Re: [RFC PATCH 10/12] khugepaged: Skip PTE range if a larger mTHP is already mapped

[Ryan Roberts <ryan.roberts@arm.com>]

On 16/12/2024 16:51, Dev Jain wrote:
> We may hit a situation wherein we have a larger folio mapped. It is incorrect
> to go ahead with the collapse since some pages will be unmapped, leading to
> the entire folio getting unmapped. Therefore, skip the corresponding range.
> 
> Signed-off-by: Dev Jain <dev.jain@arm.com>
> ---
> In the future, if at all it is required that at some point we want all the folios
> in the system to be of a specific order, we may split these larger folios.
> 
>  mm/khugepaged.c | 31 +++++++++++++++++++++++++++++++
>  1 file changed, 31 insertions(+)
> 
> diff --git a/mm/khugepaged.c b/mm/khugepaged.c
> index 8040b130e677..47e7c476b893 100644
> --- a/mm/khugepaged.c
> +++ b/mm/khugepaged.c
> @@ -33,6 +33,7 @@ enum scan_result {
>  	SCAN_PMD_NULL,
>  	SCAN_PMD_NONE,
>  	SCAN_PMD_MAPPED,
> +	SCAN_PTE_MAPPED,
>  	SCAN_EXCEED_NONE_PTE,
>  	SCAN_EXCEED_SWAP_PTE,
>  	SCAN_EXCEED_SHARED_PTE,
> @@ -609,6 +610,11 @@ static int __collapse_huge_page_isolate(struct vm_area_struct *vma,
>  		folio = page_folio(page);
>  		VM_BUG_ON_FOLIO(!folio_test_anon(folio), folio);
>  
> +		if (order !=HPAGE_PMD_ORDER && folio_order(folio) >= order) {
> +			result = SCAN_PTE_MAPPED;
> +			goto out;
> +		}
> +
>  		/* See hpage_collapse_scan_ptes(). */
>  		if (folio_likely_mapped_shared(folio)) {
>  			++shared;
> @@ -1369,6 +1375,7 @@ static int hpage_collapse_scan_ptes(struct mm_struct *mm,
>  	unsigned long orders;
>  	pte_t *pte, *_pte;
>  	spinlock_t *ptl;
> +	int found_order;
>  	pmd_t *pmd;
>  	int order;
>  
> @@ -1467,6 +1474,24 @@ static int hpage_collapse_scan_ptes(struct mm_struct *mm,
>  			goto out_unmap;
>  		}
>  
> +		found_order = folio_order(folio);
> +
> +		/*
> +		 * No point of scanning. Two options: if this folio was hit
> +		 * somewhere in the middle of the scan, then drop down the
> +		 * order. Or, completely skip till the end of this folio. The
> +		 * latter gives us a higher order to start with, with atmost
> +		 * 1 << order PTEs not collapsed; the former may force us
> +		 * to end up going below order 2 and exiting.
> +		 */
> +		if (order != HPAGE_PMD_ORDER && found_order >= order) {
> +			result = SCAN_PTE_MAPPED;
> +			_address += (PAGE_SIZE << found_order);
> +			_pte += (1UL << found_order);
> +			pte_unmap_unlock(pte, ptl);
> +			goto decide_order;
> +		}

It would be good if you can spell out the desired policy when khugepaged hits
partially unmapped large folios and unaligned large folios. I think the simple
approach is to always collapse them to fully mapped, aligned folios even if the
resulting order is smaller than the original. But I'm not sure that's definitely
going to always be the best thing.

Regardless, I'm struggling to understand the logic in this patch. Taking the
order of a folio based on having hit one of it's pages says anything about
whether the whole of that folio is mapped or not or it's alignment. And it's not
clear to me how we would get to a situation where we are scanning for a lower
order and find a (fully mapped, aligned) folio of higher order in the first place.

Let's assume the desired policy is that khugepaged should always collapse to
naturally aligned large folios. If there happens to be an existing aligned
order-4 folio that is fully mapped, we will identify that for collapse as part
of the scan for order-4. At that point, we should just notice that it is already
an aligned order-4 folio and bypass collapse. Of course we may have already
chosen to collapse it into a higher order, but we should definitely not get to a
lower order before we notice it.

Hmm... I guess if the sysfs thp settings have been changed then things could get
spicy... if order-8 was previously enabled and we have an order-8 folio, then it
get's disabled and khugepaged is scanning for order-4 (which is still enabled)
then hits the order-8; what's the expected policy? rework into 2 order-4 folios
or leave it as as single order-8?


> +
>  		/*
>  		 * We treat a single page as shared if any part of the THP
>  		 * is shared. "False negatives" from
> @@ -1550,6 +1575,10 @@ static int hpage_collapse_scan_ptes(struct mm_struct *mm,
>  		if (_address == org_address + (PAGE_SIZE << HPAGE_PMD_ORDER))
>  			goto out;
>  	}
> +	/* A larger folio was mapped; it will be skipped in next iteration */
> +	if (result == SCAN_PTE_MAPPED)
> +		goto decide_order;
> +
>  	if (result != SCAN_SUCCEED) {
>  
>  		/* Go to the next order. */
> @@ -1558,6 +1587,8 @@ static int hpage_collapse_scan_ptes(struct mm_struct *mm,
>  			goto out;
>  		goto maybe_mmap_lock;
>  	} else {
> +
> +decide_order:
>  		address = _address;
>  		pte = _pte;
>