linux-kernel - Re: [PATCH RFC 7/7] mm: better document PG

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Date:   Wed, 5 Dec 2018 06:35:10 -0800
From:   Matthew Wilcox <willy@...radead.org>
To:     David Hildenbrand <david@...hat.com>
Cc:     linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org,
        linux-m68k@...ts.linux-m68k.org, linuxppc-dev@...ts.ozlabs.org,
        linux-riscv@...ts.infradead.org, linux-s390@...r.kernel.org,
        linux-mediatek@...ts.infradead.org,
        Andrew Morton <akpm@...ux-foundation.org>,
        Stephen Rothwell <sfr@...b.auug.org.au>,
        Pavel Tatashin <pasha.tatashin@...cle.com>,
        Michal Hocko <mhocko@...e.com>,
        Alexander Duyck <alexander.h.duyck@...ux.intel.com>,
        Anthony Yznaga <anthony.yznaga@...cle.com>,
        Miles Chen <miles.chen@...iatek.com>,
        yi.z.zhang@...ux.intel.com, Dan Williams <dan.j.williams@...el.com>
Subject: Re: [PATCH RFC 7/7] mm: better document PG_reserved

On Wed, Dec 05, 2018 at 01:28:51PM +0100, David Hildenbrand wrote:
> I don't see a reason why we have to document "Some of them might not even
> exist". If there is a user, we should document it. E.g. for balloon
> drivers we now use PG_offline to indicate that a page might currently
> not be backed by memory in the hypervisor. And that is independent from
> PG_reserved.

I think you're confused by the meaning of "some of them might not even
exist".  What this means is that there might not be memory there; maybe
writes to that memory will be discarded, or maybe they'll cause a machine
check.  Maybe reads will return ~0, or 0, or cause a machine check.
We just don't know what's there, and we shouldn't try touching the memory.

> +++ b/include/linux/page-flags.h
> @@ -17,8 +17,22 @@
>  /*
>   * Various page->flags bits:
>   *
> - * PG_reserved is set for special pages, which can never be swapped out. Some
> - * of them might not even exist...
> + * PG_reserved is set for special pages. The "struct page" of such a page
> + * should in general not be touched (e.g. set dirty) except by their owner.
> + * Pages marked as PG_reserved include:
> + * - Kernel image (including vDSO) and similar (e.g. BIOS, initrd)
> + * - Pages allocated early during boot (bootmem, memblock)
> + * - Zero pages
> + * - Pages that have been associated with a zone but are not available for
> + *   the page allocator (e.g. excluded via online_page_callback())
> + * - Pages to exclude from the hibernation image (e.g. loaded kexec images)
> + * - MMIO pages (communicate with a device, special caching strategy needed)
> + * - MCA pages on ia64 (pages with memory errors)
> + * - Device memory (e.g. PMEM, DAX, HMM)
> + * Some architectures don't allow to ioremap pages that are not marked
> + * PG_reserved (as they might be in use by somebody else who does not respect
> + * the caching strategy). Consequently, PG_reserved for a page mapped into
> + * user space can indicate the zero page, the vDSO, MMIO pages or device memory.

So maybe just add one more option to the list.