Re: [PATCH 0/2] mm: madvise: make MADV_NOHUGEPAGE a no-op if !THP

[Lorenzo Stoakes <lorenzo.stoakes@oracle.com>]

On Mon, May 19, 2025 at 12:31:04PM -0700, Yang Shi wrote:
>
>
> On 5/19/25 7:43 AM, Lorenzo Stoakes wrote:
> > Andrew -
> >
> > OK, I realise there's an issue here with patch 2/2. We're not accounting
> > for the fact that madvise() will reject this _anyway_ because
> > madvise_behavior_valid() will reject it.
>
> Good catch. The purpose of this patch is to make MADV_NOHUGEPAGE a no-op, so
> we can just simply bail out early? The point of madvise behavior check is to
> avoid taking mmap_lock and walking vmas for invalid behavior, but it doesn't
> consider no-op (I treat op-op as valid but do nothing), so if we know this
> advise is a no-op, we just bail out by returning 0.

Yeah I was thinking better to just do it in mm/madvise.c to be honest.

>
> Maybe MADV_UNMERGEABLE should be no-op too, it returns 0 for !KSM anyway.

Ack, yes could maybe bundle up?

Anyway I didn't want to delay the s390 fix any longer and this is a mess now so
better to defer to next cycle I think :>)

>
> Thanks,
> Yang
>
>
> >
> > I've tried to be especially helpful here to aid Ignacio in his early
> > contributions, but I think it's best now (if you don't mind Igancio) for me
> > to figure out a better solution after the merge window.
> >
> > We're late in the cycle now so I will just resend the 1st patch (for s390)
> > separately if you're happy to take that for 6.16? It's a simple rename of
> > an entirely static identifier so should present no risk, and is approved by
> > the arch maintainers who have also agreed for it to come through the mm
> > tree.
> >
> > Apologies for the mess!
> >
> > Cheers, Lorenzo
> >
> > On Thu, May 15, 2025 at 09:15:44PM +0100, Lorenzo Stoakes wrote:
> > > Andrew -
> > >
> > > I hope the explanation below resolves your query about the header include
> > > (in [0]), let me know if doing this as a series like this works (we need to
> > > enforce the ordering here).
> > >
> > > Thanks!
> > >
> > > [0]: 20250514153648.598bb031a2e498b1ac505b60@linux-foundation.org
> > >
> > >
> > >
> > > Currently, when somebody attempts to set MADV_NOHUGEPAGE on a system that
> > > does not enable CONFIG_TRANSPARENT_HUGEPAGE the confguration option, this
> > > results in an -EINVAL error arising.
> > >
> > > This doesn't really make sense, as to do so is essentially a no-op.
> > >
> > > Additionally, the semantics of setting VM_[NO]HUGEPAGE in any case are such
> > > that, should the attribute not apply, nothing will be done.
> > >
> > > It therefore makes sense to simply make this operation a noop.
> > >
> > > However, a fly in the ointment is that, in order to do so, we must check
> > > against the MADV_NOHUGEPAGE constant. In doing so, we encounter two rather
> > > annoying issues.
> > >
> > > The first is that the usual include we would import to get hold of
> > > MADV_NOHUGEPAGE, linux/mman.h, results in a circular dependency:
> > >
> > > * If something includes linux/mman.h, we in turn include linux/mm.h prior
> > >    to declaring MADV_NOHUGEPAGE.
> > > * This then, in turn, includes linux/huge_mm.h.
> > > * linux/huge_mm.h declares hugepage_madvise(), which then tries to
> > >    reference MADV_NOHUGEPAGE, and the build fails.
> > >
> > > This can be reached in other ways too.
> > >
> > > So we work around this by including uapi/asm/mman.h instead, which allows
> > > us to keep hugepage_madvise() inline.
> > >
> > > The second issue is that the s390 arch declares PROT_NONE as a value in the
> > > enum prot_type enumeration.
> > >
> > > By updating the include in linux/huge_mm.h, we pull in the PROT_NONE
> > > declaration (unavoidably, this is ultimately in
> > > uapi/asm-generic/mman-common.h alongside MADV_NOHUGEPAGE), which collides
> > > with the enumeration value.
> > >
> > > To resolve this, we rename PROT_NONE to PROT_TYPE_DUMMY.
> > >
> > > The ordering of these patches is critical, the s390 patch must be applied
> > > prior to the MADV_NOHUGEPAGE patch, and therefore the two patches are sent
> > > as a series.
> > >
> > > v1:
> > > * Place patches in series.
> > > * Correct typo in comment as per James.
> > >
> > > previous patches:
> > > huge_mm.h patch - https://lore.kernel.org/all/20250508-madvise-nohugepage-noop-without-thp-v1-1-e7ceffb197f3@kuka.com/
> > > s390 patch - https://lore.kernel.org/all/20250514163530.119582-1-lorenzo.stoakes@oracle.com/
> > >
> > > Ignacio Moreno Gonzalez (1):
> > >    mm: madvise: make MADV_NOHUGEPAGE a no-op if !THP
> > >
> > > Lorenzo Stoakes (1):
> > >    KVM: s390: rename PROT_NONE to PROT_TYPE_DUMMY
> > >
> > >   arch/s390/kvm/gaccess.c | 8 ++++----
> > >   include/linux/huge_mm.h | 5 +++++
> > >   2 files changed, 9 insertions(+), 4 deletions(-)
> > >
> > > --
> > > 2.49.0
>
>