On 29.04.2018 23:08, Michal Hocko wrote: > 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. > kdump tools (makedumptool) don't rely on any specific value or assume anything. Using the example of musing PG_hwpoison to mapcount: If it sees PG_hwpoison: - it knows the right bit number to use - it knows the kernel uses it If it doesn't see PG_hwpoison (in the ELF info) anymore: - it cannot exclude poisoned pages anymore, potentially crashing the system during a dump If you have a better fitting name for "requires a interlocked update with tools to keep it working" than ABI, please let me know :) Anyhow, I have a new prototype based on PAGE_OFFLINE_MAPCOUNT_VALUE that I will share briefly. -- Thanks, David / dhildenb
