linux-kernel - Re: [PATCH v8 3/3] mm/madvise: optimize lazyfreeing with mTHP in madvise

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Date: Wed, 17 Apr 2024 19:05:49 +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 v8 3/3] mm/madvise: optimize lazyfreeing with mTHP in
 madvise_free

On 17.04.24 16:14, 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
> 
> Signed-off-by: Lance Yang <ioworker0@...il.com>

Some of the changes could have been moved into separate patches to ease 
review ;)

At least the folio_pte_batch() change and factoring out some stuff from 
madvise_cold_or_pageout_pte_range(). But see below on the latter.

> ---
>   mm/internal.h |  12 ++++-
>   mm/madvise.c  | 141 ++++++++++++++++++++++++++++----------------------
>   mm/memory.c   |   4 +-
>   3 files changed, 91 insertions(+), 66 deletions(-)

[...]

> diff --git a/mm/madvise.c b/mm/madvise.c
> index f5e3699e7b54..d6f1889d6308 100644
> --- a/mm/madvise.c
> +++ b/mm/madvise.c
> @@ -321,6 +321,39 @@ static inline bool can_do_file_pageout(struct vm_area_struct *vma)
>   	       file_permission(vma->vm_file, MAY_WRITE) == 0;
>   }
>   
> +static inline int madvise_folio_pte_batch(unsigned long addr, unsigned long end,
> +					  struct folio *folio, pte_t *ptep,
> +					  pte_t pte, bool *any_young,
> +					  bool *any_dirty)
> +{
> +	int max_nr = (end - addr) / PAGE_SIZE;
> +	const fpb_t fpb_flags = FPB_IGNORE_DIRTY | FPB_IGNORE_SOFT_DIRTY;

Reverse Christmas tree looks nicer ;)

> +
> +	return folio_pte_batch(folio, addr, ptep, pte, max_nr, fpb_flags, NULL,
> +			       any_young, any_dirty);
> +}
> +
> +static inline bool madvise_pte_split_folio(struct mm_struct *mm, pmd_t *pmd,
> +					   unsigned long addr,
> +					   struct folio *folio, pte_t **pte,
> +					   spinlock_t **ptl)
> +{
> +	int err;
> +
> +	if (!folio_trylock(folio))
> +		return false;
> +
> +	folio_get(folio);
> +	pte_unmap_unlock(*pte, *ptl);
> +	err = split_folio(folio);
> +	folio_unlock(folio);
> +	folio_put(folio);
> +
> +	*pte = pte_offset_map_lock(mm, pmd, addr, ptl);

Staring at this helper again, I am really not sure if we should have it. 
Calling semantics are "special" and that pte_t **pte is just ... 
"special" as well ;)

Can we just leave that part as is, in the caller? That would also mean 
less madvise_cold_or_pageout_pte_range() churn ... which i would welcome 
as part of this patch.

[...]

> @@ -741,19 +767,10 @@ static int madvise_free_pte_range(pmd_t *pmd, unsigned long addr,
>   		}
>   
>   		if (pte_young(ptent) || pte_dirty(ptent)) {
> -			/*
> -			 * Some of architecture(ex, PPC) don't update TLB
> -			 * with set_pte_at and tlb_remove_tlb_entry so for
> -			 * the portability, remap the pte with old|clean
> -			 * after pte clearing.
> -			 */
> -			ptent = ptep_get_and_clear_full(mm, addr, pte,
> -							tlb->fullmm);
> -
> -			ptent = pte_mkold(ptent);
> -			ptent = pte_mkclean(ptent);
> -			set_pte_at(mm, addr, pte, ptent);
> -			tlb_remove_tlb_entry(tlb, pte, addr);
> +			clear_young_dirty_ptes(vma, addr, pte, nr,
> +					       CYDP_CLEAR_YOUNG |
> +						       CYDP_CLEAR_DIRTY);

That indent looks odd. I suggest simply having a local variable

const cydp_t cydp_flags = CYDP_CLEAR_YOUNG | CYDP_CLEAR_DIRTY;

and then use cydp_flags here that will make this easier to read.

-- 
Cheers,

David / dhildenb