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[70.27.3.10]) by smtp.gmail.com with ESMTPSA id s24-20020a02cf38000000b0034339c2dd5fsm1464329jar.91.2022.08.10.12.52.38 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Wed, 10 Aug 2022 12:52:39 -0700 (PDT) Date: Wed, 10 Aug 2022 15:52:38 -0400 From: Peter Xu To: David Hildenbrand Cc: linux-kernel@vger.kernel.org, linux-mm@kvack.org, Andrew Morton , Mike Kravetz , Muchun Song , Peter Feiner , "Kirill A . Shutemov" Subject: Re: [PATCH v1 2/2] mm/hugetlb: support write-faults in shared mappings Message-ID: References: <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> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline In-Reply-To: Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, Aug 10, 2022 at 09:40:11PM +0200, David Hildenbrand wrote: > 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. But this specific warning is under the VM_MAYSHARE if clause just added in this patch? My understanding is any FOLL_FORCE will always constantly fail before this patch, and it should keep failing as usual and I don't see any case it'll be failing at the warn_on_once here. So again, I'm fine with having the warning, but I just want to make sure what you want to capture is what you expected.. > > > > 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 ..." Sure, no strong opinion. I'll leave that to Mike. Thanks, -- Peter Xu