linux-kernel - Re: [GIT PULL] Memory folios for v5.15

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Date:   Tue, 31 Aug 2021 13:50:35 -0500
From:   ebiederm@...ssion.com (Eric W. Biederman)
To:     Johannes Weiner <hannes@...xchg.org>
Cc:     Matthew Wilcox <willy@...radead.org>,
        "Darrick J. Wong" <djwong@...nel.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        linux-mm@...ck.org, linux-fsdevel@...r.kernel.org,
        linux-kernel@...r.kernel.org,
        Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [GIT PULL] Memory folios for v5.15

Johannes Weiner <hannes@...xchg.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