On Wed, 26 Feb 2014 16:31:44 +0100
Oleg Nesterov <[email protected]> wrote:
> On 02/26, Christian Borntraeger wrote:
> >
> > On 26/02/14 15:50, Oleg Nesterov wrote:
> > >
> > > But perhaps qemu can be changed to avoid MADV_HUGEPAGE on s390 ?
> > > Otherwise I'd suggest the change below.
> > >
> > > Oleg.
> > >
> > >
> > > --- x/mm/huge_memory.c
> > > +++ x/mm/huge_memory.c
> > > @@ -1968,8 +1968,6 @@ out:
> > > int hugepage_madvise(struct vm_area_struct *vma,
> > > unsigned long *vm_flags, int advice)
> > > {
> > > - struct mm_struct *mm = vma->vm_mm;
> > > -
> > > switch (advice) {
> > > case MADV_HUGEPAGE:
> > > /*
> > > @@ -1977,8 +1975,16 @@ int hugepage_madvise(struct vm_area_stru
> > > */
> > > if (*vm_flags & (VM_HUGEPAGE | VM_NO_THP))
> > > return -EINVAL;
> > > - if (mm->def_flags & VM_NOHUGEPAGE)
> > > +
> > > +/*
> > > + * MADV_HUGEPAGE after PRCTL_THP_DISABLE is broken on s390 because
> > > + * qemu blindly does madvise(MADV_HUGEPAGE) after s390_enable_sie().
> > > + */
> > > +#ifdef CONFIG_S390
> > > + if (vma->vm_mm->def_flags & VM_NOHUGEPAGE)
> > > return -EINVAL;
> > > +#endif
> > > +
> >
> > Ifdefs are ugly but might be the only way of not breaking existing
> > userspace.
>
> Yes, agreed. but note that this code is already s390-specific, nobody
> else puts VM_NOHUGEPAGE into ->def_flags (until this series of course).
>
> > If we come up with a solution for THP in KVM host processes on s390, we can
> > then remove that wart. We could even limit that hack to KVM only processes
> > to retain Alex' prctl capability by checking mm_has_pgste
>
> Yes, yes, I looked at mm->context.has_pgste too ;) But I wasn't sure
> we can rely on it. Thanks.
>
> > > +/*
> > > + * MADV_HUGEPAGE after PRCTL_THP_DISABLE is broken on s390 because
> > > + * qemu blindly does madvise(MADV_HUGEPAGE) after s390_enable_sie().
> > > + */
> > > +#ifdef CONFIG_S390
> > > + if ((vma->vm_mm->def_flags & VM_NOHUGEPAGE) &&
> > > mm_has_pgste(vma->vm_mm))
> > > return -EINVAL;
>
> Hmm... but why do we need to check VM_NOHUGEPAGE then? Can't the
> change below work?
Yes, good point, that should do the trick.
>
> Oleg.
>
> --- x/arch/s390/mm/pgtable.c
> +++ x/arch/s390/mm/pgtable.c
> @@ -1084,7 +1084,6 @@ static inline void thp_split_mm(struct m
> vma->vm_flags &= ~VM_HUGEPAGE;
> vma->vm_flags |= VM_NOHUGEPAGE;
> }
> - mm->def_flags |= VM_NOHUGEPAGE;
> }
> #else
> static inline void thp_split_mm(struct mm_struct *mm)
> --- x/mm/huge_memory.c
> +++ x/mm/huge_memory.c
> @@ -1968,8 +1968,6 @@ out:
> int hugepage_madvise(struct vm_area_struct *vma,
> unsigned long *vm_flags, int advice)
> {
> - struct mm_struct *mm = vma->vm_mm;
> -
> switch (advice) {
> case MADV_HUGEPAGE:
> /*
> @@ -1977,8 +1975,12 @@ int hugepage_madvise(struct vm_area_stru
> */
> if (*vm_flags & (VM_HUGEPAGE | VM_NO_THP))
> return -EINVAL;
> - if (mm->def_flags & VM_NOHUGEPAGE)
> +
> +#ifdef CONFIG_S390
> + if (mm_has_pgste(vma->vm_mm))
> return -EINVAL;
> +#endif
> +
> *vm_flags &= ~VM_NOHUGEPAGE;
> *vm_flags |= VM_HUGEPAGE;
> /*
>
--
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