On Thu, 2019-09-12 at 11:19 +0200, Michal Hocko wrote: > On Wed 11-09-19 08:12:03, Alexander Duyck wrote: > > On Wed, Sep 11, 2019 at 4:36 AM Michal Hocko <mhocko@xxxxxxxxxx> 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. > > > > How is this any different then the code that moves a page that will > > likely be merged to the tail though? > > I guess you are referring to the page shuffling. If that is the case > then this is an integral part of the allocator for a reason and it is > very well obvious in the code including the consequences. I do not > really like an idea of hiding similar constrains behind a generic > looking feature which is completely detached from the allocator and so > any future change of the allocator might subtly break it. > > > In our case the "Reported" page is likely going to be much more > > expensive to allocate and use then a standard page because it will be > > faulted back in. In such a case wouldn't it make sense for us to want > > to keep the pages that don't require faults ahead of those pages in > > the free_list so that they are more likely to be allocated? > > OK, I was suspecting this would pop out. And this is exactly why I > didn't like an idea of an external code imposing a non obvious constrains > to the allocator. You simply cannot count with any ordering with the > page allocator. We used to distinguish cache hot/cold pages in the past > and pushed pages to the specific end of the free list but that has been > removed. There are other potential changes like that possible. Shuffling > is a good recent example. > > Anyway I am not a maintainer of this code. I would really like to hear > opinions from Mel and Vlastimil here (now CCed - the thread starts > http://lkml.kernel.org/r/20190907172225.10910.34302.stgit@localhost.localdomain. One alternative I could do if we are wanting to make things more obvious would be to add yet another add_to_free_list_XXX function that would be used specifically for reported pages. The only real requirement I have is that we have to insert reported pages such that we generate a continuous block without interleaving non-reported pages in between. So as long as reported pages are always inserted at the boundary/iterator when we are actively reporting on a section then I can guarantee the list won't have gaps formed. Also as far as the concerns about this being an external user, one thing I can do is break up the headers a bit and define an internal header in mm/ that defines all the items used by the page allocator, and another in include/linux/ that defines what is used by devices when receiving the notifications. It would then help to reduce the likelihood of an outside entity messing with the page allocator too much.
