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. 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. 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? -- Oscar Salvador SUSE L3
