Johannes Weiner <hannes@xxxxxxxxxxx> 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
