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? > >> -- >> Michal Hocko >> SUSE Labs > > -- > 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> > -- 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>
