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

[Jerome Glisse]

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.

JÃrÃme