On 17.11.25 12:46, Mike Rapoport wrote:
From: "Mike Rapoport (Microsoft)" <[email protected]>
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) <[email protected]>
---
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?
+ 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?
But it also makes me wonder if the callback would ever even require the
VMA, or actually only vma->vm_file?
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).
+#endif
};
#ifdef CONFIG_NUMA_BALANCING
diff --git a/mm/shmem.c b/mm/shmem.c
index b9081b817d28..4ac122284bff 100644
--- a/mm/shmem.c
+++ b/mm/shmem.c
@@ -3260,6 +3260,20 @@ int shmem_mfill_atomic_pte(pmd_t *dst_pmd,
shmem_inode_unacct_blocks(inode, 1);
return ret;
}
+
+static struct folio *shmem_get_pagecache_folio(struct vm_area_struct *vma,
+ pgoff_t pgoff)
+{
+ struct inode *inode = file_inode(vma->vm_file);
+ struct folio *folio;
+ int err;
+
+ err = shmem_get_folio(inode, pgoff, 0, &folio, SGP_NOALLOC);
+ if (err)
+ return ERR_PTR(err);
+
+ return folio;
+}
#endif /* CONFIG_USERFAULTFD */
#ifdef CONFIG_TMPFS
@@ -5292,6 +5306,9 @@ static const struct vm_operations_struct shmem_vm_ops = {
.set_policy = shmem_set_policy,
.get_policy = shmem_get_policy,
#endif
+#ifdef CONFIG_USERFAULTFD
+ .get_pagecache_folio = shmem_get_pagecache_folio,
+#endif
};
static const struct vm_operations_struct shmem_anon_vm_ops = {
@@ -5301,6 +5318,9 @@ static const struct vm_operations_struct
shmem_anon_vm_ops = {
.set_policy = shmem_set_policy,
.get_policy = shmem_get_policy,
#endif
+#ifdef CONFIG_USERFAULTFD
+ .get_pagecache_folio = shmem_get_pagecache_folio,
+#endif
};
int shmem_init_fs_context(struct fs_context *fc)
diff --git a/mm/userfaultfd.c b/mm/userfaultfd.c
index 8dc964389b0d..60b3183a72c0 100644
--- a/mm/userfaultfd.c
+++ b/mm/userfaultfd.c
@@ -382,21 +382,17 @@ static int mfill_atomic_pte_continue(pmd_t *dst_pmd,
unsigned long dst_addr,
uffd_flags_t flags)
{
- struct inode *inode = file_inode(dst_vma->vm_file);
pgoff_t pgoff = linear_page_index(dst_vma, dst_addr);
struct folio *folio;
struct page *page;
int ret;
- ret = shmem_get_folio(inode, pgoff, 0, &folio, SGP_NOALLOC);
+ folio = dst_vma->vm_ops->get_pagecache_folio(dst_vma, pgoff);
/* Our caller expects us to return -EFAULT if we failed to find folio */
- if (ret == -ENOENT)
- ret = -EFAULT;
- if (ret)
- goto out;
- if (!folio) {
- ret = -EFAULT;
- goto out;
+ if (IS_ERR_OR_NULL(folio)) {
+ if (PTR_ERR(folio) == -ENOENT || !folio)
+ return -EFAULT;
+ return PTR_ERR(folio);
}
page = folio_file_page(folio, pgoff);
@@ -411,13 +407,12 @@ static int mfill_atomic_pte_continue(pmd_t *dst_pmd,
goto out_release;
folio_unlock(folio);
- ret = 0;
-out:
- return ret;
+ return 0;
+
out_release:
folio_unlock(folio);
folio_put(folio);
- goto out;
+ return ret;
}
/* Handles UFFDIO_POISON for all non-hugetlb VMAs. */
@@ -694,6 +689,22 @@ static __always_inline ssize_t mfill_atomic_pte(pmd_t
*dst_pmd,
return err;
}
+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;
+ else
+ return 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);
--
Cheers
David
