linux-kernel - Re: [PATCH V4 1/2] mm: add folio_migration_expected

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Message-ID: <8f24de4d-5088-498a-968d-9e8bb85201ac@redhat.com>
Date: Wed, 23 Apr 2025 09:25:05 +0200
From: David Hildenbrand <david@...hat.com>
To: Matthew Wilcox <willy@...radead.org>,
 Andrew Morton <akpm@...ux-foundation.org>
Cc: Shivank Garg <shivankg@....com>, shaggy@...nel.org,
 wangkefeng.wang@...wei.com, jane.chu@...cle.com, ziy@...dia.com,
 donettom@...ux.ibm.com, apopple@...dia.com,
 jfs-discussion@...ts.sourceforge.net, linux-kernel@...r.kernel.org,
 linux-mm@...ck.org, syzbot+8bb6fd945af4e0ad9299@...kaller.appspotmail.com
Subject: Re: [PATCH V4 1/2] mm: add folio_migration_expected_refs() as inline
 function

On 23.04.25 09:22, David Hildenbrand wrote:
> 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.

Sorry, I forgot that we still care about the reference from ->private 
here. We expect the folio to be unmapped.

-- 
Cheers,

David / dhildenb