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

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Date:   Wed, 21 Nov 2018 09:50:17 +0100
From:   David Hildenbrand <david@...hat.com>
To:     Baoquan He <bhe@...hat.com>
Cc:     linux-mm@...ck.org, Michal Hocko <mhocko@...e.com>,
        Arnd Bergmann <arnd@...db.de>, linux-pm@...r.kernel.org,
        pv-drivers@...are.com, Borislav Petkov <bp@...en8.de>,
        linux-doc@...r.kernel.org, kexec-ml <kexec@...ts.infradead.org>,
        linux-kernel@...r.kernel.org, Matthew Wilcox <willy@...radead.org>,
        Omar Sandoval <osandov@...com>,
        Kazuhito Hagio <k-hagio@...jp.nec.com>,
        "Michael S. Tsirkin" <mst@...hat.com>,
        xen-devel@...ts.xenproject.org, linux-fsdevel@...r.kernel.org,
        devel@...uxdriverproject.org, Dave Young <dyoung@...hat.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        Lianbo Jiang <lijiang@...hat.com>
Subject: Re: [PATCH v1 3/8] kexec: export PG_offline to VMCOREINFO

On 21.11.18 07:04, Baoquan He wrote:
> On 11/19/18 at 11:16am, David Hildenbrand wrote:
>> 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
> 
> This solution looks good to me. One small concern is why we don't
> export PG_offline to vmcoreinfo directly, then define
> PAGE_OFFLINE_MAPCOUNT_VALUE in makedumpfile. We have been exporting
> kernel data/MACRO directly, why this one is exceptional.
> 

1. We are much more similar to PG_buddy (in contrast to actual page
flags), and for PG_buddy it is historically handled like this (and I
think it makes sense to expose these as actual MAPCOUNT_VALUEs).

2. Right now only one page type per page is supported. Therefore only
exactly one value in mapcount indicates e.g. PageBuddy()/PageOffline().

Now, if we ever decide to change this (e.g. treat them like real flags),
it is much easier to switch to PG_offline/PG_buddy then. We can directly
see in makedumpfile that .*_MAPCOUNT_VALUE is no longer available but
instead e.g. PG_offline and PG_buddy. Instead we would no see a change
in makedumpfile and would have to rely on other properties.

If there are no strong opinions I will leave it like this.

Thanks!

> Thanks
> Baoquan
> 


-- 

Thanks,

David / dhildenb