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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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