On Sun, Apr 22, 2018 at 10:17:31AM +0200, David Hildenbrand wrote: > On 22.04.2018 05:01, Matthew Wilcox wrote: > > On Sat, Apr 21, 2018 at 06:52:18PM +0200, Vlastimil Babka wrote: > >> Sounds like your newly introduced "page types" could be useful here? I > >> don't suppose those offline pages would be using mapcount which is > >> aliased there? > > > > Oh, that's a good point! Yes, this is a perfect use for page_type. > > We have something like twenty bits available there. > > > > Now you've got me thinking that we can move PG_hwpoison and PG_reserved > > to be page_type flags too. That'll take us from 23 to 21 bits (on 32-bit, > > with PG_UNCACHED) > > Some things to clarify here. I modified the current RFC to also allow > PG_offline on allocated (ballooned) pages (e.g. virtio-balloon). > > kdump based dump tools can then easily identify which pages are not to > be dumped (either because the content is invalid or not accessible). > > I previously stated that ballooned pages would be marked as PG_reserved, > which is not true (at least not for virtio-balloon). However this allows > me to detect if all pages in a section are offline by looking at > (PG_reserved && PG_offline). So I can actually tell if a page is marked > as offline and allocated or really offline. > > > 1. The location (not the number!) of PG_hwpoison is basically ABI and > cannot be changed. Moving it around will most probably break dump tools. > (see kernel/crash_core.c) It's not ABI. It already changed after 4.9 when PG_waiters was introduced by commit 62906027091f. > 2. Exposing PG_offline via kdump will make it ABI as well. And we don't > want any complicated validity checks ("is the bit valid or not?"), > because that would imply having to make these bits ABI as well. So > having PG_offline just like PG_hwpoison part of page_flags is the right > thing to do. (see patch nr 4) > > 3. For determining if all pages of a section are offline (see patch nr > 5), I will have to be able to check 1. PG_offline and 2. PG_reserved on > any page. Will this be possible by moving e.g. PG_reserved to page > types? (especially if some field is suddenly aliased?) It's possible to tell whether the field is in use as mapcount or page_types; mapcount should always be non-negative, and page_types reserves a few bits to detect under/overflow of mapcount. The slab/slob users of the field will also be positive uses.