Re: [PATCH 02/13] mm/rmap: update to new mmu_notifier semantic

[Nadav Amit]

Nadav Amit <nadav.amit@xxxxxxxxx> wrote:

> Jerome Glisse <jglisse@xxxxxxxxxx> wrote:
> 
>> On Tue, Aug 29, 2017 at 07:46:07PM -0700, Nadav Amit wrote:
>>> JÃrÃme Glisse <jglisse@xxxxxxxxxx> 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: JÃrÃme Glisse <jglisse@xxxxxxxxxx>
>>>> Cc: Dan Williams <dan.j.williams@xxxxxxxxx>
>>>> Cc: Ross Zwisler <ross.zwisler@xxxxxxxxxxxxxxx>
>>>> Cc: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
>>>> Cc: Bernhard Held <berny156@xxxxxx>
>>>> Cc: Adam Borowski <kilobyte@xxxxxxxxxx>
>>>> Cc: Andrea Arcangeli <aarcange@xxxxxxxxxx>
>>>> Cc: Radim KrÄmÃÅ <rkrcmar@xxxxxxxxxx>
>>>> Cc: Wanpeng Li <kernellwp@xxxxxxxxx>
>>>> Cc: Paolo Bonzini <pbonzini@xxxxxxxxxx>
>>>> Cc: Takashi Iwai <tiwai@xxxxxxx>
>>>> Cc: Nadav Amit <nadav.amit@xxxxxxxxx>
>>>> Cc: Mike Galbraith <efault@xxxxxx>
>>>> Cc: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx>
>>>> Cc: axie <axie@xxxxxxx>
>>>> Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
>>>> ---
>>>> 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 donât 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