linux-kernel - Re: [PATCH v5] mm/gup: disallow GUP writing to file-backed mappings by default

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Date:   Fri, 28 Apr 2023 11:26:59 -0700
From:   John Hubbard <jhubbard@...dia.com>
To:     David Hildenbrand <david@...hat.com>,
        Lorenzo Stoakes <lstoakes@...il.com>
Cc:     Peter Xu <peterx@...hat.com>,
        "Kirill A . Shutemov" <kirill@...temov.name>,
        Jason Gunthorpe <jgg@...dia.com>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org,
        Andrew Morton <akpm@...ux-foundation.org>,
        Jens Axboe <axboe@...nel.dk>,
        Matthew Wilcox <willy@...radead.org>,
        Dennis Dalessandro <dennis.dalessandro@...nelisnetworks.com>,
        Leon Romanovsky <leon@...nel.org>,
        Christian Benvenuti <benve@...co.com>,
        Nelson Escobar <neescoba@...co.com>,
        Bernard Metzler <bmt@...ich.ibm.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>,
        Arnaldo Carvalho de Melo <acme@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        Jiri Olsa <jolsa@...nel.org>,
        Namhyung Kim <namhyung@...nel.org>,
        Ian Rogers <irogers@...gle.com>,
        Adrian Hunter <adrian.hunter@...el.com>,
        Bjorn Topel <bjorn@...nel.org>,
        Magnus Karlsson <magnus.karlsson@...el.com>,
        Maciej Fijalkowski <maciej.fijalkowski@...el.com>,
        Jonathan Lemon <jonathan.lemon@...il.com>,
        "David S . Miller" <davem@...emloft.net>,
        Eric Dumazet <edumazet@...gle.com>,
        Jakub Kicinski <kuba@...nel.org>,
        Paolo Abeni <pabeni@...hat.com>,
        Christian Brauner <brauner@...nel.org>,
        Richard Cochran <richardcochran@...il.com>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Jesper Dangaard Brouer <hawk@...nel.org>,
        John Fastabend <john.fastabend@...il.com>,
        linux-fsdevel@...r.kernel.org, linux-perf-users@...r.kernel.org,
        netdev@...r.kernel.org, bpf@...r.kernel.org,
        Oleg Nesterov <oleg@...hat.com>, Jan Kara <jack@...e.cz>,
        Pavel Begunkov <asml.silence@...il.com>,
        Mika Penttila <mpenttil@...hat.com>,
        David Howells <dhowells@...hat.com>,
        Christoph Hellwig <hch@....de>
Subject: Re: [PATCH v5] mm/gup: disallow GUP writing to file-backed mappings
 by default

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