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

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Message-ID: <34bc4f3c-8dfe-46ec-9f9b-358cdf4c37e3@lucifer.local>
Date:   Fri, 28 Apr 2023 18:42:41 +0100
From:   Lorenzo Stoakes <lstoakes@...il.com>
To:     Jason Gunthorpe <jgg@...dia.com>
Cc:     David Hildenbrand <david@...hat.com>,
        "Kirill A . Shutemov" <kirill@...temov.name>,
        Peter Xu <peterx@...hat.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>,
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        John Fastabend <john.fastabend@...il.com>,
        linux-fsdevel@...r.kernel.org, linux-perf-users@...r.kernel.org,
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        Oleg Nesterov <oleg@...hat.com>,
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        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 Fri, Apr 28, 2023 at 02:31:38PM -0300, Jason Gunthorpe wrote:
> On Fri, Apr 28, 2023 at 07:02:22PM +0200, David Hildenbrand wrote:
>
> > > No. VMA cannot get away before PTEs are unmapped and TLB is flushed. And
> > > TLB flushing is serialized against GUP_fast().
> >
> > The whole CONFIG_MMU_GATHER_RCU_TABLE_FREE handling makes the situation more
> > complicated.
>
> Yeah, you have to think of gup_fast as RCU with a hacky pre-RCU implementation
> on most architectures.
>
> We could make page->mapping safe under RCU, for instance.
>
> Jason

Does it really require a change though? I might be missing some details,
but afaict with interrupts disabled we should be ok to deref page->mapping
to check PageAnon and a_ops before handing back a page right?