On Mon, May 22, 2017 at 05:52:47PM +0200, Vlastimil Babka wrote: > On 05/22/2017 04:29 PM, Mike Rapoport wrote: > > On Mon, May 22, 2017 at 03:55:48PM +0200, Michal Hocko wrote: > >> On Mon 22-05-17 16:36:00, Mike Rapoport wrote: > >>> On Mon, May 22, 2017 at 02:42:43PM +0300, Kirill A. Shutemov wrote: > >>>> On Mon, May 22, 2017 at 09:12:42AM +0300, Mike Rapoport wrote: > >>>>> Currently applications can explicitly enable or disable THP for a memory > >>>>> region using MADV_HUGEPAGE or MADV_NOHUGEPAGE. However, once either of > >>>>> these advises is used, the region will always have > >>>>> VM_HUGEPAGE/VM_NOHUGEPAGE flag set in vma->vm_flags. > >>>>> The MADV_CLR_HUGEPAGE resets both these flags and allows managing THP in > >>>>> the region according to system-wide settings. > >>>> > >>>> Seems reasonable. But could you describe an use-case when it's useful in > >>>> real world. > >>> > >>> My use-case was combination of pre- and post-copy migration of containers > >>> with CRIU. > >>> In this case we populate a part of a memory region with data that was saved > >>> during the pre-copy stage. Afterwards, the region is registered with > >>> userfaultfd and we expect to get page faults for the parts of the region > >>> that were not yet populated. However, khugepaged collapses the pages and > >>> the page faults we would expect do not occur. > >> > >> I am not sure I undestand the problem. Do I get it right that the > >> khugepaged will effectivelly corrupt the memory by collapsing a range > >> which is not yet fully populated? If yes shouldn't that be fixed in > >> khugepaged rather than adding yet another madvise command? Also how do > >> you prevent on races? (say you VM_NOHUGEPAGE, khugepaged would be in the > >> middle of the operation and sees a collapsable vma and you get the same > >> result) > > > > 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. -- Sincerely yours, Mike. -- 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>
