On Tue, Feb 07, 2017 at 11:14:56AM -0600, Zi Yan wrote: > > > Kirill A. Shutemov wrote: > > On Tue, Feb 07, 2017 at 09:11:05AM -0600, Zi Yan wrote: > >>>> This causes memory leak or kernel crashing, if VM_BUG_ON() is enabled. > >>> The problem is that numabalancing calls change_huge_pmd() under > >>> down_read(mmap_sem), not down_write(mmap_sem) as the rest of users do. > >>> It makes numabalancing the only code path beyond page fault that can turn > >>> pmd_none() into pmd_trans_huge() under down_read(mmap_sem). > >>> > >>> This can lead to race when MADV_DONTNEED miss THP. That's not critical for > >>> pagefault vs. MADV_DONTNEED race as we will end up with clear page in that > >>> case. Not so much for change_huge_pmd(). > >>> > >>> Looks like we need pmdp_modify() or something to modify protection bits > >>> inplace, without clearing pmd. > >>> > >>> Not sure how to get crash scenario. > >>> > >>> BTW, Zi, have you observed the crash? Or is it based on code inspection? > >>> Any backtraces? > >> The problem should be very rare in the upstream kernel. I discover the > >> problem in my customized kernel which does very frequent page migration > >> and uses numa_protnone. > >> > >> The crash scenario I guess is like: > >> 1. A huge page pmd entry is in the middle of being changed into either a > >> pmd_protnone or a pmd_migration_entry. It is cleared to pmd_none. > >> > >> 2. At the same time, the application frees the vma this page belongs to. > > > > Em... no. > > > > This shouldn't be possible: your 1. must be done under down_read(mmap_sem). > > And we only be able to remove vma under down_write(mmap_sem), so the > > scenario should be excluded. > > > > What do I miss? > > You are right. This problem will not happen in the upstream kernel. > > The problem comes from my customized kernel, where I migrate pages away > instead of reclaiming them when memory is under pressure. I did not take > any mmap_sem when I migrate pages. So I got this error. > > It is a false alarm. Sorry about that. Thanks for clarifying the problem. I think there's still a race between MADV_DONTNEED and change_huge_pmd(.prot_numa=1) resulting in skipping THP by zap_pmd_range(). It need to be addressed. And MADV_FREE requires a fix. So, minus one non-bug, plus two bugs. -- Kirill A. Shutemov -- 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>
