linux-kernel - Re: [PATCH 34/46] hugetlb: userfaultfd: when using MADV_SPLIT, round addresses to PAGE

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Date:   Fri, 6 Jan 2023 10:13:14 -0500
From:   Peter Xu <peterx@...hat.com>
To:     James Houghton <jthoughton@...gle.com>
Cc:     Mike Kravetz <mike.kravetz@...cle.com>,
        Muchun Song <songmuchun@...edance.com>,
        David Hildenbrand <david@...hat.com>,
        David Rientjes <rientjes@...gle.com>,
        Axel Rasmussen <axelrasmussen@...gle.com>,
        Mina Almasry <almasrymina@...gle.com>,
        Zach O'Keefe <zokeefe@...gle.com>,
        Manish Mishra <manish.mishra@...anix.com>,
        Naoya Horiguchi <naoya.horiguchi@....com>,
        "Dr . David Alan Gilbert" <dgilbert@...hat.com>,
        "Matthew Wilcox (Oracle)" <willy@...radead.org>,
        Vlastimil Babka <vbabka@...e.cz>,
        Baolin Wang <baolin.wang@...ux.alibaba.com>,
        Miaohe Lin <linmiaohe@...wei.com>,
        Yang Shi <shy828301@...il.com>,
        Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 34/46] hugetlb: userfaultfd: when using MADV_SPLIT, round
 addresses to PAGE_SIZE

On Thu, Jan 05, 2023 at 10:18:32AM +0000, James Houghton wrote:
> MADV_SPLIT enables HugeTLB HGM which allows for UFFDIO_CONTINUE in
> PAGE_SIZE chunks. If a huge-page-aligned address were to be provided,
> userspace would be completely unable to take advantage of HGM. That
> would then require userspace to know to provide
> UFFD_FEATURE_EXACT_ADDRESS.
> 
> This patch would make it harder to make a mistake. Instead of requiring
> userspace to provide UFFD_FEATURE_EXACT_ADDRESS, always provide a usable
> address.
> 
> Signed-off-by: James Houghton <jthoughton@...gle.com>
> ---
>  mm/hugetlb.c | 31 +++++++++++++++----------------
>  1 file changed, 15 insertions(+), 16 deletions(-)
> 
> diff --git a/mm/hugetlb.c b/mm/hugetlb.c
> index 5af6db52f34e..5b6215e03fe1 100644
> --- a/mm/hugetlb.c
> +++ b/mm/hugetlb.c
> @@ -5936,28 +5936,27 @@ static inline vm_fault_t hugetlb_handle_userfault(struct vm_area_struct *vma,
>  						  unsigned long addr,
>  						  unsigned long reason)
>  {
> +	u32 hash;
> +	struct vm_fault vmf;
> +
>  	/*
>  	 * Don't use the hpage-aligned address if the user has explicitly
>  	 * enabled HGM.
>  	 */
>  	if (hugetlb_hgm_advised(vma) && reason == VM_UFFD_MINOR)
> -		haddr = address & PAGE_MASK;
> -
> -	u32 hash;
> -	struct vm_fault vmf = {
> -		.vma = vma,
> -		.address = haddr,
> -		.real_address = addr,
> -		.flags = flags,
> +		haddr = addr & PAGE_MASK;
>  
> -		/*
> -		 * Hard to debug if it ends up being
> -		 * used by a callee that assumes
> -		 * something about the other
> -		 * uninitialized fields... same as in
> -		 * memory.c
> -		 */
> -	};
> +	vmf.vma = vma;
> +	vmf.address = haddr;
> +	vmf.real_address = addr;
> +	vmf.flags = flags;

Const fields here:

mm/hugetlb.c: In function ‘hugetlb_handle_userfault’:
mm/hugetlb.c:5961:17: error: assignment of member ‘vma’ in read-only object
 5961 |         vmf.vma = vma;
      |                 ^ 
mm/hugetlb.c:5962:21: error: assignment of member ‘address’ in read-only object
 5962 |         vmf.address = haddr;   
      |                     ^      
mm/hugetlb.c:5963:26: error: assignment of member ‘real_address’ in read-only object                                                                                                          
 5963 |         vmf.real_address = addr;

> +	/*
> +	 * Hard to debug if it ends up being
> +	 * used by a callee that assumes
> +	 * something about the other
> +	 * uninitialized fields... same as in
> +	 * memory.c
> +	 */

PS: I think we can drop this along the way.

>  
>  	/*
>  	 * vma_lock and hugetlb_fault_mutex must be dropped before handling
> -- 
> 2.39.0.314.g84b9a713c41-goog
> 

-- 
Peter Xu