linux-kernel - Re: [PATCH v1 3/8] kexec: export PG

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Message-ID: <20181120205822-mutt-send-email-mst@kernel.org>
Date:   Tue, 20 Nov 2018 20:59:43 -0500
From:   "Michael S. Tsirkin" <mst@...hat.com>
To:     David Hildenbrand <david@...hat.com>
Cc:     linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        linux-doc@...r.kernel.org, devel@...uxdriverproject.org,
        linux-fsdevel@...r.kernel.org, linux-pm@...r.kernel.org,
        xen-devel@...ts.xenproject.org,
        kexec-ml <kexec@...ts.infradead.org>, pv-drivers@...are.com,
        Andrew Morton <akpm@...ux-foundation.org>,
        Dave Young <dyoung@...hat.com>,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        Baoquan He <bhe@...hat.com>, Omar Sandoval <osandov@...com>,
        Arnd Bergmann <arnd@...db.de>,
        Matthew Wilcox <willy@...radead.org>,
        Michal Hocko <mhocko@...e.com>,
        Lianbo Jiang <lijiang@...hat.com>,
        Borislav Petkov <bp@...en8.de>,
        Kazuhito Hagio <k-hagio@...jp.nec.com>
Subject: Re: [PATCH v1 3/8] kexec: export PG_offline to VMCOREINFO

On Mon, Nov 19, 2018 at 11:16:11AM +0100, David Hildenbrand wrote:
> Right now, pages inflated as part of a balloon driver will be dumped
> by dump tools like makedumpfile. While XEN is able to check in the
> crash kernel whether a certain pfn is actuall backed by memory in the
> hypervisor (see xen_oldmem_pfn_is_ram) and optimize this case, dumps of
> other balloon inflated memory will essentially result in zero pages getting
> allocated by the hypervisor and the dump getting filled with this data.
> 
> The allocation and reading of zero pages can directly be avoided if a
> dumping tool could know which pages only contain stale information not to
> be dumped.
> 
> We now have PG_offline which can be (and already is by virtio-balloon)
> used for marking pages as logically offline. Follow up patches will
> make use of this flag also in other balloon implementations.
> 
> Let's export PG_offline via PAGE_OFFLINE_MAPCOUNT_VALUE, so
> makedumpfile can directly skip pages that are logically offline and the
> content therefore stale.
> 
> Please note that this is also helpful for a problem we were seeing under
> Hyper-V: Dumping logically offline memory (pages kept fake offline while
> onlining a section via online_page_callback) would under some condicions
> result in a kernel panic when dumping them.
> 
> Cc: Andrew Morton <akpm@...ux-foundation.org>
> Cc: Dave Young <dyoung@...hat.com>
> Cc: "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
> Cc: Baoquan He <bhe@...hat.com>
> Cc: Omar Sandoval <osandov@...com>
> Cc: Arnd Bergmann <arnd@...db.de>
> Cc: Matthew Wilcox <willy@...radead.org>
> Cc: Michal Hocko <mhocko@...e.com>
> Cc: "Michael S. Tsirkin" <mst@...hat.com>
> Cc: Lianbo Jiang <lijiang@...hat.com>
> Cc: Borislav Petkov <bp@...en8.de>
> Cc: Kazuhito Hagio <k-hagio@...jp.nec.com>
> Signed-off-by: David Hildenbrand <david@...hat.com>

Acked-by: Michael S. Tsirkin <mst@...hat.com>

> ---
>  kernel/crash_core.c | 2 ++
>  1 file changed, 2 insertions(+)
> 
> diff --git a/kernel/crash_core.c b/kernel/crash_core.c
> index 933cb3e45b98..093c9f917ed0 100644
> --- a/kernel/crash_core.c
> +++ b/kernel/crash_core.c
> @@ -464,6 +464,8 @@ static int __init crash_save_vmcoreinfo_init(void)
>  	VMCOREINFO_NUMBER(PAGE_BUDDY_MAPCOUNT_VALUE);
>  #ifdef CONFIG_HUGETLB_PAGE
>  	VMCOREINFO_NUMBER(HUGETLB_PAGE_DTOR);
> +#define PAGE_OFFLINE_MAPCOUNT_VALUE	(~PG_offline)
> +	VMCOREINFO_NUMBER(PAGE_OFFLINE_MAPCOUNT_VALUE);
>  #endif
>  
>  	arch_crash_save_vmcoreinfo();
> -- 
> 2.17.2