On Wed 11-09-19 15:03:39, David Hildenbrand wrote: > On 11.09.19 14:54, Michal Hocko wrote: > > 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. > > > > Something slightly similar is being performed by Nitesh's patch set. On > every free of a certain granularity, he records it in the bitmap. These > bits are "hints of free pages". > > A thread then walks over the bitmap and tries to allocate the "hints". > If the pages were already reused, the bit is silently cleared. > > Instead of allocating/freeing, we could only try to isolate the > pageblock, then test if free. (One of the usual issues to work around is > MAX_ORDER-1 crossing pageblocks, that might need special care) OK, cool that I have reinvented the wheel ;). Allocation is indeed not necessary as long as pages are isolated because nobody will allocate them. > I think you should have a look at the rough idea of Nitesh's patch set > to see if something like that is going into a better direction. The > bitmap part is in place to do bulk reporting and avoid duplicate reports. Let's see how much time I can find for that in my endless inbox whack a mole. > I think main points we want (and what I am missing from callback idea > being discussed) are > 1. Do bulk reporting only when a certain threshold is reached Is a time based approach too coarse? > 2. Report only bigger granularities (especially, avoid THP splits in the > hypervisor - >= 2MB proofed to be effective) the callback has supported order based scan in some of its iteration. > 3. Avoid reporting what has just been reported. Is the overhead of checking a pfn range in a bitmask that much of an overhead to really care? > 4. Continuously report, not the "one time report everything" approach. So you mean the allocator reporting this rather than an external code to poll right? I do not know, how much this is nice to have than must have? -- Michal Hocko SUSE Labs
