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

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Date:   Wed, 17 Aug 2022 15:27:32 -0400
From:   Peter Xu <peterx@...hat.com>
To:     Nadav Amit <nadav.amit@...il.com>
Cc:     "Huang, Ying" <ying.huang@...el.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

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();
        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,

-- 
Peter Xu