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Date: Thu, 1 Feb 2024 17:38:42 +0000
From: Alexandru Elisei <alexandru.elisei@arm.com>
To: Anshuman Khandual <anshuman.khandual@arm.com>
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Subject: Re: [PATCH RFC v3 31/35] khugepaged: arm64: Don't collapse MTE
 enabled VMAs
Message-ID: <ZbvXImzAJNKQvamJ@raptor>
References: <20240125164256.4147-1-alexandru.elisei@arm.com>
 <20240125164256.4147-32-alexandru.elisei@arm.com>
 <599769c3-0aef-4c5b-ac98-f109649862f7@arm.com>
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In-Reply-To: <599769c3-0aef-4c5b-ac98-f109649862f7@arm.com>

On Thu, Feb 01, 2024 at 01:42:08PM +0530, Anshuman Khandual wrote:
> 
> 
> 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 ?

Exactly!

> 
> > 
> >  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 ?

Yeah, that's strange, I don't know why I used 1 there. Will change it to true,
thanks for spotting it.

> 
> > +#endif
> 
> Right, above construct is much better than __HAVE_ARCH_XXXX based one.

Thanks!

Alex

> 
> > +
> >  #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.