Re: [PATCH 2/2] mm: thp: kill the bogus ->def_flags check in hugepage_madvise()

[Oleg Nesterov <oleg@redhat.com>]

On 01/22, Hugh Dickins wrote:
>
> On Wed, 22 Jan 2014, Oleg Nesterov wrote:
>
> > hugepage_madvise() checks "mm->def_flags & VM_NOHUGEPAGE" but
> > this can be never true, currently mm->def_flags can only have
> > VM_LOCKED.
>
> But line 1087 of arch/s390/mm/pgtable.c says
> 	mm->def_flags |= VM_NOHUGEPAGE;
> from 3eabaee998c787e7e1565574821652548f7fc003
> "KVM: s390: allow sie enablement for multi-threaded programs".

Argh. Thanks Hugh!

Another case when I forgot about /bin/grep. So the patch is wrong,
at least the changelog is certainly is. I am stupid.

But, perhaps, this all still can work? Looks like, s390 already
implements PR_SET_THP_DISABLE using the same idea, it would be
nice to avoid another hack.

Gerald, any chance we can revert 8e72033f2a489 "thp: make MADV_HUGEPAGE
check for mm->def_flags" ? The changelog says "In order to also prevent
MADV_HUGEPAGE on such an mm", is it really important? I mean, if the
application calls madvise(MADV_HUGEPAGE) it should probably know what
it does and, this can be useful after if PR_SET_THP_DISABLE or
KVM_S390_ENABLE_SIE. Of course I do not understand this code, perhaps
MADV_HUGEPAGE is simply impossible.

Another question, can't fork/exec preserve VM_NOHUGEPAGE in ->def_flags?
This is what PR_SET_THP_DISABLE wants.

I'll send you mbox with the previous discussion privately.

Thanks,

Oleg.