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Date: Thu, 1 Feb 2024 13:42:08 +0530
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Subject: Re: [PATCH RFC v3 31/35] khugepaged: arm64: Don't collapse MTE
 enabled VMAs
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To: Alexandru Elisei <alexandru.elisei@arm.com>, catalin.marinas@arm.com,
 will@kernel.org, oliver.upton@linux.dev, maz@kernel.org,
 james.morse@arm.com, suzuki.poulose@arm.com, yuzenghui@huawei.com,
 arnd@arndb.de, akpm@linux-foundation.org, mingo@redhat.com,
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 mhiramat@kernel.org, rppt@kernel.org, hughd@google.com
Cc: pcc@google.com, steven.price@arm.com, vincenzo.frascino@arm.com,
 david@redhat.com, eugenis@google.com, kcc@google.com, hyesoo.yu@samsung.com,
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References: <20240125164256.4147-1-alexandru.elisei@arm.com>
 <20240125164256.4147-32-alexandru.elisei@arm.com>
From: Anshuman Khandual <anshuman.khandual@arm.com>
In-Reply-To: <20240125164256.4147-32-alexandru.elisei@arm.com>
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On 1/25/24 22:12, Alexandru Elisei wrote:
> copy_user_highpage() will do memory allocation if there are saved tags for
> the destination page, and the page is missing tag storage.
> 
> After commit a349d72fd9ef ("mm/pgtable: add rcu_read_lock() and
> rcu_read_unlock()s"), collapse_huge_page() calls
> __collapse_huge_page_copy() -> .. -> copy_user_highpage() with the RCU lock
> held, which means that copy_user_highpage() can only allocate memory using
> GFP_ATOMIC or equivalent.
> 
> Get around this by refusing to collapse pages into a transparent huge page
> if the VMA is MTE-enabled.

Makes sense when copy_user_highpage() will allocate memory for tag storage.

> 
> Signed-off-by: Alexandru Elisei <alexandru.elisei@arm.com>
> ---
> 
> Changes since rfc v2:
> 
> * New patch. I think an agreement on whether copy*_user_highpage() should be
> always allowed to sleep, or should not be allowed, would be useful.

This is a good question ! Even after preventing the collapse of MTE VMA here,
there still might be more paths where a sleeping (i.e memory allocating)
copy*_user_highpage() becomes problematic ?

> 
>  arch/arm64/include/asm/pgtable.h    | 3 +++
>  arch/arm64/kernel/mte_tag_storage.c | 5 +++++
>  include/linux/khugepaged.h          | 5 +++++
>  mm/khugepaged.c                     | 4 ++++
>  4 files changed, 17 insertions(+)
> 
> diff --git a/arch/arm64/include/asm/pgtable.h b/arch/arm64/include/asm/pgtable.h
> index 87ae59436162..d0473538c926 100644
> --- a/arch/arm64/include/asm/pgtable.h
> +++ b/arch/arm64/include/asm/pgtable.h
> @@ -1120,6 +1120,9 @@ static inline bool arch_alloc_cma(gfp_t gfp_mask)
>  	return true;
>  }
>  
> +bool arch_hugepage_vma_revalidate(struct vm_area_struct *vma, unsigned long address);
> +#define arch_hugepage_vma_revalidate arch_hugepage_vma_revalidate
> +
>  #endif /* CONFIG_ARM64_MTE_TAG_STORAGE */
>  #endif /* CONFIG_ARM64_MTE */
>  
> diff --git a/arch/arm64/kernel/mte_tag_storage.c b/arch/arm64/kernel/mte_tag_storage.c
> index ac7b9c9c585c..a99959b70573 100644
> --- a/arch/arm64/kernel/mte_tag_storage.c
> +++ b/arch/arm64/kernel/mte_tag_storage.c
> @@ -636,3 +636,8 @@ void arch_alloc_page(struct page *page, int order, gfp_t gfp)
>  	if (tag_storage_enabled() && alloc_requires_tag_storage(gfp))
>  		reserve_tag_storage(page, order, gfp);
>  }
> +
> +bool arch_hugepage_vma_revalidate(struct vm_area_struct *vma, unsigned long address)
> +{
> +	return !(vma->vm_flags & VM_MTE);
> +}
> diff --git a/include/linux/khugepaged.h b/include/linux/khugepaged.h
> index f68865e19b0b..461e4322dff2 100644
> --- a/include/linux/khugepaged.h
> +++ b/include/linux/khugepaged.h
> @@ -38,6 +38,11 @@ static inline void khugepaged_exit(struct mm_struct *mm)
>  	if (test_bit(MMF_VM_HUGEPAGE, &mm->flags))
>  		__khugepaged_exit(mm);
>  }
> +
> +#ifndef arch_hugepage_vma_revalidate
> +#define arch_hugepage_vma_revalidate(vma, address) 1

Please replace s/1/true as arch_hugepage_vma_revalidate() returns bool ?

> +#endif

Right, above construct is much better than __HAVE_ARCH_XXXX based one.

> +
>  #else /* CONFIG_TRANSPARENT_HUGEPAGE */
>  static inline void khugepaged_fork(struct mm_struct *mm, struct mm_struct *oldmm)
>  {
> diff --git a/mm/khugepaged.c b/mm/khugepaged.c
> index 2b219acb528e..cb9a9ddb4d86 100644
> --- a/mm/khugepaged.c
> +++ b/mm/khugepaged.c
> @@ -935,6 +935,10 @@ static int hugepage_vma_revalidate(struct mm_struct *mm, unsigned long address,
>  	 */
>  	if (expect_anon && (!(*vmap)->anon_vma || !vma_is_anonymous(*vmap)))
>  		return SCAN_PAGE_ANON;
> +
> +	if (!arch_hugepage_vma_revalidate(vma, address))
> +		return SCAN_VMA_CHECK;
> +
>  	return SCAN_SUCCEED;
>  }
>  

Otherwise this LGTM.