linux-kernel - Re: Struct page proposal

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Date:   Mon, 27 Sep 2021 20:16:15 +0200
From:   David Hildenbrand <david@...hat.com>
To:     Matthew Wilcox <willy@...radead.org>,
        Kent Overstreet <kent.overstreet@...il.com>
Cc:     Vlastimil Babka <vbabka@...e.cz>, linux-fsdevel@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        Johannes Weiner <hannes@...xchg.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        "Darrick J. Wong" <djwong@...nel.org>,
        Christoph Hellwig <hch@...radead.org>,
        David Howells <dhowells@...hat.com>
Subject: Re: Struct page proposal

On 27.09.21 20:12, Matthew Wilcox wrote:
> On Mon, Sep 27, 2021 at 02:09:49PM -0400, Kent Overstreet wrote:
>> On Mon, Sep 27, 2021 at 07:05:26PM +0100, Matthew Wilcox wrote:
>>> On Mon, Sep 27, 2021 at 07:48:15PM +0200, Vlastimil Babka wrote:
>>>> On 9/23/21 03:21, Kent Overstreet wrote:
>>>>> So if we have this:
>>>>>
>>>>> struct page {
>>>>> 	unsigned long	allocator;
>>>>> 	unsigned long	allocatee;
>>>>> };
>>>>>
>>>>> The allocator field would be used for either a pointer to slab/slub's state, if
>>>>> it's a slab page, or if it's a buddy allocator page it'd encode the order of the
>>>>> allocation - like compound order today, and probably whether or not the
>>>>> (compound group of) pages is free.
>>>>
>>>> The "free page in buddy allocator" case will be interesting to implement.
>>>> What the buddy allocator uses today is:
>>>>
>>>> - PageBuddy - determine if page is free; a page_type (part of mapcount
>>>> field) today, could be a bit in "allocator" field that would have to be 0 in
>>>> all other "page is allocated" contexts.
>>>> - nid/zid - to prevent merging accross node/zone boundaries, now part of
>>>> page flags
>>>> - buddy order
>>>> - a list_head (reusing the "lru") to hold the struct page on the appropriate
>>>> free list, which has to be double-linked so page can be taken from the
>>>> middle of the list instantly
>>>>
>>>> Won't be easy to cram all that into two unsigned long's, or even a single
>>>> one. We should avoid storing anything in the free page itself. Allocating
>>>> some external structures to track free pages is going to have funny
>>>> bootstrap problems. Probably a major redesign would be needed...
>>>
>>> Wait, why do we want to avoid using the memory that we're allocating?
>>
>> The issue is where to stick the state for free pages. If that doesn't fit in two
>> ulongs, then we'd need a separate allocation, which means slab needs to be up
>> and running before free pages are initialized.
> 
> But the thing we're allocating is at least PAGE_SIZE bytes in size.
> Why is "We should avoid storing anything in the free page itself" true?
> 

Immediately comes to mind:
* Free page reporting via virtio-balloon
* CMA on s390x (see arch_free_page())
* Free page poisoning
* init_on_free

-- 
Thanks,

David / dhildenb