Re: [PATCH V4 1/2] mm: add folio_migration_expected_refs() as inline function

[David Hildenbrand <david@redhat.com>]

On 23.04.25 02:36, Matthew Wilcox wrote:
> On Tue, Apr 22, 2025 at 04:41:11PM -0700, Andrew Morton wrote:
>>> +/**
>>> + * folio_migrate_expected_refs - Count expected references for an unmapped folio.
>>
>> "folio_migration_expected_refs"
> 
> Please run make W=1 fs/jfs/ in order to run kernel-doc on this file.
> It'll flag this kind of error.
> 
>> It's concerning that one particular filesystem needs this - one
>> suspects that it is doing something wrong, or that the present API
>> offerings were misdesigned.  It would be helpful if the changelogs were
>> to explain what is special about JFS.
> 
> It doesn't surprise me at all.  Almost no filesystem implements its own
> migrate_folio operation.  Without going into too much detail, almost
> all filesystems can use filemap_migrate_folio(), buffer_migrate_folio()
> or buffer_migrate_folio_norefs().  So this is not an indication that
> jfs is doing anything wrong (except maybe it's misdesigned in that the
> per-folio metadata caches the address of the folio, but changing that
> seems very much too much work to ask someone to do).
> 
> What I do wonder is whether we want to have such a specialised
> function existing.  We have can_split_folio() in huge_memory.c
> which is somewhat more comprehensive and doesn't require the folio to be
> unmapped first.

I was debating with myself whether we should do the usual "refs from 
->private, refs from page table mappings" .. dance, and look up the 
mapping from the folio instead of passing it in.

I concluded that for this (migration) purpose the function is good 
enough as it is: if abused in wrong context (e.g., still ->private, 
still page table mappings), it would not fake that there are no 
unexpected references.

Because references from ->private and page tables would be unexpected at 
this point.

So I'm fine with this.

A more generic function might be helpful, but in general it is more 
prone to races (e.g., page table mappings concurrently going away), so 
it gets trickier to document that properly.

-- 
Cheers,

David / dhildenb