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From:   Suren Baghdasaryan <surenb@google.com>
Date:   Tue, 22 Dec 2020 09:48:43 -0800
Message-ID: <CAJuCfpGiVS69kznSrAdosxnRd-zgXPJd8MXou=gd8K8j7xLMjA@mail.gmail.com>
Subject: Re: [PATCH 1/2] mm/madvise: allow process_madvise operations on
 entire memory range
To:     Christoph Hellwig <hch@infradead.org>
Cc:     Jann Horn <jannh@google.com>, Minchan Kim <minchan@kernel.org>,
        Christoph Hellwig <hch@lst.de>,
        Andrew Morton <akpm@linux-foundation.org>,
        Michal Hocko <mhocko@kernel.org>,
        Michal Hocko <mhocko@suse.com>,
        David Rientjes <rientjes@google.com>,
        Matthew Wilcox <willy@infradead.org>,
        Johannes Weiner <hannes@cmpxchg.org>,
        Roman Gushchin <guro@fb.com>, Rik van Riel <riel@surriel.com>,
        Christian Brauner <christian@brauner.io>,
        Oleg Nesterov <oleg@redhat.com>,
        Tim Murray <timmurray@google.com>,
        Linux API <linux-api@vger.kernel.org>,
        Linux-MM <linux-mm@kvack.org>,
        kernel list <linux-kernel@vger.kernel.org>,
        kernel-team <kernel-team@android.com>
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On Tue, Dec 22, 2020 at 5:44 AM Christoph Hellwig <hch@infradead.org> wrote:
>
> On Fri, Dec 11, 2020 at 09:27:46PM +0100, Jann Horn wrote:
> > > Can we just use one element in iovec to indicate entire address rather
> > > than using up the reserved flags?
> > >
> > >         struct iovec {
> > >                 .iov_base = NULL,
> > >                 .iov_len = (~(size_t)0),
> > >         };
> >
> > In addition to Suren's objections, I think it's also worth considering
> > how this looks in terms of compat API. If a compat process does
> > process_madvise() on another compat process, it would be specifying
> > the maximum 32-bit number, rather than the maximum 64-bit number, so
> > you'd need special code to catch that case, which would be ugly.
> >
> > And when a compat process uses this API on a non-compat process, it
> > semantically gets really weird: The actual address range covered would
> > be larger than the address range specified.
> >
> > And if we want different access checks for the two flavors in the
> > future, gating that different behavior on special values in the iovec
> > would feel too magical to me.
> >
> > And the length value SIZE_MAX doesn't really make sense anyway because
> > the length of the whole address space would be SIZE_MAX+1, which you
> > can't express.
> >
> > So I'm in favor of a new flag, and strongly against using SIZE_MAX as
> > a magic number here.
>
> Yes, using SIZE_MAX is a horrible interface in this case.  I'm not
> a huge fan of a flag either.  What is the use case for the madvise
> to all of a processes address space anyway?

Thanks for the feedback! The use case is userspace memory reaping
similar to oom-reaper. Detailed justification is here:
https://lore.kernel.org/linux-mm/20201124053943.1684874-1-surenb@google.com