On Mon 21-10-19 15:48:48, Oscar Salvador wrote: > On Fri, Oct 18, 2019 at 02:39:01PM +0200, Michal Hocko wrote: > > > > I am sorry but I got lost in the above description and I cannot really > > make much sense from the code either. Let me try to outline the way how > > I think about this. > > > > Say we have a pfn to hwpoison. We have effectivelly three possibilities > > - page is poisoned already - done nothing to do > > - page is managed by the buddy allocator - excavate from there > > - page is in use > > > > The last category is the most interesting one. There are essentially > > three classes of pages > > - freeable > > - migrateable > > - others > > > > We cannot do really much about the last one, right? Do we mark them > > HWPoison anyway? > > We can only perform actions on LRU/Movable pages or hugetlb pages. What would prevent other pages mapped via page tables to be handled as well? > So unless the page does not fall into those areas, we do not do anything > with them. > > > Freeable should be simply marked HWPoison and freed. > > For all those migrateable, we simply do migrate and mark HWPoison. > > Now the main question is how to handle HWPoison page when it is freed > > - aka last reference is dropped. The main question is whether the last > > reference is ever dropped. If yes then the free_pages_prepare should > > never release it to the allocator (some compound destructors would have > > to special case as well, e.g. hugetlb would have to hand over to the > > allocator rather than a pool). If not then the page would be lingering > > potentially with some state bound to it (e.g. memcg charge). So I > > suspect you want the former. > > For non-hugetlb pages, we do not call put_page in the migration path, > but we do it in page_handle_poison, after the page has been flagged as > hwpoison. > Then the check in free_papes_prepare will see that the page is hwpoison > and will bail out, so the page is not released into the allocator/pcp lists. > > Hugetlb pages follow a different methodology. > They are dissolved, and then we split the higher-order page and take the > page off the buddy. > The problem is that while it is easy to hold a non-hugetlb page, > doing the same for hugetlb pages is not that easy: > > 1) we would need to hook in enqueue_hugetlb_page so the page is not enqueued > into hugetlb freelists > 2) when trying to free a hugetlb page, we would need to do as we do for gigantic > pages now, and that is breaking down the pages into order-0 pages and release > them to the buddy (so the check in free_papges_prepare would skip the > hwpoison page). > Trying to handle a higher-order hwpoison page in free_pages_prepare is > a bit complicated. I am not sure I see the problem. If you dissolve the hugetlb page then there is no hugetlb page anymore and so you make it a regular high-order page. > There is one thing I was unsure though. > Bailing out at the beginning of free_pages_prepare if the page is hwpoison > means that the calls to > > - __memcg_kmem_uncharge > - page_cpupid_reset_last > - reset_page_owner > - ... > > will not be performed. > I thought this is right because the page is not really "free", it is just unusable, > so.. it should be still charged to the memcg? If the page is free then it shouldn't pin the memcg or any other state. -- Michal Hocko SUSE Labs
