Re: [PATCH] mm/gup: fix GUP-fast fallback for NULL-mapping order-0 folios

[Zi Yan]

On 9 Apr 2026, at 11:11, David Hildenbrand (Arm) wrote:

> On 4/9/26 17:05, Zi Yan wrote:
>> On 9 Apr 2026, at 3:52, David Hildenbrand (Arm) wrote:
>>
>>> On 4/9/26 03:46, John Hubbard wrote:
>>>> Since commit f002882ca369 ("mm: merge folio_is_secretmem() and
>>>> folio_fast_pin_allowed() into gup_fast_folio_allowed()"),
>>>> gup_fast_folio_allowed() falls back to the slow path for any order-0
>>>> folio with a NULL mapping when CONFIG_SECRETMEM=y. This causes a
>>>> performance regression for drivers that allocate pages with alloc_page()
>>>> and insert them into VMAs via vm_insert_page(). These pages legitimately
>>>> have a NULL folio->mapping, but they cannot be secretmem pages.
>>>>
>>>> Secretmem pages are always added to the secretmem inode's page cache via
>>>> filemap_add_folio(), which sets folio->mapping to the inode's i_mapping.
>>>> A folio with a NULL mapping can never be a secretmem folio. The
>>>> NULL-mapping check was intended to handle truncated file-backed pages (a
>>>> reject_file_backed concern), not secretmem detection.
>>>>
>>>> When only check_secretmem is true (and reject_file_backed is false), a
>>>> NULL mapping is sufficient to prove the folio is not secretmem, so the
>>>> fast path can proceed.
>>>
>>> Hm, what if secretmem folio just got truncated? I hate to rely on some
>>> handling in the caller to detect truncation differently during GUP-fast,
>>> but this function returning "true".
>>>
>>> Zi is working on a way to distinguish folios from non-folio things: that
>>> we can identify whatever was added through vm_insert_page().
>>
>> The idea is to give every allocated page a NotRmappable page_type and
>> 1) let page_rmappable_folio() remove the page_type by setting mapcount
>> to 0, since page_type overlaps with mapcount;
>> 2) allow callers of alloc_page*() to change page_type from NotRmappable
>> to other types.
>
> Could we instead just set the special type in vm_insert_page(), after
> verifying that it does not have any other type?

You mean in validate_page_before_insert() we set the page to NotRmappable
if it can be inserted, then in the rmap code we reject NotRmappable
page/folios? And NotRmappable will remain until the page/folio is freed.
That should work. So the purpose is to avoid setting the page_type if
a page/folio is never going through vm_insert_page()?

>
>>
>> After these, rmappable folios with any order is a folio without any
>> page_type. HugeTLB is an exception, but rmap code has special handling
>> for them, so it should be fine.
>>
>> I have a draft[1], but have not had time to refresh it yet. I am planning
>> to work on this after my two other jobs (make READ_ONLY_THP_FOR_FS enabled
>> for all FS with large folio support and ensure ->private is 0 for tail pages).
>>
>> [1] https://github.com/x-y-z/linux-dev/tree/remove_mapcount_for_non_folio-2026-02-16-23-27
>>
>>>
>>> Because that's really the key problem here: vm_insert_page() pages are
>>> not actually folios, they just look like a folio today, but looking at
>>> fields like ->mapping does not make any sense.
>>
>> Based on my understanding of some callers of alloc_page()/get_free_pages(),
>> vm_insert_page(), they use GFP_COMP to get a compound page and vm_insert_page()
>> individual pages of the compound page. These pages might be pinned.
>> During past THP cabal meeting, we discussed about it if
>> 1. we should disallow such uses and only allow order-0 allocations, or
>
> Right.
>
>> 2. we need a new type like new_type_folio for them and the type does not
>> have folio fields except ->pincount.
>
> Right.
>
> -- 
> Cheers,
>
> David


Best Regards,
Yan, Zi