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Date:   Tue, 29 Aug 2017 19:46:07 -0700
From:   Nadav Amit <nadav.amit@...il.com>
To:     Jérôme Glisse <jglisse@...hat.com>
Cc:     Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        "open list:MEMORY MANAGEMENT" <linux-mm@...ck.org>,
        Dan Williams <dan.j.williams@...el.com>,
        Ross Zwisler <ross.zwisler@...ux.intel.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Bernhard Held <berny156@....de>,
        Adam Borowski <kilobyte@...band.pl>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Radim Krčmář <rkrcmar@...hat.com>,
        Wanpeng Li <kernellwp@...il.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Takashi Iwai <tiwai@...e.de>, Mike Galbraith <efault@....de>,
        "Kirill A . Shutemov" <kirill.shutemov@...ux.intel.com>,
        axie <axie@....com>, Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH 02/13] mm/rmap: update to new mmu_notifier semantic

Jérôme Glisse <jglisse@...hat.com> wrote:

> Replacing all mmu_notifier_invalidate_page() by mmu_notifier_invalidat_range()
> and making sure it is bracketed by call to mmu_notifier_invalidate_range_start/
> end.
> 
> Note that because we can not presume the pmd value or pte value we have to
> assume the worse and unconditionaly report an invalidation as happening.
> 
> Signed-off-by: Jérôme Glisse <jglisse@...hat.com>
> Cc: Dan Williams <dan.j.williams@...el.com>
> Cc: Ross Zwisler <ross.zwisler@...ux.intel.com>
> Cc: Linus Torvalds <torvalds@...ux-foundation.org>
> Cc: Bernhard Held <berny156@....de>
> Cc: Adam Borowski <kilobyte@...band.pl>
> Cc: Andrea Arcangeli <aarcange@...hat.com>
> Cc: Radim Krčmář <rkrcmar@...hat.com>
> Cc: Wanpeng Li <kernellwp@...il.com>
> Cc: Paolo Bonzini <pbonzini@...hat.com>
> Cc: Takashi Iwai <tiwai@...e.de>
> Cc: Nadav Amit <nadav.amit@...il.com>
> Cc: Mike Galbraith <efault@....de>
> Cc: Kirill A. Shutemov <kirill.shutemov@...ux.intel.com>
> Cc: axie <axie@....com>
> Cc: Andrew Morton <akpm@...ux-foundation.org>
> ---
> mm/rmap.c | 44 +++++++++++++++++++++++++++++++++++++++++---
> 1 file changed, 41 insertions(+), 3 deletions(-)
> 
> diff --git a/mm/rmap.c b/mm/rmap.c
> index c8993c63eb25..da97ed525088 100644
> --- a/mm/rmap.c
> +++ b/mm/rmap.c
> @@ -887,11 +887,21 @@ static bool page_mkclean_one(struct page *page, struct vm_area_struct *vma,
> 		.address = address,
> 		.flags = PVMW_SYNC,
> 	};
> +	unsigned long start = address, end;
> 	int *cleaned = arg;
> 
> +	/*
> +	 * We have to assume the worse case ie pmd for invalidation. Note that
> +	 * the page can not be free from this function.
> +	 */
> +	end = min(vma->vm_end, (start & PMD_MASK) + PMD_SIZE);
> +	mmu_notifier_invalidate_range_start(vma->vm_mm, start, end);
> +
> 	while (page_vma_mapped_walk(&pvmw)) {
> +		unsigned long cstart, cend;
> 		int ret = 0;
> -		address = pvmw.address;
> +
> +		cstart = address = pvmw.address;
> 		if (pvmw.pte) {
> 			pte_t entry;
> 			pte_t *pte = pvmw.pte;
> @@ -904,6 +914,7 @@ static bool page_mkclean_one(struct page *page, struct vm_area_struct *vma,
> 			entry = pte_wrprotect(entry);
> 			entry = pte_mkclean(entry);
> 			set_pte_at(vma->vm_mm, address, pte, entry);
> +			cend = cstart + PAGE_SIZE;
> 			ret = 1;
> 		} else {
> #ifdef CONFIG_TRANSPARENT_HUGE_PAGECACHE
> @@ -918,6 +929,8 @@ static bool page_mkclean_one(struct page *page, struct vm_area_struct *vma,
> 			entry = pmd_wrprotect(entry);
> 			entry = pmd_mkclean(entry);
> 			set_pmd_at(vma->vm_mm, address, pmd, entry);
> +			cstart &= PMD_MASK;
> +			cend = cstart + PMD_SIZE;
> 			ret = 1;
> #else
> 			/* unexpected pmd-mapped page? */
> @@ -926,11 +939,13 @@ static bool page_mkclean_one(struct page *page, struct vm_area_struct *vma,
> 		}
> 
> 		if (ret) {
> -			mmu_notifier_invalidate_page(vma->vm_mm, address);
> +			mmu_notifier_invalidate_range(vma->vm_mm, cstart, cend);
> 			(*cleaned)++;
> 		}
> 	}
> 
> +	mmu_notifier_invalidate_range_end(vma->vm_mm, start, end);
> +
> 	return true;
> }
> 
> @@ -1324,6 +1339,7 @@ static bool try_to_unmap_one(struct page *page, struct vm_area_struct *vma,
> 	pte_t pteval;
> 	struct page *subpage;
> 	bool ret = true;
> +	unsigned long start = address, end;
> 	enum ttu_flags flags = (enum ttu_flags)arg;
> 
> 	/* munlock has nothing to gain from examining un-locked vmas */
> @@ -1335,6 +1351,14 @@ static bool try_to_unmap_one(struct page *page, struct vm_area_struct *vma,
> 				flags & TTU_MIGRATION, page);
> 	}
> 
> +	/*
> +	 * We have to assume the worse case ie pmd for invalidation. Note that
> +	 * the page can not be free in this function as call of try_to_unmap()
> +	 * must hold a reference on the page.
> +	 */
> +	end = min(vma->vm_end, (start & PMD_MASK) + PMD_SIZE);
> +	mmu_notifier_invalidate_range_start(vma->vm_mm, start, end);
> +
> 	while (page_vma_mapped_walk(&pvmw)) {
> 		/*
> 		 * If the page is mlock()d, we cannot swap it out.
> @@ -1408,6 +1432,8 @@ static bool try_to_unmap_one(struct page *page, struct vm_area_struct *vma,
> 				set_huge_swap_pte_at(mm, address,
> 						     pvmw.pte, pteval,
> 						     vma_mmu_pagesize(vma));
> +				mmu_notifier_invalidate_range(mm, address,
> +					address + vma_mmu_pagesize(vma));

I don’t think that the notifier should be called after the PTE is set, but
after the PTE is cleared, PTE permissions are demoted (e.g., RW->RO) or
access/dirty bits are cleared. [There is an exception: if the PFN in the PTE
is changed without clearing the PTE before, but it does not apply here, and
there is a different notifier for this case.]

Therefore, IIUC, try_to_umap_one() should only call
mmu_notifier_invalidate_range() after ptep_get_and_clear() and
ptep_clear_flush() are called. All the other calls to
mmu_notifier_invalidate_range() in this function can be removed.

Regards,
Nadav

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