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(p200300cbc7071600a3ceb459ef577b93.dip0.t-ipconnect.de. [2003:cb:c707:1600:a3ce:b459:ef57:7b93]) by smtp.gmail.com with ESMTPSA id t24-20020a1c7718000000b003a501ad8648sm3192271wmi.40.2022.08.10.12.40.11 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Wed, 10 Aug 2022 12:40:12 -0700 (PDT) Message-ID: Date: Wed, 10 Aug 2022 21:40:11 +0200 MIME-Version: 1.0 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:91.0) Gecko/20100101 Thunderbird/91.11.0 Subject: Re: [PATCH v1 2/2] mm/hugetlb: support write-faults in shared mappings Content-Language: en-US To: Peter Xu Cc: linux-kernel@vger.kernel.org, linux-mm@kvack.org, Andrew Morton , Mike Kravetz , Muchun Song , Peter Feiner , "Kirill A . Shutemov" References: <20220805110329.80540-1-david@redhat.com> <20220805110329.80540-3-david@redhat.com> <4f644ac5-c40b-32d4-3234-c1dac3d09f83@redhat.com> <8b317ac7-f80e-4aab-4ad1-4e19a1a0740f@redhat.com> <12c65d91-5fc0-cb2e-c415-2b3447960b43@redhat.com> From: David Hildenbrand Organization: Red Hat In-Reply-To: Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 10.08.22 21:29, Peter Xu wrote: > On Wed, Aug 10, 2022 at 11:37:13AM +0200, David Hildenbrand wrote: >> On 09.08.22 00:08, Peter Xu wrote: >>> On Mon, Aug 08, 2022 at 04:21:39PM -0400, Peter Xu wrote: >>>> On Mon, Aug 08, 2022 at 06:25:21PM +0200, David Hildenbrand wrote: >>>>>>> Relying on VM_SHARED to detect MAP_PRIVATE vs. MAP_SHARED is >>>>>>> unfortunately wrong. >>>>>>> >>>>>>> If you're curious, take a look at f83a275dbc5c ("mm: account for >>>>>>> MAP_SHARED mappings using VM_MAYSHARE and not VM_SHARED in hugetlbfs") >>>>>>> and mmap() code. >>>>>>> >>>>>>> Long story short: if the file is read-only, we only have VM_MAYSHARE but >>>>>>> not VM_SHARED (and consequently also not VM_MAYWRITE). >>>>>> >>>>>> To ask in another way: if file is RO but mapped RW (mmap() will have >>>>>> VM_SHARED cleared but VM_MAYSHARE set), then if we check VM_MAYSHARE here >>>>>> won't we grant write bit errornously while we shouldn't? As the user >>>>>> doesn't really have write permission to the file. >>>>> >>>>> Thus the VM_WRITE check. :) >>>>> >>>>> I wonder if we should just do it cleanly and introduce the maybe_mkwrite >>>>> semantics here as well. Then there is no need for additional VM_WRITE >>>>> checks and hugetlb will work just like !hugetlb. >>>> >>>> Hmm yeah I think the VM_MAYSHARE check is correct, since we'll need to fail >>>> the cases where MAYSHARE && !SHARE - we used to silently let it pass. >>> >>> Sorry I think this is a wrong statement I made.. IIUC we'll fail correctly >>> with/without the patch on any write to hugetlb RO regions. >>> >>> Then I just don't see a difference on checking VM_SHARED or VM_MAYSHARE >>> here, it's just that VM_MAYSHARE check should work too like VM_SHARED so I >>> don't see a problem. >>> >>> It also means I can't think of any valid case of having VM_WRITE when >>> reaching here, then the WARN_ON_ONCE() is okay but maybe also redundant. >>> Using maybe_mkwrite() seems misleading to me if FOLL_FORCE not ready for >>> hugetlbfs after all. >>> >> >> The main reason we'd have it would be to scream out lout and fail >> gracefully if someone would -- for example -- use it for something like >> FOLL_FORCE. > > Having that WARN_ON_ONCE() is okay to me, but just to double check we're on > the same page: why there's concern on using FOLL_FORCE? IIUC we're talking > about shared mappings here, then no FOLL_FORCE possible at all? IOW, > "!is_cow_mapping()" should fail in check_vma_flags() already. This code path also covers the anon case. > > The other thing is I'm wondering whether patch 2 should be postponed anyway > so that we can wait for a full resolution of the problem from Mike. To make the code more robust and avoid any other such surprises I prefer to have this in rather earlier than later. As the commit says "Let's add a safety net ..." -- Thanks, David / dhildenb