On Thu 19-05-22 08:02:10, CGEL wrote: > On Thu, May 19, 2022 at 09:35:30AM +0200, Michal Hocko wrote: > > On Thu 19-05-22 06:23:30, CGEL wrote: > > > On Wed, May 18, 2022 at 02:12:26PM +0200, Michal Hocko wrote: > > > > On Wed 18-05-22 02:47:06, CGEL wrote: > > > > > On Tue, May 17, 2022 at 04:04:38PM +0200, Michal Hocko wrote: > > > > > > [CCing Hugh and linux-api] > > > > > > > > > > > > On Tue 17-05-22 09:27:01, cgel.zte@xxxxxxxxx wrote: > > > > > > per mm but the actual implementation currently relies on the per-vma > > > > > > flags. That means that one can explicitly disallow merging by madvise > > > > > > for a range. Is it wise to override that by a per-process knob? I mean > > > > > > there might be a very good reason why a particular memory ranges should > > > > > > never be merged but a per-process knob could easily ignore that hint > > > > > > from the application. Or am I just confuse? > > > > > For now, there is no any hints for letting KSM never merge some memory > > > > > ranges. > > > > > > > > I am not sure I understand. Could you be more specific? > > > > > > Not like THP, KSM doesn't have anything like VM_NOHUGEPAGE, so apps > > > cann't explicitly disallow merging by madvise. If it is really necessary for > > > a particular meory ranges of a process to be never merged, we have to submit > > > one more patch to achieve that. > > > > What about MADV_UNMERGEABLE? > > MADV_UNMERGEABLE and MADV_MERGEABLE usually appear in pairs, MADV_UNMERGEABLE cannot > appear alone. That might be the case currently because KSM is an opt-in feature that has to be explicitly enabled. The existing interface only allows to enable it by MADV_MERGEABLE but now you are proposing an extension when there would be other way to achieve the same (with a wider scope but that is not really all that important). MADV_UNMERGEABLE has a well defined behavior even on VMAs which are not marked for merging. Let's say that somebody would like to use a process wide setup except for few special mappings because merging is not really desirable for whatever reason. How do you achieve that? > I mean MADV_UNMERGEABLE is used to unmerges whatever it merged in the > specifed range, not to disallow merging. I disagree. It clearly drops the mergeable flag so it effectivelly disallow merging. -- Michal Hocko SUSE Labs
