linux-kernel - Re: [PATCH v9 4/4] mm/madvise: optimize lazyfreeing with mTHP in madvise

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Date: Thu, 18 Apr 2024 14:03:05 +0200
From: David Hildenbrand <david@...hat.com>
To: Lance Yang <ioworker0@...il.com>, akpm@...ux-foundation.org
Cc: ryan.roberts@....com, 21cnbao@...il.com, mhocko@...e.com,
 fengwei.yin@...el.com, zokeefe@...gle.com, shy828301@...il.com,
 xiehuan09@...il.com, wangkefeng.wang@...wei.com, songmuchun@...edance.com,
 peterx@...hat.com, minchan@...nel.org, linux-mm@...ck.org,
 linux-kernel@...r.kernel.org
Subject: Re: [PATCH v9 4/4] mm/madvise: optimize lazyfreeing with mTHP in
 madvise_free

On 18.04.24 12:57, Lance Yang wrote:
> This patch optimizes lazyfreeing with PTE-mapped mTHP[1]
> (Inspired by David Hildenbrand[2]). We aim to avoid unnecessary folio
> splitting if the large folio is fully mapped within the target range.
> 
> If a large folio is locked or shared, or if we fail to split it, we just
> leave it in place and advance to the next PTE in the range. But note that
> the behavior is changed; previously, any failure of this sort would cause
> the entire operation to give up. As large folios become more common,
> sticking to the old way could result in wasted opportunities.
> 
> On an Intel I5 CPU, lazyfreeing a 1GiB VMA backed by PTE-mapped folios of
> the same size results in the following runtimes for madvise(MADV_FREE) in
> seconds (shorter is better):
> 
> Folio Size |   Old    |   New    | Change
> ------------------------------------------
>        4KiB | 0.590251 | 0.590259 |    0%
>       16KiB | 2.990447 | 0.185655 |  -94%
>       32KiB | 2.547831 | 0.104870 |  -95%
>       64KiB | 2.457796 | 0.052812 |  -97%
>      128KiB | 2.281034 | 0.032777 |  -99%
>      256KiB | 2.230387 | 0.017496 |  -99%
>      512KiB | 2.189106 | 0.010781 |  -99%
>     1024KiB | 2.183949 | 0.007753 |  -99%
>     2048KiB | 0.002799 | 0.002804 |    0%
> 
> [1] https://lkml.kernel.org/r/20231207161211.2374093-5-ryan.roberts@arm.com
> [2] https://lore.kernel.org/linux-mm/20240214204435.167852-1-david@redhat.com
> 
> Reviewed-by: Ryan Roberts <ryan.roberts@....com>
> Signed-off-by: Lance Yang <ioworker0@...il.com>
> ---
>   mm/madvise.c | 85 +++++++++++++++++++++++++++-------------------------
>   1 file changed, 44 insertions(+), 41 deletions(-)
> 
> diff --git a/mm/madvise.c b/mm/madvise.c
> index 4597a3568e7e..375ab3234603 100644
> --- a/mm/madvise.c
> +++ b/mm/madvise.c
> @@ -643,6 +643,7 @@ static int madvise_free_pte_range(pmd_t *pmd, unsigned long addr,
>   				unsigned long end, struct mm_walk *walk)
>   
>   {
> +	const cydp_t cydp_flags = CYDP_CLEAR_YOUNG | CYDP_CLEAR_DIRTY;
>   	struct mmu_gather *tlb = walk->private;
>   	struct mm_struct *mm = tlb->mm;
>   	struct vm_area_struct *vma = walk->vma;
> @@ -697,44 +698,57 @@ static int madvise_free_pte_range(pmd_t *pmd, unsigned long addr,
>   			continue;
>   
>   		/*
> -		 * If pmd isn't transhuge but the folio is large and
> -		 * is owned by only this process, split it and
> -		 * deactivate all pages.
> +		 * If we encounter a large folio, only split it if it is not
> +		 * fully mapped within the range we are operating on. Otherwise
> +		 * leave it as is so that it can be marked as lazyfree. If we
> +		 * fail to split a folio, leave it in place and advance to the
> +		 * next pte in the range.
>   		 */
>   		if (folio_test_large(folio)) {
> -			int err;
> +			bool any_young, any_dirty;
>   
> -			if (folio_likely_mapped_shared(folio))
> -				break;
> -			if (!folio_trylock(folio))
> -				break;
> -			folio_get(folio);
> -			arch_leave_lazy_mmu_mode();
> -			pte_unmap_unlock(start_pte, ptl);
> -			start_pte = NULL;
> -			err = split_folio(folio);
> -			folio_unlock(folio);
> -			folio_put(folio);
> -			if (err)
> -				break;
> -			start_pte = pte =
> -				pte_offset_map_lock(mm, pmd, addr, &ptl);
> -			if (!start_pte)
> -				break;
> -			arch_enter_lazy_mmu_mode();
> -			pte--;
> -			addr -= PAGE_SIZE;
> -			continue;
> +			nr = madvise_folio_pte_batch(addr, end, folio, pte,
> +						     ptent, &any_young, NULL);
> +
> +			if (nr < folio_nr_pages(folio)) {
> +				int err;
> +
> +				if (folio_likely_mapped_shared(folio))
> +					continue;
> +				if (!folio_trylock(folio))
> +					continue;
> +				folio_get(folio);
> +				arch_leave_lazy_mmu_mode();
> +				pte_unmap_unlock(start_pte, ptl);
> +				start_pte = NULL;
> +				err = split_folio(folio);
> +				folio_unlock(folio);
> +				folio_put(folio);
> +				start_pte = pte =
> +					pte_offset_map_lock(mm, pmd, addr, &ptl);

I'd just put it on a single line.

> +				if (!start_pte)
> +					break;
> +				arch_enter_lazy_mmu_mode();
> +				if (!err)
> +					nr = 0;
> +				continue;
> +			}
> +
> +			if (any_young)
> +				ptent = pte_mkyoung(ptent);
> +			if (any_dirty)

any_dirty is never set, likely missed to pass it to 
madvise_folio_pte_batch().

Apart from that LGTM and this patch is much easier to review now!


With above:

Acked-by: David Hildenbrand <david@...hat.com>

-- 
Cheers,

David / dhildenb