On Wed 11-09-19 14:42:41, David Hildenbrand wrote: > On 11.09.19 14:25, Michal Hocko wrote: > > On Wed 11-09-19 14:19:41, Michal Hocko wrote: > >> On Wed 11-09-19 08:08:38, Michael S. Tsirkin wrote: > >>> On Wed, Sep 11, 2019 at 01:36:19PM +0200, Michal Hocko wrote: > >>>> On Tue 10-09-19 14:23:40, Alexander Duyck wrote: > >>>> [...] > >>>>> We don't put any limitations on the allocator other then that it needs to > >>>>> clean up the metadata on allocation, and that it cannot allocate a page > >>>>> that is in the process of being reported since we pulled it from the > >>>>> free_list. If the page is a "Reported" page then it decrements the > >>>>> reported_pages count for the free_area and makes sure the page doesn't > >>>>> exist in the "Boundary" array pointer value, if it does it moves the > >>>>> "Boundary" since it is pulling the page. > >>>> > >>>> This is still a non-trivial limitation on the page allocation from an > >>>> external code IMHO. I cannot give any explicit reason why an ordering on > >>>> the free list might matter (well except for page shuffling which uses it > >>>> to make physical memory pattern allocation more random) but the > >>>> architecture seems hacky and dubious to be honest. It shoulds like the > >>>> whole interface has been developed around a very particular and single > >>>> purpose optimization. > >>>> > >>>> I remember that there was an attempt to report free memory that provided > >>>> a callback mechanism [1], which was much less intrusive to the internals > >>>> of the allocator yet it should provide a similar functionality. Did you > >>>> see that approach? How does this compares to it? Or am I completely off > >>>> when comparing them? > >>>> > >>>> [1] mostly likely not the latest version of the patchset > >>>> http://lkml.kernel.org/r/1502940416-42944-5-git-send-email-wei.w.wang@xxxxxxxxx > >>> > >>> Linus nacked that one. He thinks invoking callbacks with lots of > >>> internal mm locks is too fragile. > >> > >> I would be really curious how much he would be happy about injecting > >> other restrictions on the allocator like this patch proposes. This is > >> more intrusive as it has a higher maintenance cost longterm IMHO. > > > > Btw. I do agree that callbacks with internal mm locks are not great > > either. We do have a model for that in mmu_notifiers and it is something > > I do consider PITA, on the other hand it is mostly sleepable part of the > > interface which makes it the real pain. The above callback mechanism was > > explicitly documented with restrictions and that the context is > > essentially atomic with no access to particular struct pages and no > > expensive operations possible. So in the end I've considered it > > acceptably painful. Not that I want to override Linus' nack but if > > virtualization usecases really require some form of reporting and no > > other way to do that push people to invent even more interesting > > approaches then we should simply give them/you something reasonable > > and least intrusive to our internals. > > > > The issue with "[PATCH v14 4/5] mm: support reporting free page blocks" > is that it cannot really handle the use case we have here if I am not > wrong. While a page is getting processed by the hypervisor (e.g. > MADV_DONTNEED), it must not get reused. What prevents to use the callback to get a list of pfn ranges to work on and then use something like start_isolate_page_range on the collected pfn ranges to make sure nobody steals pages from under your feet, do your thing and drop the isolated state afterwards. I am saying somethig like because you wouldn't really want a generic has_unmovable_pages but rather if (!page_ref_count(page)) { if (PageBuddy(page)) iter += (1 << page_order(page)) - 1; continue; } subset of it. -- Michal Hocko SUSE Labs
