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Date:   Fri, 28 Apr 2023 11:26:59 -0700
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Subject: Re: [PATCH v5] mm/gup: disallow GUP writing to file-backed mappings
 by default
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To:     David Hildenbrand <david@redhat.com>,
        Lorenzo Stoakes <lstoakes@gmail.com>
Cc:     Peter Xu <peterx@redhat.com>,
        "Kirill A . Shutemov" <kirill@shutemov.name>,
        Jason Gunthorpe <jgg@nvidia.com>, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org,
        Andrew Morton <akpm@linux-foundation.org>,
        Jens Axboe <axboe@kernel.dk>,
        Matthew Wilcox <willy@infradead.org>,
        Dennis Dalessandro <dennis.dalessandro@cornelisnetworks.com>,
        Leon Romanovsky <leon@kernel.org>,
        Christian Benvenuti <benve@cisco.com>,
        Nelson Escobar <neescoba@cisco.com>,
        Bernard Metzler <bmt@zurich.ibm.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Ingo Molnar <mingo@redhat.com>,
        Arnaldo Carvalho de Melo <acme@kernel.org>,
        Mark Rutland <mark.rutland@arm.com>,
        Alexander Shishkin <alexander.shishkin@linux.intel.com>,
        Jiri Olsa <jolsa@kernel.org>,
        Namhyung Kim <namhyung@kernel.org>,
        Ian Rogers <irogers@google.com>,
        Adrian Hunter <adrian.hunter@intel.com>,
        Bjorn Topel <bjorn@kernel.org>,
        Magnus Karlsson <magnus.karlsson@intel.com>,
        Maciej Fijalkowski <maciej.fijalkowski@intel.com>,
        Jonathan Lemon <jonathan.lemon@gmail.com>,
        "David S . Miller" <davem@davemloft.net>,
        Eric Dumazet <edumazet@google.com>,
        Jakub Kicinski <kuba@kernel.org>,
        Paolo Abeni <pabeni@redhat.com>,
        Christian Brauner <brauner@kernel.org>,
        Richard Cochran <richardcochran@gmail.com>,
        Alexei Starovoitov <ast@kernel.org>,
        Daniel Borkmann <daniel@iogearbox.net>,
        Jesper Dangaard Brouer <hawk@kernel.org>,
        John Fastabend <john.fastabend@gmail.com>,
        linux-fsdevel@vger.kernel.org, linux-perf-users@vger.kernel.org,
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        Oleg Nesterov <oleg@redhat.com>, Jan Kara <jack@suse.cz>,
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References: <077c4b21-8806-455f-be98-d7052a584259@lucifer.local>
 <62ec50da-5f73-559c-c4b3-bde4eb215e08@redhat.com>
 <6ddc7ac4-4091-632a-7b2c-df2005438ec4@redhat.com>
 <20230428160925.5medjfxkyvmzfyhq@box.shutemov.name>
 <39cc0f26-8fc2-79dd-2e84-62238d27fd98@redhat.com>
 <20230428162207.o3ejmcz7rzezpt6n@box.shutemov.name> <ZEv2196tk5yWvgW5@x1n>
 <173337c0-14f4-3246-15ff-7fbf03861c94@redhat.com>
 <40fc128f-1978-42db-b9c1-77ac3c2cebfe@lucifer.local>
 <3d7fcfab-e445-1dc7-f000-9fbe7bea04c0@redhat.com>
 <bd470e63-e2e0-4532-8aab-cffe326688b6@lucifer.local>
 <7e096879-4578-36df-4809-3b04f4c20587@redhat.com>
From:   John Hubbard <jhubbard@nvidia.com>
In-Reply-To: <7e096879-4578-36df-4809-3b04f4c20587@redhat.com>
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On 4/28/23 10:29, David Hildenbrand wrote:
...
>> Is that an issue with interrupts disabled though? Will block page tables being
>> removed and as Kirill says (sorry I maybe misinterpreted you) we should be ok.
> 
> Let's rule out page table freeing. If our VMA only spans a single page 
> and falls into the same PMD as another VMA, an munmap() would not even 
> free a single page table.
> 
> However, if unmapping a page (flushing the TLB) would imply an IPI as 
> Kirill said, we'd be fine. I recall that that's not the case for all 
> architectures, but I might be just wrong.
> 

Some architectures use IPIs, and others use an RCU-like approach instead.

thanks,
-- 
John Hubbard
NVIDIA