Re: [PATCH] mm: introduce MADV_CLR_HUGEPAGE

[Michal Hocko]

On Wed 24-05-17 13:39:48, Mike Rapoport wrote:
> On Wed, May 24, 2017 at 09:58:06AM +0200, Vlastimil Babka wrote:
> > On 05/24/2017 09:50 AM, Mike Rapoport wrote:
> > > On Mon, May 22, 2017 at 05:52:47PM +0200, Vlastimil Babka wrote:
> > >> On 05/22/2017 04:29 PM, Mike Rapoport wrote:
> > >>>
> > >>> Probably I didn't explained it too well.
> > >>>
> > >>> The range is intentionally not populated. When we combine pre- and
> > >>> post-copy for process migration, we create memory pre-dump without stopping
> > >>> the process, then we freeze the process without dumping the pages it has
> > >>> dirtied between pre-dump and freeze, and then, during restore, we populate
> > >>> the dirtied pages using userfaultfd.
> > >>>
> > >>> When CRIU restores a process in such scenario, it does something like:
> > >>>
> > >>> * mmap() memory region
> > >>> * fill in the pages that were collected during the pre-dump
> > >>> * do some other stuff
> > >>> * register memory region with userfaultfd
> > >>> * populate the missing memory on demand
> > >>>
> > >>> khugepaged collapses the pages in the partially populated regions before we
> > >>> have a chance to register these regions with userfaultfd, which would
> > >>> prevent the collapse.
> > >>>
> > >>> We could have used MADV_NOHUGEPAGE right after the mmap() call, and then
> > >>> there would be no race because there would be nothing for khugepaged to
> > >>> collapse at that point. But the problem is that we have no way to reset
> > >>> *HUGEPAGE flags after the memory restore is complete.
> > >>
> > >> Hmm, I wouldn't be that sure if this is indeed race-free. Check that
> > >> this scenario is indeed impossible?
> > >>
> > >> - you do the mmap
> > >> - khugepaged will choose the process' mm to scan
> > >> - khugepaged will get to the vma in question, it doesn't have
> > >> MADV_NOHUGEPAGE yet
> > >> - you set MADV_NOHUGEPAGE on the vma
> > >> - you start populating the vma
> > >> - khugepaged sees the vma is non-empty, collapses
> > >>
> > >> unless I'm wrong, the racers will have mmap_sem for reading only when
> > >> setting/checking the MADV_NOHUGEPAGE? Might be actually considered a bug.
> > >>
> > >> However, can't you use prctl(PR_SET_THP_DISABLE) instead? "If arg2 has a
> > >> nonzero value, the flag is set, otherwise it is cleared." says the
> > >> manpage. Do it before the mmap and you avoid the race as well?
> > > 
> > > Unfortunately, prctl(PR_SET_THP_DISABLE) didn't help :(
> > > When I've tried to use it, I've ended up with VM_NOHUGEPAGE set on all VMAs
> > > created after prctl(). This returns me to the state when checkpoint-restore
> > > alters the application vma->vm_flags although it shouldn't and I do not see
> > > a way to fix it using existing interfaces.
> > 
> > [CC linux-api, should have been done in the initial posting already]
> 
> Sorry, missed that.
>  
> > Hm so the prctl does:
> > 
> >                 if (arg2)
> >                         me->mm->def_flags |= VM_NOHUGEPAGE;
> >                 else
> >                         me->mm->def_flags &= ~VM_NOHUGEPAGE;
> > 
> > That's rather lazy implementation IMHO. Could we change it so the flag
> > is stored elsewhere in the mm, and the code that decides to (not) use
> > THP will check both the per-vma flag and the per-mm flag?
> 
> I afraid I don't understand how that can help.
> What we need is an ability to temporarily disable collapse of the pages in
> VMAs that do not have VM_*HUGEPAGE flags set and that after we re-enable
> THP, the vma->vm_flags for those VMAs will remain intact.

Why cannot khugepaged simply skip over all VMAs which have userfault
regions registered? This would sound like a less error prone approach to
me.
-- 
Michal Hocko
SUSE Labs