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