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

[Jann Horn <jannh@google.com>]

On Sat, Dec 12, 2020 at 12:01 AM Minchan Kim <minchan@kernel.org> wrote:
> On Fri, Dec 11, 2020 at 09:27:46PM +0100, Jann Horn wrote:
> > +CC Christoph Hellwig for opinions on compat
> >
> > On Thu, Nov 26, 2020 at 12:22 AM Minchan Kim <minchan@kernel.org> wrote:
> > > On Mon, Nov 23, 2020 at 09:39:42PM -0800, Suren Baghdasaryan wrote:
> > > > process_madvise requires a vector of address ranges to be provided for
> > > > its operations. When an advice should be applied to the entire process,
> > > > the caller process has to obtain the list of VMAs of the target process
> > > > by reading the /proc/pid/maps or some other way. The cost of this
> > > > operation grows linearly with increasing number of VMAs in the target
> > > > process. Even constructing the input vector can be non-trivial when
> > > > target process has several thousands of VMAs and the syscall is being
> > > > issued during high memory pressure period when new allocations for such
> > > > a vector would only worsen the situation.
> > > > In the case when advice is being applied to the entire memory space of
> > > > the target process, this creates an extra overhead.
> > > > Add PMADV_FLAG_RANGE flag for process_madvise enabling the caller to
> > > > advise a memory range of the target process. For now, to keep it simple,
> > > > only the entire process memory range is supported, vec and vlen inputs
> > > > in this mode are ignored and can be NULL and 0.
> > > > Instead of returning the number of bytes that advice was successfully
> > > > applied to, the syscall in this mode returns 0 on success. This is due
> > > > to the fact that the number of bytes would not be useful for the caller
> > > > that does not know the amount of memory the call is supposed to affect.
> > > > Besides, the ssize_t return type can be too small to hold the number of
> > > > bytes affected when the operation is applied to a large memory range.
> > >
> > > 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.
>
> Can't we simply pass NULL as iovec as special id, then?

AFAIK in theory NULL can be a valid userspace pointer (although that
basically never happens and, on MMU systems, requires root to
explicitly do it). On the other hand, there are some parts of the UAPI
that already special-case NULL, so maybe that's considered acceptable?

Also, special-casing NULL slightly increases the chance that userspace
messes up and accidentally triggers completely different behavior
because an allocation failed or something like that.

So while I'm not strongly against using NULL here, I don't exactly
like the idea.