On Fri, 2 Jun 2017 22:31:47 +0200 Vlastimil Babka <vbabka@xxxxxxx> wrote: > On 06/02/2017 09:50 PM, Andrew Morton wrote: > > On Fri, 2 Jun 2017 18:03:22 +0300 "Mike Rapoport" <rppt@xxxxxxxxxxxxxxxxxx> wrote: > > > >> PR_SET_THP_DISABLE has a rather subtle semantic. It doesn't affect any > >> existing mapping because it only updated mm->def_flags which is a template > >> for new mappings. The mappings created after prctl(PR_SET_THP_DISABLE) have > >> VM_NOHUGEPAGE flag set. This can be quite surprising for all those > >> applications which do not do prctl(); fork() & exec() and want to control > >> their own THP behavior. > >> > >> Another usecase when the immediate semantic of the prctl might be useful is > >> a combination of pre- and post-copy migration of containers with CRIU. In > >> this case CRIU populates a part of a memory region with data that was saved > >> during the pre-copy stage. Afterwards, the region is registered with > >> userfaultfd and CRIU expects to get page faults for the parts of the region > >> that were not yet populated. However, khugepaged collapses the pages and > >> the expected page faults do not occur. > >> > >> In more general case, the prctl(PR_SET_THP_DISABLE) could be used as a > >> temporary mechanism for enabling/disabling THP process wide. > >> > >> Implementation wise, a new MMF_DISABLE_THP flag is added. This flag is > >> tested when decision whether to use huge pages is taken either during page > >> fault of at the time of THP collapse. > >> > >> It should be noted, that the new implementation makes PR_SET_THP_DISABLE > >> master override to any per-VMA setting, which was not the case previously. > >> > >> Fixes: a0715cc22601 ("mm, thp: add VM_INIT_DEF_MASK and PRCTL_THP_DISABLE") > > > > "Fixes" is a bit strong. I'd say "alters". And significantly altering > > the runtime behaviour of a three-year-old interface is rather a worry, > > no? > > > > Perhaps we should be adding new prctl modes to select this new > > behaviour and leave the existing PR_SET_THP_DISABLE behaviour as-is? > > I think we can reasonably assume that most users of the prctl do just > the fork() & exec() thing, so they will be unaffected. That sounds optimistic. Perhaps people are using the current behaviour to set on particular mapping to MMF_DISABLE_THP, with prctl(PR_SET_THP_DISABLE) mmap() prctl(PR_CLR_THP_DISABLE) ? Seems a reasonable thing to do. But who knows - people do all sorts of inventive things. > And as usual, if > somebody does complain in the end, we revert and try the other way? But by then it's too late - the new behaviour will be out in the field. -- To unsubscribe from this list: send the line "unsubscribe linux-api" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html