On Fri 23-12-16 23:01:31, Minchan Kim wrote: > On Fri, Dec 23, 2016 at 12:54:21PM +0100, Michal Hocko wrote: > > On Fri 23-12-16 18:53:36, Minchan Kim wrote: [...] > > > stucks until VM marked the pmd dirty. > > > > > > How the emulation work depends on the architecture. In case of arm64, > > > when it set up pte firstly, it sets pte PTE_RDONLY to get a chance to > > > mark the pte dirty via triggering page fault when store access happens. > > > Once the page fault occurs, VM marks the pte dirty and arch code for > > > setting pte will clear PTE_RDONLY for application to proceed. > > > > > > IOW, if VM doesn't mark the pte dirty, application hangs forever by > > > repeated fault(i.e., store op but the pte is PTE_RDONLY). > > > > > > This patch enables dirty-bit emulation for those architectures. > > > > Yes this is helpful and much more clear, thank you. One thing that is > > still not clear to me is why cannot we handle that in the arch specific > > code. I mean what is the side effect of doing pmd_mkdirty for > > architectures which do not need it? > > For architecture which supports H/W access/dirty bit, it couldn't be > reached there code path so there is no side effect, I think. ahh, I knew I was missing something. It definitely wasn't obvious to me and my x86 config it simply generates code to call huge_pmd_set_accessed. > A thing > I can think of is just increasing code size little bit. Maybe, we > could optimize away some ifdef magic but not sure worth it. it is not -- 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>