linux-kernel - Re: [PATCH 07/19] mm/fork: Accept huge pfnmap entries

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Message-ID: <d7fcec73-16f6-4d54-b334-6450a29e0a1d@redhat.com>
Date: Fri, 9 Aug 2024 18:32:44 +0200
From: David Hildenbrand <david@...hat.com>
To: Peter Xu <peterx@...hat.com>, linux-mm@...ck.org,
 linux-kernel@...r.kernel.org
Cc: Sean Christopherson <seanjc@...gle.com>,
 Oscar Salvador <osalvador@...e.de>, Jason Gunthorpe <jgg@...dia.com>,
 Axel Rasmussen <axelrasmussen@...gle.com>,
 linux-arm-kernel@...ts.infradead.org, x86@...nel.org,
 Will Deacon <will@...nel.org>, Gavin Shan <gshan@...hat.com>,
 Paolo Bonzini <pbonzini@...hat.com>, Zi Yan <ziy@...dia.com>,
 Andrew Morton <akpm@...ux-foundation.org>,
 Catalin Marinas <catalin.marinas@....com>, Ingo Molnar <mingo@...hat.com>,
 Alistair Popple <apopple@...dia.com>, Borislav Petkov <bp@...en8.de>,
 Thomas Gleixner <tglx@...utronix.de>, kvm@...r.kernel.org,
 Dave Hansen <dave.hansen@...ux.intel.com>,
 Alex Williamson <alex.williamson@...hat.com>, Yan Zhao <yan.y.zhao@...el.com>
Subject: Re: [PATCH 07/19] mm/fork: Accept huge pfnmap entries

On 09.08.24 18:08, Peter Xu wrote:
> Teach the fork code to properly copy pfnmaps for pmd/pud levels.  Pud is
> much easier, the write bit needs to be persisted though for writable and
> shared pud mappings like PFNMAP ones, otherwise a follow up write in either
> parent or child process will trigger a write fault.
> 
> Do the same for pmd level.
> 
> Signed-off-by: Peter Xu <peterx@...hat.com>
> ---
>   mm/huge_memory.c | 27 ++++++++++++++++++++++++---
>   1 file changed, 24 insertions(+), 3 deletions(-)
> 
> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> index 6568586b21ab..015c9468eed5 100644
> --- a/mm/huge_memory.c
> +++ b/mm/huge_memory.c
> @@ -1375,6 +1375,22 @@ int copy_huge_pmd(struct mm_struct *dst_mm, struct mm_struct *src_mm,
>   	pgtable_t pgtable = NULL;
>   	int ret = -ENOMEM;
>   
> +	pmd = pmdp_get_lockless(src_pmd);
> +	if (unlikely(pmd_special(pmd))) {
> +		dst_ptl = pmd_lock(dst_mm, dst_pmd);
> +		src_ptl = pmd_lockptr(src_mm, src_pmd);
> +		spin_lock_nested(src_ptl, SINGLE_DEPTH_NESTING);
> +		/*
> +		 * No need to recheck the pmd, it can't change with write
> +		 * mmap lock held here.
> +		 */
> +		if (is_cow_mapping(src_vma->vm_flags) && pmd_write(pmd)) {
> +			pmdp_set_wrprotect(src_mm, addr, src_pmd);
> +			pmd = pmd_wrprotect(pmd);
> +		}
> +		goto set_pmd;
> +	}
> +

I strongly assume we should be using using vm_normal_page_pmd() instead 
of pmd_page() further below. pmd_special() should be mostly limited to 
GUP-fast and vm_normal_page_pmd().

Again, we should be doing this similar to how we handle PTEs.

I'm a bit confused about the "unlikely(!pmd_trans_huge(pmd)" check, 
below: what else should we have here if it's not a migration entry but a 
present entry?

Likely this function needs a bit of rework.

-- 
Cheers,

David / dhildenb