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Date:   Thu, 22 Dec 2022 16:47:21 -0500
From:   Peter Xu <peterx@redhat.com>
To:     James Houghton <jthoughton@google.com>
Cc:     Mike Kravetz <mike.kravetz@oracle.com>,
        Muchun Song <songmuchun@bytedance.com>,
        David Hildenbrand <david@redhat.com>,
        David Rientjes <rientjes@google.com>,
        Axel Rasmussen <axelrasmussen@google.com>,
        Mina Almasry <almasrymina@google.com>,
        Zach O'Keefe <zokeefe@google.com>,
        Manish Mishra <manish.mishra@nutanix.com>,
        Naoya Horiguchi <naoya.horiguchi@nec.com>,
        "Dr . David Alan Gilbert" <dgilbert@redhat.com>,
        "Matthew Wilcox (Oracle)" <willy@infradead.org>,
        Vlastimil Babka <vbabka@suse.cz>,
        Baolin Wang <baolin.wang@linux.alibaba.com>,
        Miaohe Lin <linmiaohe@huawei.com>,
        Yang Shi <shy828301@gmail.com>,
        Andrew Morton <akpm@linux-foundation.org>, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org
Subject: Re: [RFC PATCH v2 35/47] userfaultfd: require
 UFFD_FEATURE_EXACT_ADDRESS when using HugeTLB HGM
Message-ID: <Y6TQaZaB+PKObfNs@x1n>
References: <20221021163703.3218176-1-jthoughton@google.com>
 <20221021163703.3218176-36-jthoughton@google.com>
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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