Re: [PATCH v2 5/6] mm: tlb: Provide flush_*_tlb_range wrappers

[Catalin Marinas <catalin.marinas@arm.com>]

Hi Zhenyu,

On Sat, May 30, 2020 at 06:24:21PM +0800, Zhenyu Ye wrote:
> On 2020/5/26 22:52, Catalin Marinas wrote:
> > On Mon, May 25, 2020 at 03:19:42PM +0800, Zhenyu Ye wrote:
> >> tlb_flush_##_pxx##_range() is used to set tlb->cleared_*,
> >> flush_##_pxx##_tlb_range() will actually flush the TLB entry.
> >>
> >> In arch64, tlb_flush_p?d_range() is defined as:
> >>
> >> 	#define flush_pmd_tlb_range(vma, addr, end)	flush_tlb_range(vma, addr, end)
> >> 	#define flush_pud_tlb_range(vma, addr, end)	flush_tlb_range(vma, addr, end)
> > 
> > Currently, flush_p??_tlb_range() are generic and defined as above. I
> > think in the generic code they can remain an alias for
> > flush_tlb_range().
> > 
> > On arm64, we can redefine them as:
> > 
> > #define flush_pte_tlb_range(vma, addr, end)	__flush_tlb_range(vma, addr, end, 3)
> > #define flush_pmd_tlb_range(vma, addr, end)	__flush_tlb_range(vma, addr, end, 2)
> > #define flush_pud_tlb_range(vma, addr, end)	__flush_tlb_range(vma, addr, end, 1)
> > #define flush_p4d_tlb_range(vma, addr, end)	__flush_tlb_range(vma, addr, end, 0)
> > 
> > (unless the compiler optimises away all the mmu_gather stuff in your
> > macro above but they don't look trivial to me)
> 
> I changed generic code before considering that other structures may also
> use this feature, such as Power9. And Peter may want to replace all
> flush_tlb_range() by tlb_flush() in the future, see [1] for details.
> 
> If only enable this feature on aarch64, your codes are better.
> 
> [1] https://lore.kernel.org/linux-arm-kernel/20200402163849.GM20713@hirez.programming.kicks-ass.net/

But we change the semantics slightly if we implement these as
mmu_gather. For example, tlb_end_vma() -> tlb_flush_mmu_tlbonly() ends
up calling mmu_notifier_invalidate_range() which it didn't before. I
think we end up invoking the notifier unnecessarily in some cases (see
the comment in __split_huge_pmd()) or we end up calling the notifier
twice (e.g. pmdp_huge_clear_flush_notify()).

> > Also, I don't see the new flush_pte_* and flush_p4d_* macros used
> > anywhere and I don't think they are needed. The pte equivalent is
> > flush_tlb_page() (we need to make sure it's not used on a pmd in the
> > hugetlb context).
> 
> flush_tlb_page() is used to flush only one page.  If we add the
> flush_pte_tlb_range(), then we can use it to flush a range of pages in
> the future.

If we know flush_tlb_page() is only called on a small page, could we add
TTL information here as well?

> But flush_pte_* and flush_p4d_* macros are really not used anywhere. I
> will remove them in next version of series, and add them if someone
> needs.

I think it makes sense.

-- 
Catalin