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Date:   Tue, 29 Aug 2017 20:16:44 -0700
From:   Nadav Amit <nadav.amit@...il.com>
To:     Jerome 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

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

> On Tue, Aug 29, 2017 at 07:46:07PM -0700, Nadav Amit wrote:
>> 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.
> 
> Yes it would simplify the patch, i was trying to optimize for the case
> where we restore the pte to its original value after ptep_clear_flush()
> or ptep_get_and_clear() as in this case there is no need to invalidate
> any secondary page table but that's an overkill optimization that just
> add too much complexity.

Interesting. Actually, prior to your changes, it seems that the break
statements would skip mmu_notifier_invalidate_page() when the PTE is not
changed. So why not just change mmu_notifier_invalidate_page() into
mmu_notifier_invalidate_range() ?

Regards,
Nadav

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