On Mon, Nov 17, 2025 at 06:08:57PM +0100, David Hildenbrand (Red Hat) wrote:
On 17.11.25 12:46, Mike Rapoport wrote:
From: "Mike Rapoport (Microsoft)" rppt@kernel.org
When userspace resolves a page fault in a shmem VMA with UFFDIO_CONTINUE it needs to get a folio that already exists in the pagecache backing that VMA.
Instead of using shmem_get_folio() for that, add a get_pagecache_folio() method to 'struct vm_operations_struct' that will return a folio if it exists in the VMA's pagecache at given pgoff.
Implement get_pagecache_folio() method for shmem and slightly refactor userfaultfd's mfill_atomic() and mfill_atomic_pte_continue() to support this new API.
Signed-off-by: Mike Rapoport (Microsoft) rppt@kernel.org
include/linux/mm.h | 9 +++++++ mm/shmem.c | 20 ++++++++++++++++ mm/userfaultfd.c | 60 ++++++++++++++++++++++++++++++---------------- 3 files changed, 69 insertions(+), 20 deletions(-)
diff --git a/include/linux/mm.h b/include/linux/mm.h index d16b33bacc32..c35c1e1ac4dd 100644 --- a/include/linux/mm.h +++ b/include/linux/mm.h @@ -690,6 +690,15 @@ struct vm_operations_struct { struct page *(*find_normal_page)(struct vm_area_struct *vma, unsigned long addr); #endif /* CONFIG_FIND_NORMAL_PAGE */ +#ifdef CONFIG_USERFAULTFD
- /*
* Called by userfault to resolve UFFDIO_CONTINUE request.* Should return the folio found at pgoff in the VMA's pagecache if it* exists or ERR_PTR otherwise.*/What are the locking +refcount rules? Without looking at the code, I would assume we return with a folio reference held and the folio locked?
Right, will add it to the comment
- struct folio *(*get_pagecache_folio)(struct vm_area_struct *vma,
pgoff_t pgoff);The combination of VMA + pgoff looks weird at first. Would vma + addr or vma+vma_offset into vma be better?
Copied from map_pages() :)
But it also makes me wonder if the callback would ever even require the VMA, or actually only vma->vm_file?
It's actually inode, I'm going to pass that instead of vma.
Thinking out loud, I wonder if one could just call that "get_folio" or "get_shared_folio" (IOW, never an anon folio in a MAP_PRIVATE mapping).
Naming is hard :)
get_shared_folio() sounds good to me so unless there other suggestions I'll stick with it.
+#endif }; #ifdef CONFIG_NUMA_BALANCING
...
+static __always_inline bool vma_can_mfill_atomic(struct vm_area_struct *vma,
uffd_flags_t flags)+{
- if (uffd_flags_mode_is(flags, MFILL_ATOMIC_CONTINUE)) {
if (vma->vm_ops && vma->vm_ops->get_pagecache_folio)return true;elsereturn false;Probably easier to read is
return vma->vm_ops && vma->vm_ops->get_pagecache_folio;
- }
- if (vma_is_anonymous(vma) || vma_is_shmem(vma))
return true;- return false;
Could also be simplified to:
return vma_is_anonymous(vma) || vma_is_shmem(vma);
Agree with for both of them.
-- Cheers
David