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 -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>
