Re: [PATCH] mm: pgtable: make ptep_clear() non-atomic

[Jann Horn <jannh@google.com>]

On Tue, Nov 19, 2024 at 10:10 AM Qi Zheng <zhengqi.arch@bytedance.com> wrote:
> In the generic ptep_get_and_clear() implementation, it is just a simple
> combination of ptep_get() and pte_clear(). But for some architectures
> (such as x86 and arm64, etc), the hardware will modify the A/D bits of the
> page table entry, so the ptep_get_and_clear() needs to be overwritten
> and implemented as an atomic operation to avoid contention, which has a
> performance cost.
>
> The commit d283d422c6c4 ("x86: mm: add x86_64 support for page table
> check") adds the ptep_clear() on the x86, and makes it call
> ptep_get_and_clear() when CONFIG_PAGE_TABLE_CHECK is enabled. The page
> table check feature does not actually care about the A/D bits, so only
> ptep_get() + pte_clear() should be called. But considering that the page
> table check is a debug option, this should not have much of an impact.
>
> But then the commit de8c8e52836d ("mm: page_table_check: add hooks to
> public helpers") changed ptep_clear() to unconditionally call
> ptep_get_and_clear(), so that the CONFIG_PAGE_TABLE_CHECK check can be
> put into the page table check stubs (in include/linux/page_table_check.h).
> This also cause performance loss to the kernel without
> CONFIG_PAGE_TABLE_CHECK enabled, which doesn't make sense.
>
> Currently ptep_clear() is only used in debug code and in khugepaged
> collapse paths, which are fairly expensive. So the cost of an extra atomic
> RMW operation does not matter. But this may be used for other paths in the
> future. After all, for the present pte entry, we need to call ptep_clear()
> instead of pte_clear() to ensure that PAGE_TABLE_CHECK works properly.
>
> So to be more precise, just calling ptep_get() and pte_clear() in the
> ptep_clear().
>
> Signed-off-by: Qi Zheng <zhengqi.arch@bytedance.com>

Yeah, as you mentioned on the other thread, this seems like a
reasonable cleanup.

Reviewed-by: Jann Horn <jannh@google.com>