linux-kernel - Re: [PATCH v3 2/2] mm: Allocate THP on hugezeropage wp-fault

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Message-ID: <783a0d91-2910-4446-a979-c681dde402ec@redhat.com>
Date: Wed, 11 Sep 2024 11:36:33 +0200
From: David Hildenbrand <david@...hat.com>
To: Dev Jain <dev.jain@....com>, akpm@...ux-foundation.org,
 willy@...radead.org, kirill.shutemov@...ux.intel.com
Cc: ryan.roberts@....com, anshuman.khandual@....com, catalin.marinas@....com,
 cl@...two.org, vbabka@...e.cz, mhocko@...e.com, apopple@...dia.com,
 dave.hansen@...ux.intel.com, will@...nel.org, baohua@...nel.org,
 jack@...e.cz, mark.rutland@....com, hughd@...gle.com,
 aneesh.kumar@...nel.org, yang@...amperecomputing.com, peterx@...hat.com,
 ioworker0@...il.com, jglisse@...gle.com, wangkefeng.wang@...wei.com,
 ziy@...dia.com, linux-arm-kernel@...ts.infradead.org,
 linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [PATCH v3 2/2] mm: Allocate THP on hugezeropage wp-fault

On 11.09.24 08:56, Dev Jain wrote:
> Introduce do_huge_zero_wp_pmd() to handle wp-fault on a hugezeropage and
> replace it with a PMD-mapped THP. Change the helper introduced in the
> previous patch to flush TLB entry corresponding to the hugezeropage.
> In case of failure, fallback to splitting the PMD.
> 
> Signed-off-by: Dev Jain <dev.jain@....com>
> ---
>   mm/huge_memory.c | 52 +++++++++++++++++++++++++++++++++++++++++++++---
>   1 file changed, 49 insertions(+), 3 deletions(-)
> 
> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> index b96a1ff2bf40..3e28946a805f 100644
> --- a/mm/huge_memory.c
> +++ b/mm/huge_memory.c
> @@ -987,16 +987,20 @@ static void __pmd_thp_fault_success_stats(struct vm_area_struct *vma)
>   static void map_pmd_thp(struct folio *folio, struct vm_fault *vmf,
>   			struct vm_area_struct *vma, unsigned long haddr)
>   {
> -	pmd_t entry;
> +	pmd_t entry, old_pmd;
> +	bool is_pmd_none = pmd_none(*vmf->pmd);
>   
>   	entry = mk_huge_pmd(&folio->page, vma->vm_page_prot);
>   	entry = maybe_pmd_mkwrite(pmd_mkdirty(entry), vma);
>   	folio_add_new_anon_rmap(folio, vma, haddr, RMAP_EXCLUSIVE);
>   	folio_add_lru_vma(folio, vma);
> +	if (!is_pmd_none)
> +		old_pmd = pmdp_huge_clear_flush(vma, haddr, vmf->pmd);

This should likely be done in the caller.

>   	set_pmd_at(vma->vm_mm, haddr, vmf->pmd, entry);
>   	update_mmu_cache_pmd(vma, vmf->address, vmf->pmd);
>   	add_mm_counter(vma->vm_mm, MM_ANONPAGES, HPAGE_PMD_NR);
> -	mm_inc_nr_ptes(vma->vm_mm);
> +	if (is_pmd_none)
> +		mm_inc_nr_ptes(vma->vm_mm);

And this as well.

No need to make this function deal with this if the callers exactly know 
what they are doing.

>   }
>   
>   static vm_fault_t __do_huge_pmd_anonymous_page(struct vm_fault *vmf)
> @@ -1576,6 +1580,41 @@ void huge_pmd_set_accessed(struct vm_fault *vmf)
>   	spin_unlock(vmf->ptl);
>   }
>   
> +static vm_fault_t do_huge_zero_wp_pmd(struct vm_fault *vmf, unsigned long haddr)

Is there a need to pass in "haddr" if we have the vmf?

> +{
> +	struct vm_area_struct *vma = vmf->vma;
> +	gfp_t gfp = vma_thp_gfp_mask(vma);
> +	struct mmu_notifier_range range;
> +	struct folio *folio;
> +	vm_fault_t ret = 0;
> +
> +	folio = pmd_thp_fault_alloc(gfp, vma, haddr, vmf->address);
> +	if (unlikely(!folio)) {
> +		ret = VM_FAULT_FALLBACK;
> +		goto out;
> +	}
> +
> +	mmu_notifier_range_init(&range, MMU_NOTIFY_CLEAR, 0, vma->vm_mm, haddr,
> +				haddr + HPAGE_PMD_SIZE);
> +	mmu_notifier_invalidate_range_start(&range);
> +	vmf->ptl = pmd_lock(vma->vm_mm, vmf->pmd);
> +	if (unlikely(!pmd_same(pmdp_get(vmf->pmd), vmf->orig_pmd)))
> +		goto release;
> +	ret = check_stable_address_space(vma->vm_mm);
> +	if (ret)
> +		goto release;

The clear+flush really belongs here.

> +	map_pmd_thp(folio, vmf, vma, haddr);
> +	__pmd_thp_fault_success_stats(vma);
> +	goto unlock;
> +release:
> +	folio_put(folio);
> +unlock:
> +	spin_unlock(vmf->ptl);
> +	mmu_notifier_invalidate_range_end(&range);
> +out:
> +	return ret;
> +}
> +

-- 
Cheers,

David / dhildenb