On Fri, Dec 28, 2018 at 01:29:33PM -0800, David Rientjes wrote: > On Fri, 28 Dec 2018, Michal Hocko wrote: > > > > > And if the > > > > application later enables THP by the prctl then existing mappings would > > > > never get the THP enabled status back. > > > > > > > > > > If do prctl(PR_SET_THP_DISABLE, 0) later to clear the disabling, my patch > > > is a no-op, smaps reports whether the vma is eligible for hugepages > > > depending on MADV_HUGEPAGE or not, which also matches the implementation > > > of transparent_hugepage_enabled(). > > > > > > Where is the issue? > > > > How do you clear the NOHUGE flag in pre-existing vmas to the original > > value? You cannot tell vmas with explicit NOHUGE flag set by madvise and > > those reported due the prctl value. > > > > In other words. Consider a CRIU-like usecase when you snapshot an application > > at some point of time, restore it and let the application run again > > and it does (PR_SET_THP_DISABLE, 0). All the pre-existing vmas will > > retain their NOHUGE vma flag and as such will be excluded from THP. > > Application will behave differently from before the snaphost which looks > > like a bug, right? > > > > Smaps reveals the state of each vma at the moment it is read, not the > future, this has always been the case before and after the broken commit; > it's not intended to be something that can specify "thp is currently > disabled, but if we did prctl(PR_SET_THP_DISABLE, 0), this vma has done > madvise(MADV_HUGEPAGE), and we are on a thp enabled == madvise host, so > it would be eligible for thp." The point of CRIU is that it tries to save and re-create the state of the process. With your fix, "nh" in VMA flags can mean two different things: - MADV_NOHUGEPAGE was called for the VMA, or - prctl(PR_SET_THP_DISABLE) was called for the whole process Current CRIU code assumes that "nh" means MADV_NOHUGEPAGE. But if it was due to prctl(PR_SET_THP_DISABLE) the process state will not be restored correctly: all VMAs will be restored with MADV_NOHUGEPAGE and future re-enabling THP with prctl(PR_SET_THP_DISABLE) for restored process will not have any effect. VMAs stay not eligble for THP. -- Kirill A. Shutemov
