From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-qk0-f198.google.com (mail-qk0-f198.google.com [209.85.220.198]) by kanga.kvack.org (Postfix) with ESMTP id 0A6A86B0007 for ; Sun, 22 Apr 2018 11:13:58 -0400 (EDT) Received: by mail-qk0-f198.google.com with SMTP id z128so9148871qka.8 for ; Sun, 22 Apr 2018 08:13:58 -0700 (PDT) Received: from mx1.redhat.com (mx3-rdu2.redhat.com. [66.187.233.73]) by mx.google.com with ESMTPS id t53-v6si14737670qta.167.2018.04.22.08.13.56 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Sun, 22 Apr 2018 08:13:57 -0700 (PDT) Subject: Re: [PATCH RFC 2/8] mm: introduce PG_offline References: <20180413131632.1413-1-david@redhat.com> <20180413131632.1413-3-david@redhat.com> <20180413171120.GA1245@bombadil.infradead.org> <89329958-2ff8-9447-408e-fd478b914ec4@suse.cz> <20180422030130.GG14610@bombadil.infradead.org> <7db70df4-c714-574c-5b14-898c1cf49af6@redhat.com> <20180422140246.GA30714@bombadil.infradead.org> From: David Hildenbrand Message-ID: <903ab7f7-88ce-9bc3-036b-261cce1bb26c@redhat.com> Date: Sun, 22 Apr 2018 17:13:52 +0200 MIME-Version: 1.0 In-Reply-To: <20180422140246.GA30714@bombadil.infradead.org> Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 8bit Sender: owner-linux-mm@kvack.org List-ID: To: Matthew Wilcox Cc: Vlastimil Babka , linux-mm@kvack.org, Steven Rostedt , Ingo Molnar , Andrew Morton , Michal Hocko , Huang Ying , Greg Kroah-Hartman , Pavel Tatashin , Miles Chen , Mel Gorman , Rik van Riel , James Hogan , "Levin, Alexander (Sasha Levin)" , open list 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) > >> 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. > Thanks for the info! -- Thanks, David / dhildenb