Re: [RFC v2 0/3] Decoupling large folios dependency on THP

[David Hildenbrand (Arm)]

On 2/27/26 16:19, Matthew Wilcox wrote:
> On Fri, Feb 27, 2026 at 09:45:07AM +0100, David Hildenbrand (Arm) wrote:
>> I guess it would be rather trivial to just replace
>> add_to_page_cache_lru() by filemap_add_folio() in below code.
> 
> In the Ottawa interpretation, that's true, but I'd prefer not to revisit
> this code when transitioning to the New York interpretation.  This is
> the NOMMU code after all, and the less time we spend on it, the better.
> 
>>> So either we need to reimplement all the good stuff that folio_split()
>>> does for us, or we need to make folio_split() available on nommu.
>>
>> folio splitting usually involves unmapping pages, which is rather
>> cumbersome on nommu ;) So we'd have to think about that and the
>> implications.
> 
> Depending on your point of view, either everything is mapped on nommu,
> or nothing is mapped ;-)  In any case, the folio is freshly-allocated
> and locked, so there's no chance anybody has mapped it yet.
> 
>> ramfs_nommu_expand_for_mapping() is all about allocating memory, not
>> splitting something that might already in use somewhere.
>>
>> So I folio_split() on nommu is a bit weird in that context.
> 
> Well, it is, but it's also exactly what we need to do -- frees folios
> which are now entirely beyond i_size.  And it's code that's also used on
> MMU systems, and the more code that's shared, the better.
> 
>> When it comes to allocating memory, I would assume that it would be
>> better (and faster!) to
>>
>> a) allocate a frozen high-order page
>>
>> b) Create the (large) folios directly on chunks of the frozen page, and
>> add them through filemap_add_folio().
>>
>> We'd have a function that consumes a suitable page range and turns it
>> into a folio (later allocates memdesc).
>>
>> c) Return all unused frozen bits to the page allocator
> 
> Right, we could do that.  But that's more code and special code in the
> nommu codebase.

I guess I'd have to see the frankenstein folio_split() to judge if it is
really better :)

If it's really just about skipping the unmap+remap the end result would
indeed be nice.

-- 
Cheers,

David