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From:   ebiederm@xmission.com (Eric W. Biederman)
To:     Johannes Weiner <hannes@cmpxchg.org>
Cc:     Matthew Wilcox <willy@infradead.org>,
        "Darrick J. Wong" <djwong@kernel.org>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        linux-mm@kvack.org, linux-fsdevel@vger.kernel.org,
        linux-kernel@vger.kernel.org,
        Andrew Morton <akpm@linux-foundation.org>
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Date:   Tue, 31 Aug 2021 13:50:35 -0500
In-Reply-To: <YS0/GHBG15+2Mglk@cmpxchg.org> (Johannes Weiner's message of
        "Mon, 30 Aug 2021 16:27:04 -0400")
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Subject: Re: [GIT PULL] Memory folios for v5.15
Precedence: bulk

Johannes Weiner <hannes@cmpxchg.org> writes:

> On Mon, Aug 30, 2021 at 07:22:25PM +0100, Matthew Wilcox wrote:
>> On Mon, Aug 30, 2021 at 01:32:55PM -0400, Johannes Weiner wrote:
>> > > The mistake you're making is coupling "minimum mapping granularity" with
>> > > "minimum allocation granularity".  We can happily build a system which
>> > > only allocates memory on 2MB boundaries and yet lets you map that memory
>> > > to userspace in 4kB granules.
>> > 
>> > Yeah, but I want to do it without allocating 4k granule descriptors
>> > statically at boot time for the entirety of available memory.
>> 
>> Even that is possible when bumping the PAGE_SIZE to 16kB.  It needs a
>> bit of fiddling:
>> 
>> static int insert_page_into_pte_locked(struct mm_struct *mm, pte_t *pte,
>>                         unsigned long addr, struct page *page, pgprot_t prot)
>> {
>>         if (!pte_none(*pte))
>>                 return -EBUSY;
>>         /* Ok, finally just insert the thing.. */
>>         get_page(page);
>>         inc_mm_counter_fast(mm, mm_counter_file(page));
>>         page_add_file_rmap(page, false);
>>         set_pte_at(mm, addr, pte, mk_pte(page, prot));
>>         return 0;
>> }
>> 
>> mk_pte() assumes that a struct page refers to a single pte.  If we
>> revamped it to take (page, offset, prot), it could construct the
>> appropriate pte for the offset within that page.
>
> Right, page tables only need a pfn. The struct page is for us to
> maintain additional state about the object.
>
> For the objects that are subpage sized, we should be able to hold that
> state (shrinker lru linkage, referenced bit, dirtiness, ...) inside
> ad-hoc allocated descriptors.
>
> Descriptors which could well be what struct folio {} is today, IMO. As
> long as it doesn't innately assume, or will assume, in the API the
> 1:1+ mapping to struct page that is inherent to the compound page.

struct buffer_head any one?

I am being silly but when you say you want something that isn't a page
for caching that could be less than a page in size, it really sounds
like you want struct buffer_head.

The only actual problem I am aware of with struct buffer_head is that
it is a block device abstraction and does not map well to other
situations.  Which makes network filesystems unable to use struct
buffer_head.

Eric