_VALID is clear, so this is moved to overlay PTE_UXN
> in patch 1, freeing up the SW bit. Bit 63 is marked as "IGNORED" in the Arm ARM,
> but it does not currently indicate "reserved for SW use" like it does for the
> other SW bits. I've confirmed with the spec owner that this is an oversight; the
> bit is intended to be reserved for SW use and the spec will clarify this in a
> future update.
> 
> So we have our two bits; patch 2 enables uffd-wp, patch 3 enables soft-dirty and
> patches 4 and 5 sort out the selftests so that the soft-dirty tests are compiled
> for, and run on arm64.
> 
> That said, these are the last 2 SW bits and we may want to keep 1 bit in reserve
> for future use. soft-dirty is only used for CRIU to my knowledge, and it is
> thought that their use case could be solved with the more generic uffd-wp. So
> unless somebody makes a clear case for the inclusion of soft-dirty support, we
> are probably better off dropping patches 3 and 4 and keeping bit 63 for future
> use. Although note that the most recent attempt to add soft-dirty for arm64 was
> last month [1] so I'd like to give Shivansh Vij the opportunity to make the
> case.

Ugh, forgot to mention that this applies on top of v6.9-rc3, and all the uffd-wp
and soft-dirty tests in the mm selftests suite run and pass. And no regressions
are observed in any of the other selftests.


> 
> ---8<---
> As an appendix, I've also experimented with adding an "extended SW bits" region
> linked by the `struct ptdesc` (which you can always find from the `pte_t *`). If
> demonstrated to work, this would act as an insurance policy in case we ever need
> more SW bits in future, giving us confidence to merge soft-dirty now.
> Unfortunately this approach suffers from 2 problems; 1) its slow; my fork()
> microbenchmark takes 40% longer in the worst case. 2) it is not possible to read
> the HW pte and the extended SW bits atomically so it is impossible to implement
> ptep_get_lockess() in its current form. So I've abandoned this experiment. (I
> can provide more details if there is interest).
> ---8<---
> 
> [1] https://lore.kernel.org/linux-arm-kernel/MW4PR12MB687563EFB56373E8D55DDEABB92B2@MW4PR12MB6875.namprd12.prod.outlook.com/
> 
> Thanks,
> Ryan
> 
> 
> Ryan Roberts (5):
>   arm64/mm: Move PTE_PROT_NONE and PMD_PRESENT_INVALID
>   arm64/mm: Add uffd write-protect support
>   arm64/mm: Add soft-dirty page tracking support
>   selftests/mm: Enable soft-dirty tests on arm64
>   selftests/mm: soft-dirty should fail if a testcase fails
> 
>  arch/arm64/Kconfig                         |   2 +
>  arch/arm64/include/asm/pgtable-prot.h      |  20 +++-
>  arch/arm64/include/asm/pgtable.h           | 118 +++++++++++++++++++--
>  arch/arm64/mm/contpte.c                    |   6 +-
>  arch/arm64/mm/fault.c                      |   3 +-
>  arch/arm64/mm/hugetlbpage.c                |   6 +-
>  tools/testing/selftests/mm/Makefile        |   5 +-
>  tools/testing/selftests/mm/madv_populate.c |  26 +----
>  tools/testing/selftests/mm/run_vmtests.sh  |   5 +-
>  tools/testing/selftests/mm/soft-dirty.c    |   2 +-
>  10 files changed, 141 insertions(+), 52 deletions(-)
> 
> --
> 2.25.1
>