On 11/28/2016 12:40 AM, Aaron Lu wrote:

As suggested by Linus, the same mmu gather logic could be used for tlb
flush in mremap and this patch just did that.

Note that since there is no page added to "struct mmu_gather" for free
during mremap, when tlb needs to be flushed, we can use tlb_flush_mmu or
tlb_flush_mmu_tlbonly. Using tlb_flush_mmu could also avoid exporting
tlb_flush_mmu_tlbonly. But tlb_flush_mmu_tlbonly *looks* more clear and
straightforward so I end up using it.

OK, so the code before this patch was passing around a pointer to
'need_flush', and we basically just pass around an mmu_gather instead.
It doesn't really simplify the code _too_ much, although it does make it
less confusing than when we saw 'need_flush' mixed with 'force_flush' in
the code.

tlb_flush_mmu_tlbonly() has exactly one other use: zap_pte_range() for
flushing the TLB before pte_unmap_unlock():

        if (force_flush)
                tlb_flush_mmu_tlbonly(tlb);
        pte_unmap_unlock(start_pte, ptl);

But, both call-sites are still keeping 'force_flush' to store the
information about whether we ever saw a dirty pte.  If we moved _that_
logic into the x86 mmu_gather code, we could get rid of all the
'force_flush' tracking in both call sites.  It also makes us a bit more
future-proof against these page_mkclean() races if we ever grow a third
site for clearing ptes.

Instead of exporting and calling tlb_flush_mmu_tlbonly(), we'd need
something like tlb_flush_mmu_before_ptl_release() (but with a better
name, of course :).

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