Re: [PATCH] mm: thp: Deny THP for guest_memfd and secretmem in file_thp_enabled()
From: David Hildenbrand (Arm)
Date: Mon Feb 09 2026 - 14:45:13 EST
On 2/9/26 19:22, Ackerley Tng wrote:
Deepanshu Kartikey <kartikey406@xxxxxxxxx> writes:
On Mon, Feb 9, 2026 at 4:12 PM David Hildenbrand (Arm) <david@xxxxxxxxxx> wrote:
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.
i_writecount means "the file is open with write access" IIUC. So one can mmap(PROT_WRITE) it etc.
And that's kind of the thing: the virtual file is open with write access. That's why I am still wondering whether mimicking that is actually the right fix.
- 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.
The thing is that CONFIG_READ_ONLY_THP_FOR_FS explicitly bypasses these folio order checks. Changing it would degrade filesystems that do not support large folios yet. IOW, it would be similar to ripping out CONFIG_READ_ONLY_THP_FOR_FS. Which we plan for one of the next releases :)
Right, it does not really make sense
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?
+ 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.
It could be used at least for the time being and to fix the issue.
--
Cheers,
David