linux-kernel - Re: [PATCH 2/2] mremap: use mmu gather logic for tlb flush in mremap

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Date:   Mon, 28 Nov 2016 09:32:35 -0800
From:   Dave Hansen <dave.hansen@...el.com>
To:     Aaron Lu <aaron.lu@...el.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Linux Memory Management List <linux-mm@...ck.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        Huang Ying <ying.huang@...el.com>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/2] mremap: use mmu gather logic for tlb flush in mremap

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 :).