Re: [PATCH v6 5/6] khugepaged: enable collapse pmd for pte-mapped THP

["Kirill A. Shutemov" <kirill@shutemov.name>]

On Tue, Jun 25, 2019 at 12:33:03PM +0000, Song Liu wrote:
> 
> 
> > On Jun 25, 2019, at 3:34 AM, Kirill A. Shutemov <kirill@shutemov.name> wrote:
> > 
> > On Mon, Jun 24, 2019 at 02:25:42PM +0000, Song Liu wrote:
> >> 
> >> 
> >>> On Jun 24, 2019, at 6:19 AM, Kirill A. Shutemov <kirill@shutemov.name> wrote:
> >>> 
> >>> On Sat, Jun 22, 2019 at 10:48:28PM -0700, Song Liu wrote:
> >>>> khugepaged needs exclusive mmap_sem to access page table. When it fails
> >>>> to lock mmap_sem, the page will fault in as pte-mapped THP. As the page
> >>>> is already a THP, khugepaged will not handle this pmd again.
> >>>> 
> >>>> This patch enables the khugepaged to retry retract_page_tables().
> >>>> 
> >>>> A new flag AS_COLLAPSE_PMD is introduced to show the address_space may
> >>>> contain pte-mapped THPs. When khugepaged fails to trylock the mmap_sem,
> >>>> it sets AS_COLLAPSE_PMD. Then, at a later time, khugepaged will retry
> >>>> compound pages in this address_space.
> >>>> 
> >>>> Since collapse may happen at an later time, some pages may already fault
> >>>> in. To handle these pages properly, it is necessary to prepare the pmd
> >>>> before collapsing. prepare_pmd_for_collapse() is introduced to prepare
> >>>> the pmd by removing rmap, adjusting refcount and mm_counter.
> >>>> 
> >>>> prepare_pmd_for_collapse() also double checks whether all ptes in this
> >>>> pmd are mapping to the same THP. This is necessary because some subpage
> >>>> of the THP may be replaced, for example by uprobe. In such cases, it
> >>>> is not possible to collapse the pmd, so we fall back.
> >>>> 
> >>>> Signed-off-by: Song Liu <songliubraving@fb.com>
> >>>> ---
> >>>> include/linux/pagemap.h |  1 +
> >>>> mm/khugepaged.c         | 69 +++++++++++++++++++++++++++++++++++------
> >>>> 2 files changed, 60 insertions(+), 10 deletions(-)
> >>>> 
> >>>> diff --git a/include/linux/pagemap.h b/include/linux/pagemap.h
> >>>> index 9ec3544baee2..eac881de2a46 100644
> >>>> --- a/include/linux/pagemap.h
> >>>> +++ b/include/linux/pagemap.h
> >>>> @@ -29,6 +29,7 @@ enum mapping_flags {
> >>>> 	AS_EXITING	= 4, 	/* final truncate in progress */
> >>>> 	/* writeback related tags are not used */
> >>>> 	AS_NO_WRITEBACK_TAGS = 5,
> >>>> +	AS_COLLAPSE_PMD = 6,	/* try collapse pmd for THP */
> >>>> };
> >>>> 
> >>>> /**
> >>>> diff --git a/mm/khugepaged.c b/mm/khugepaged.c
> >>>> index a4f90a1b06f5..9b980327fd9b 100644
> >>>> --- a/mm/khugepaged.c
> >>>> +++ b/mm/khugepaged.c
> >>>> @@ -1254,7 +1254,47 @@ static void collect_mm_slot(struct mm_slot *mm_slot)
> >>>> }
> >>>> 
> >>>> #if defined(CONFIG_SHMEM) && defined(CONFIG_TRANSPARENT_HUGE_PAGECACHE)
> >>>> -static void retract_page_tables(struct address_space *mapping, pgoff_t pgoff)
> >>>> +
> >>>> +/* return whether the pmd is ready for collapse */
> >>>> +bool prepare_pmd_for_collapse(struct vm_area_struct *vma, pgoff_t pgoff,
> >>>> +			      struct page *hpage, pmd_t *pmd)
> >>>> +{
> >>>> +	unsigned long haddr = page_address_in_vma(hpage, vma);
> >>>> +	unsigned long addr;
> >>>> +	int i, count = 0;
> >>>> +
> >>>> +	/* step 1: check all mapped PTEs are to this huge page */
> >>>> +	for (i = 0, addr = haddr; i < HPAGE_PMD_NR; i++, addr += PAGE_SIZE) {
> >>>> +		pte_t *pte = pte_offset_map(pmd, addr);
> >>>> +
> >>>> +		if (pte_none(*pte))
> >>>> +			continue;
> >>>> +
> >>>> +		if (hpage + i != vm_normal_page(vma, addr, *pte))
> >>>> +			return false;
> >>>> +		count++;
> >>>> +	}
> >>>> +
> >>>> +	/* step 2: adjust rmap */
> >>>> +	for (i = 0, addr = haddr; i < HPAGE_PMD_NR; i++, addr += PAGE_SIZE) {
> >>>> +		pte_t *pte = pte_offset_map(pmd, addr);
> >>>> +		struct page *page;
> >>>> +
> >>>> +		if (pte_none(*pte))
> >>>> +			continue;
> >>>> +		page = vm_normal_page(vma, addr, *pte);
> >>>> +		page_remove_rmap(page, false);
> >>>> +	}
> >>>> +
> >>>> +	/* step 3: set proper refcount and mm_counters. */
> >>>> +	page_ref_sub(hpage, count);
> >>>> +	add_mm_counter(vma->vm_mm, mm_counter_file(hpage), -count);
> >>>> +	return true;
> >>>> +}
> >>>> +
> >>>> +extern pid_t sysctl_dump_pt_pid;
> >>>> +static void retract_page_tables(struct address_space *mapping, pgoff_t pgoff,
> >>>> +				struct page *hpage)
> >>>> {
> >>>> 	struct vm_area_struct *vma;
> >>>> 	unsigned long addr;
> >>>> @@ -1273,21 +1313,21 @@ static void retract_page_tables(struct address_space *mapping, pgoff_t pgoff)
> >>>> 		pmd = mm_find_pmd(vma->vm_mm, addr);
> >>>> 		if (!pmd)
> >>>> 			continue;
> >>>> -		/*
> >>>> -		 * We need exclusive mmap_sem to retract page table.
> >>>> -		 * If trylock fails we would end up with pte-mapped THP after
> >>>> -		 * re-fault. Not ideal, but it's more important to not disturb
> >>>> -		 * the system too much.
> >>>> -		 */
> >>>> 		if (down_write_trylock(&vma->vm_mm->mmap_sem)) {
> >>>> 			spinlock_t *ptl = pmd_lock(vma->vm_mm, pmd);
> >>>> -			/* assume page table is clear */
> >>>> +
> >>>> +			if (!prepare_pmd_for_collapse(vma, pgoff, hpage, pmd)) {
> >>>> +				spin_unlock(ptl);
> >>>> +				up_write(&vma->vm_mm->mmap_sem);
> >>>> +				continue;
> >>>> +			}
> >>>> 			_pmd = pmdp_collapse_flush(vma, addr, pmd);
> >>>> 			spin_unlock(ptl);
> >>>> 			up_write(&vma->vm_mm->mmap_sem);
> >>>> 			mm_dec_nr_ptes(vma->vm_mm);
> >>>> 			pte_free(vma->vm_mm, pmd_pgtable(_pmd));
> >>>> -		}
> >>>> +		} else
> >>>> +			set_bit(AS_COLLAPSE_PMD, &mapping->flags);
> >>>> 	}
> >>>> 	i_mmap_unlock_write(mapping);
> >>>> }
> >>>> @@ -1561,7 +1601,7 @@ static void collapse_file(struct mm_struct *mm,
> >>>> 		/*
> >>>> 		 * Remove pte page tables, so we can re-fault the page as huge.
> >>>> 		 */
> >>>> -		retract_page_tables(mapping, start);
> >>>> +		retract_page_tables(mapping, start, new_page);
> >>>> 		*hpage = NULL;
> >>>> 
> >>>> 		khugepaged_pages_collapsed++;
> >>>> @@ -1622,6 +1662,7 @@ static void khugepaged_scan_file(struct mm_struct *mm,
> >>>> 	int present, swap;
> >>>> 	int node = NUMA_NO_NODE;
> >>>> 	int result = SCAN_SUCCEED;
> >>>> +	bool collapse_pmd = false;
> >>>> 
> >>>> 	present = 0;
> >>>> 	swap = 0;
> >>>> @@ -1640,6 +1681,14 @@ static void khugepaged_scan_file(struct mm_struct *mm,
> >>>> 		}
> >>>> 
> >>>> 		if (PageTransCompound(page)) {
> >>>> +			if (collapse_pmd ||
> >>>> +			    test_and_clear_bit(AS_COLLAPSE_PMD,
> >>>> +					       &mapping->flags)) {
> >>> 
> >>> Who said it's the only PMD range that's subject to collapse? The bit has
> >>> to be per-PMD, not per-mapping.
> >> 
> >> I didn't assume this is the only PMD range that subject to collapse. 
> >> So once we found AS_COLLAPSE_PMD, it will continue scan the whole mapping:
> >> retract_page_tables(), then continue. 
> > 
> > I still don't get it.
> > 
> > Assume we have two ranges that subject to collapse. khugepaged_scan_file()
> > sees and clears AS_COLLAPSE_PMD. Tries to collapse the first range, fails
> > and set the bit again. khugepaged_scan_file() sees the second range,
> > clears the bit, but this time collapse is successful: the bit is still not
> > set, but it should be.
> 
> Yeah, you are right. Current logic only covers multiple THPs within single
> call of khugepaged_scan_file(). I missed the case you just described. 
> 
> What do you think about the first 4 patches of set and the other set? If 
> these patches look good, how about we get them in first? I will work on 
> proper fix (or re-design) for 5/6 and 6/6 in the meanwhile. 

You can use

Acked-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>

for the first 4 patches.

-- 
 Kirill A. Shutemov