"Huang, Ying" [off-list ref] writes:

Peter Xu [off-list ref] writes:

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On Thu, Aug 18, 2022 at 02:34:45PM +0800, Huang, Ying wrote:

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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.

I agree. For migration the TLB flush is only important if the PTE is
present, and in that case we do a TLB flush anyway.

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Could you elaborate?

As you have said below, we don't use non-present PTEs and flush present
PTEs before we access the pages.

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Can you find any issue if we don't use the above 4 lines?

It seems okay to me to leave stall tlb at least within the scope of this
function. It only collects present ptes and flush propoerly for them.  I
don't quickly see any other implications to other not touched ptes - unlike
e.g. mprotect(), there's a strong barrier of not allowing further write
after mprotect() returns.

Yes.  I think so too.

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Still I don't know whether there'll be any side effect of having stall tlbs
in !present ptes because I'm not familiar enough with the private dev swap
migration code.  But I think having them will be safe, even if redundant.

What side-effect were you thinking of? I don't see any issue with not
TLB flushing stale device-private TLBs prior to the migration because
they're not accessible anyway and shouldn't be in any TLB.

I don't think it's a good idea to be redundant.  That may hide the real
issue.

Best Regards,
Huang, Ying

Thanks all for the discussion. Having done some more reading I agree
that it's safe to assume HW dirty bits are write-through, so will remove
the ptep_clear_flush() and use ptep_get_and_clear() instead. Will split
out the TLB flushing fix into a separate patch in this series.

 - Alistair