linux-kernel - Re: [PATCH v2 1/2] mm/migrate_device.c: Copy pte dirty bit to page

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Message-ID: <87czcyawl6.fsf@yhuang6-desk2.ccr.corp.intel.com>
Date:   Thu, 18 Aug 2022 14:34:45 +0800
From:   "Huang, Ying" <ying.huang@...el.com>
To:     Peter Xu <peterx@...hat.com>
Cc:     Nadav Amit <nadav.amit@...il.com>,
        Alistair Popple <apopple@...dia.com>,
        huang ying <huang.ying.caritas@...il.com>,
        Linux MM <linux-mm@...ck.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        LKML <linux-kernel@...r.kernel.org>,
        "Sierra Guiza, Alejandro (Alex)" <alex.sierra@....com>,
        Felix Kuehling <Felix.Kuehling@....com>,
        Jason Gunthorpe <jgg@...dia.com>,
        John Hubbard <jhubbard@...dia.com>,
        David Hildenbrand <david@...hat.com>,
        Ralph Campbell <rcampbell@...dia.com>,
        Matthew Wilcox <willy@...radead.org>,
        Karol Herbst <kherbst@...hat.com>,
        Lyude Paul <lyude@...hat.com>, Ben Skeggs <bskeggs@...hat.com>,
        Logan Gunthorpe <logang@...tatee.com>, paulus@...abs.org,
        linuxppc-dev@...ts.ozlabs.org, stable@...r.kernel.org
Subject: Re: [PATCH v2 1/2] mm/migrate_device.c: Copy pte dirty bit to page

Peter Xu <peterx@...hat.com> writes:

> On Wed, Aug 17, 2022 at 02:41:19AM -0700, Nadav Amit wrote:
>> 4. Having multiple TLB flushing infrastructures makes all of these
>> discussions very complicated and unmaintainable. I need to convince myself
>> in every occasion (including this one) whether calls to
>> flush_tlb_batched_pending() and tlb_flush_pending() are needed or not.
>> 
>> What I would like to have [3] is a single infrastructure that gets a
>> “ticket” (generation when the batching started), the old PTE and the new PTE
>> and checks whether a TLB flush is needed based on the arch behavior and the
>> current TLB generation. If needed, it would update the “ticket” to the new
>> generation. Andy wanted a ring for pending TLB flushes, but I think it is an
>> overkill with more overhead and complexity than needed.
>> 
>> But the current situation in which every TLB flush is a basis for long
>> discussions and prone to bugs is impossible.
>> 
>> I hope it helps. Let me know if you want me to revive the patch-set or other
>> feedback.
>> 
>> [1] https://lore.kernel.org/all/20220711034615.482895-5-21cnbao@gmail.com/
>> [2] https://lore.kernel.org/all/20220718120212.3180-13-namit@vmware.com/
>> [3] https://lore.kernel.org/all/20210131001132.3368247-16-namit@vmware.com/
>
> I need more reading on tlb code and also [3] which looks useful to me.
> It's definitely sad to make tlb flushing so complicated.  It'll be great if
> things can be sorted out someday.
>
> In this specific case, the only way to do safe tlb batching in my mind is:
>
> 	pte_offset_map_lock();
> 	arch_enter_lazy_mmu_mode();
>         // If any pending tlb, do it now
>         if (mm_tlb_flush_pending())
> 		flush_tlb_range(vma, start, end);
>         else
>                 flush_tlb_batched_pending();

I don't think we need the above 4 lines.  Because we will flush TLB
before we access the pages.  Can you find any issue if we don't use the
above 4 lines?

Best Regards,
Huang, Ying

>         loop {
>                 ...
>                 pte = ptep_get_and_clear();
>                 ...
>                 if (pte_present())
>                         unmapped++;
>                 ...
>         }
> 	if (unmapped)
> 		flush_tlb_range(walk->vma, start, end);
> 	arch_leave_lazy_mmu_mode();
> 	pte_unmap_unlock();
>
> I may miss something, but even if not it already doesn't look pretty.
>
> Thanks,