linux-kernel - Re: [PATCH] mm: thp: Deny THP for guest_memfd and secretmem in file_thp

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Message-ID: <CAEvNRgEoPs5ia+u0ODra6N4BdOwpYa04j8nDxwfNVyfCOWnM6g@mail.gmail.com>
Date: Mon, 9 Feb 2026 10:22:06 -0800
From: Ackerley Tng <ackerleytng@...gle.com>
To: Deepanshu Kartikey <kartikey406@...il.com>, "David Hildenbrand (Arm)" <david@...nel.org>
Cc: akpm@...ux-foundation.org, lorenzo.stoakes@...cle.com, 
	baolin.wang@...ux.alibaba.com, Liam.Howlett@...cle.com, npache@...hat.com, 
	ryan.roberts@....com, dev.jain@....com, baohua@...nel.org, seanjc@...gle.com, 
	pbonzini@...hat.com, michael.roth@....com, vannapurve@...gle.com, 
	ziy@...dia.com, linux-mm@...ck.org, linux-kernel@...r.kernel.org, 
	syzbot+33a04338019ac7e43a44@...kaller.appspotmail.com
Subject: Re: [PATCH] mm: thp: Deny THP for guest_memfd and secretmem in file_thp_enabled()

Deepanshu Kartikey <kartikey406@...il.com> writes:

> On Mon, Feb 9, 2026 at 4:12 PM David Hildenbrand (Arm) <david@...nel.org> wrote:
>>
>> > Maybe introduce a AS_NO_READ_ONLY_THP_FOR_FS, which we can just easily
>> > rip out along with CONFIG_READ_ONLY_THP_FOR_FS later?
>>
>> On second thought, why do we pass the
>>
>>         !inode_is_open_for_write(inode)
>>
>> in file_thp_enabled()?
>>
>> Isn't that the main problem for these memfd things?
>>
>> Maybe a get_write_access() is missing somewhere?
>>
>
> Hi David,
>
> Thanks for the suggestion. I looked into the get_write_access() path.
>
> Both guest_memfd and secretmem use alloc_file_pseudo() which skips
> calling get_write_access(), so i_writecount stays 0. That's why
> file_thp_enabled() sees them as read-only files.
>
> We could add get_write_access() after alloc_file_pseudo() in both, but
> I think that would be a hack rather than a proper fix:
>
> - i_writecount has a specific semantic: tracking how many fds have the
> file open for writing. We'd be bumping it just to influence
> file_thp_enabled() behavior.
>

I agree re-using i_writecount feels odd since it is abusing the idea of
being written to. I might have misunderstood the full context of
i_writecount though.

> - It doesn't express the actual intent. The real issue is that
> CONFIG_READ_ONLY_THP_FOR_FS was never meant for pseudo-filesystem
> backed files.
>
> I think the AS_NO_READ_ONLY_THP_FOR_FS flag you suggested earlier is
> the cleaner approach. It is explicit, has no side effects, and is easy
> to rip out when CONFIG_READ_ONLY_THP_FOR_FS goes away.
>

I was considering other address space flags and I think the best might
be to make khugepaged respect AS_FOLIO_ORDER_MAX and have somewhere in
__vma_thp_allowable_orders() check the maximum allowed order for the
address space.

khugepaged is about consolidating memory to huge pages, so if the
address space doesn't allow a larger folio order, then khugepaged should
not operate on that memory.

The other options are

+ AS_UNEVICTABLE: Sounds like khugepaged should respect AS_UNEVICTABLE,
  but IIUC evictability is more closely related to swapping and
  khugepaged might operate on swappable memory? Both guest_memfd and
  secretmem set AS_UNEVICTABLE.
+ AS_INACCESSIBLE: This is only used by guest_memfd, and is mostly used
  to block migration. khugepaged kind of migrates the memory contents
  too, but someday we want guest_memfd to support migration, and at that
  time we would still want to block khugepaged, so I don't think we want
  to reuse a flag that couples khugepaged to migration.

>
> [...snip...]
>