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? -- 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