linux-kernel - Re: [RFC PATCH v2 35/47] userfaultfd: require UFFD_FEATURE_EXACT

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Message-ID: <Y6TQaZaB+PKObfNs@x1n>
Date:   Thu, 22 Dec 2022 16:47:21 -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: [RFC PATCH v2 35/47] userfaultfd: require
 UFFD_FEATURE_EXACT_ADDRESS when using HugeTLB HGM

On Fri, Oct 21, 2022 at 04:36:51PM +0000, James Houghton wrote:
> @@ -1990,6 +1990,17 @@ static int userfaultfd_api(struct userfaultfd_ctx *ctx,
>  		~(UFFD_FEATURE_MINOR_HUGETLBFS | UFFD_FEATURE_MINOR_SHMEM);
>  #ifndef CONFIG_HUGETLB_HIGH_GRANULARITY_MAPPING
>  	uffdio_api.features &= ~UFFD_FEATURE_MINOR_HUGETLBFS_HGM;
> +#else
> +
> +	ret = -EINVAL;
> +	if ((uffdio_api.features & UFFD_FEATURE_MINOR_HUGETLBFS_HGM) &&
> +	    !(uffdio_api.features & UFFD_FEATURE_EXACT_ADDRESS))

This check needs to be done upon "features" or "ctx_features", rather than
"uffdio_api.features".  The latter is the one we'll report to the user only.

> +		/*
> +		 * UFFD_FEATURE_MINOR_HUGETLBFS_HGM is mostly
> +		 * useless without UFFD_FEATURE_EXACT_ADDRESS,
> +		 * so require userspace to provide both.
> +		 */
> +		goto err_out;
>  #endif  /* CONFIG_HUGETLB_HIGH_GRANULARITY_MAPPING */
>  #endif  /* CONFIG_HAVE_ARCH_USERFAULTFD_MINOR */
>  
> -- 
> 2.38.0.135.g90850a2211-goog
> 
> 

-- 
Peter Xu