linux-kernel - Re: [RFC v3 2/4] mm: move PG_slab flag to page

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Date:   Wed, 8 Feb 2023 11:13:57 +0100
From:   David Hildenbrand <david@...hat.com>
To:     Mike Rapoport <rppt@...nel.org>
Cc:     Matthew Wilcox <willy@...radead.org>,
        Hyeonggon Yoo <42.hyeyoo@...il.com>,
        Vlastimil Babka <vbabka@...e.cz>,
        Christoph Lameter <cl@...ux.com>,
        Pekka Enberg <penberg@...nel.org>,
        David Rientjes <rientjes@...gle.com>,
        Joonsoo Kim <iamjoonsoo.kim@....com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Roman Gushchin <roman.gushchin@...ux.dev>,
        HORIGUCHI NAOYA(堀口 直也) 
        <naoya.horiguchi@....com>, Joe Perches <joe@...ches.com>,
        Petr Mladek <pmladek@...e.com>,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        Alexander Potapenko <glider@...gle.com>,
        Marco Elver <elver@...gle.com>
Subject: Re: [RFC v3 2/4] mm: move PG_slab flag to page_type

On 08.02.23 10:44, Mike Rapoport wrote:
> On Fri, Feb 03, 2023 at 05:04:01PM +0100, David Hildenbrand wrote:
>> On 30.01.23 06:11, Matthew Wilcox wrote:
>>> On Mon, Jan 30, 2023 at 01:34:59PM +0900, Hyeonggon Yoo wrote:
>>>>> Seems like quite some changes to page_type to accomodate SLAB, which is
>>>>> hopefully going away soon(TM). Could we perhaps avoid that?
>>>>
>>>> If it could be done with less changes, I'll try to avoid that.
>>>
>>> Let me outline the idea I had for removing PG_slab:
>>>
>>> Observe that PG_reserved and PG_slab are mutually exclusive.  Also,
>>
>> I recall that there are SetPageReserved() calls on pages allocated via slab.
> 
> I did a quick scan, and it seems that all allocated reserved pages come
> from alloc_pages() or vmalloc().
>   

Thanks for checking, good that my memory was wrong :)

> BTW, looking at the current usage of SetPageReserved, it seems that some
> are bogus/historical and some would justify a different page type if we are
> to have 15 bits for PG_kernel.

I remember raising in the past that something like PG_hole might be helpful.

I also recall that most SetPageReserved usage in drivers is to make 
ioremap happy. PG_ioremappable or smth like that would be better.

So yes, that would even help to cleanup the existing PG_reserved usage.

-- 
Thanks,

David / dhildenb