Messages in this thread | | | Subject | Re: [PATCH v1 0/2] mm/kdump: exclude reserved pages in dumps | From | David Hildenbrand <> | Date | Thu, 26 Jul 2018 10:45:54 +0200 |
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On 26.07.2018 10:30, Michal Hocko wrote: > On Thu 26-07-18 10:22:41, David Hildenbrand wrote: >> On 24.07.2018 09:22, Michal Hocko wrote: >>> On Mon 23-07-18 19:12:58, David Hildenbrand wrote: >>>> On 23.07.2018 13:45, Vlastimil Babka wrote: >>>>> On 07/20/2018 02:34 PM, David Hildenbrand wrote: >>>>>> Dumping tools (like makedumpfile) right now don't exclude reserved pages. >>>>>> So reserved pages might be access by dump tools although nobody except >>>>>> the owner should touch them. >>>>> >>>>> Are you sure about that? Or maybe I understand wrong. Maybe it changed >>>>> recently, but IIRC pages that are backing memmap (struct pages) are also >>>>> PG_reserved. And you definitely do want those in the dump. >>>> >>>> I proposed a new flag/value to mask pages that are logically offline but >>>> Michal wanted me to go into this direction. >>>> >>>> While we can special case struct pages in dump tools ("we have to >>>> read/interpret them either way, so we can also dump them"), it smells >>>> like my original attempt was cleaner. Michal? >>> >>> But we do not have many page flags spare and even if we have one or two >>> this doesn't look like the use for them. So I still think we should try >>> the PageReserved way. >>> >> >> So as a summary, the only real approach that would be acceptable is >> using PageReserved + some other identifier to mark pages as "logically >> offline". >> >> I wonder what identifier could be used, as this has to be consistent for >> all reserved pages (to avoid false positives). >> >> Using other pageflags in combination might be possible, but then we have >> to make assumptions about all users of PageReserved right now. >> >> As far as I can see (and as has been discussed), page_type could be >> used. If we don't want to consume a new bit, we could overload/reuse the >> "PG_balloon" bit. >> >> >> E.g. "PG_balloon" set -> exclude page from dump > > Does each user of PG_balloon check for PG_reserved? If this is the case > then yes this would be OK. >
I can only spot one user of PageBalloon() at all (fs/proc/page.c) , which makes me wonder if this bit is actually still relevant. I think the last "real" user was removed with
commit b1123ea6d3b3da25af5c8a9d843bd07ab63213f4 Author: Minchan Kim <minchan@kernel.org> Date: Tue Jul 26 15:23:09 2016 -0700
mm: balloon: use general non-lru movable page feature
Now, VM has a feature to migrate non-lru movable pages so balloon doesn't need custom migration hooks in migrate.c and compaction.c.
The only user of PG_balloon in general is "include/linux/balloon_compaction.h", used effectively only by virtio_balloon.
All such pages are allocated via balloon_page_alloc() and never set reserved.
So to me it looks like PG_balloon could be easily reused, especially to also exclude virtio-balloon pages from dumps.
--
Thanks,
David / dhildenb
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