Re: [PATCH] mm: introduce MADV_CLR_HUGEPAGE

[Mike Rapoport <rppt@linux.vnet.ibm.com>]

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.

--
Sincerely yours,
Mike.

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