On Sun 22-04-18 17:13:52, David Hildenbrand wrote: > On 22.04.2018 16:02, Matthew Wilcox wrote: > > 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. > > It is, please have a look at the file I pointed you to. > > We export the *value* of PG_hwpoison in the ELF file, therefore the > *value* can change, but the *location* (page_flags, mapcount, whatever) > must not change. Or am I missing something here? I don't think we can > move PG_hwpoison that easily. > > Also, I can read "For pages that are never mapped to userspace, > page->mapcount may be used for storing extra information about page > type" - is that true for PG_hwpoison/PG_reserved? I am skeptical. > > And we need something similar for PG_offline, because it will become > ABI. (I can see that PAGE_BUDDY_MAPCOUNT_VALUE is also exported in an > ELF file, so maybe a new page type might work for marking a page offline > - but I have to look at the details first tomorrow) Wait wait wait. Who is relying on this? Kdump? Page flags have always been an internal implementation detail and _nobody_ outside of the kernel should ever rely on the specific value. Well, kdump has been cheating but that is because kdump is inherently tight to a specific kernel implementation but that doesn't make it a stable ABI IMHO. Restricting the kernel internals because of a debugging tool would be quite insane. -- Michal Hocko SUSE Labs
