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Date: Tue, 29 Aug 2017 20:18:41 -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
Nadav Amit <nadav.amit@...il.com> wrote:
> 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() ?
Sorry - I noticed it was actually wrong before (as you noted) at least
in one case:
(unlikely(PageSwapBacked(page) != PageSwapCache(page)))
Regards,
Nadav
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