linux-kernel - Re: [PATCH 1/2] mm/madvise: allow process_madvise operations on entire memory range

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Message-ID: <20201222134438.GA7170@infradead.org>
Date:   Tue, 22 Dec 2020 13:44:38 +0000
From:   Christoph Hellwig <hch@...radead.org>
To:     Jann Horn <jannh@...gle.com>
Cc:     Minchan Kim <minchan@...nel.org>, Christoph Hellwig <hch@....de>,
        Suren Baghdasaryan <surenb@...gle.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Michal Hocko <mhocko@...nel.org>,
        Michal Hocko <mhocko@...e.com>,
        David Rientjes <rientjes@...gle.com>,
        Matthew Wilcox <willy@...radead.org>,
        Johannes Weiner <hannes@...xchg.org>,
        Roman Gushchin <guro@...com>, Rik van Riel <riel@...riel.com>,
        Christian Brauner <christian@...uner.io>,
        Oleg Nesterov <oleg@...hat.com>,
        Tim Murray <timmurray@...gle.com>,
        Linux API <linux-api@...r.kernel.org>,
        Linux-MM <linux-mm@...ck.org>,
        kernel list <linux-kernel@...r.kernel.org>,
        kernel-team <kernel-team@...roid.com>
Subject: Re: [PATCH 1/2] mm/madvise: allow process_madvise operations on
 entire memory range

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?