[PATCH 10/16] mm/hugetlb: update hugetlbfs to use mmap_prepare, mmap_complete

Lorenzo Stoakes
2025-09-08T11:12:23Z

We can now update hugetlb to make sure of the new .mmap_prepare() hook, by
deferring the reservation of pages until the VMA is fully established and
handle this in the f_op->mmap_complete() hook.

We hold the VMA write lock throughout so we can't race with faults. rmap
can discover the VMA, but this should not cause a problem.

Signed-off-by: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
---
fs/hugetlbfs/inode.c | 86 ++++++++++++++++++++++++--------------------
1 file changed, 47 insertions(+), 39 deletions(-)

diff --git a/fs/hugetlbfs/inode.c b/fs/hugetlbfs/inode.c
index 3cfdf4091001..46d1ddc654c2 100644
--- a/fs/hugetlbfs/inode.c
+++ b/fs/hugetlbfs/inode.c
@@ -96,39 +96,14 @@ static const struct fs_parameter_spec hugetlb_fs_parameters[] = {
#define PGOFF_LOFFT_MAX \
(((1UL << (PAGE_SHIFT + 1)) - 1) << (BITS_PER_LONG - (PAGE_SHIFT + 1)))

-static int hugetlbfs_file_mmap(struct file *file, struct vm_area_struct *vma)
+static int hugetlb_file_mmap_complete(struct file *file, struct vm_area_struct *vma,
+ const void *context)
{
struct inode *inode = file_inode(file);
- loff_t len, vma_len;
- int ret;
struct hstate *h = hstate_file(file);
- vm_flags_t vm_flags;
-
- /*
- * vma address alignment (but not the pgoff alignment) has
- * already been checked by prepare_hugepage_range. If you add
- * any error returns here, do so after setting VM_HUGETLB, so
- * is_vm_hugetlb_page tests below unmap_region go the right
- * way when do_mmap unwinds (may be important on powerpc
- * and ia64).
- */
- vm_flags_set(vma, VM_HUGETLB | VM_DONTEXPAND);
- vma->vm_ops = &hugetlb_vm_ops;
-
- /*
- * page based offset in vm_pgoff could be sufficiently large to
- * overflow a loff_t when converted to byte offset. This can
- * only happen on architectures where sizeof(loff_t) ==
- * sizeof(unsigned long). So, only check in those instances.
- */
- if (sizeof(unsigned long) == sizeof(loff_t)) {
- if (vma->vm_pgoff & PGOFF_LOFFT_MAX)
- return -EINVAL;
- }
-
- /* must be huge page aligned */
- if (vma->vm_pgoff & (~huge_page_mask(h) >> PAGE_SHIFT))
- return -EINVAL;
+ vm_flags_t vm_flags = vma->vm_flags;
+ loff_t len, vma_len;
+ int ret = 0;

vma_len = (loff_t)(vma->vm_end - vma->vm_start);
len = vma_len + ((loff_t)vma->vm_pgoff << PAGE_SHIFT);
@@ -139,9 +114,6 @@ static int hugetlbfs_file_mmap(struct file *file, struct vm_area_struct *vma)
inode_lock(inode);
file_accessed(file);

- ret = -ENOMEM;
-
- vm_flags = vma->vm_flags;
/*
* for SHM_HUGETLB, the pages are reserved in the shmget() call so skip
* reserving here. Note: only for SHM hugetlbfs file, the inode
@@ -151,20 +123,55 @@ static int hugetlbfs_file_mmap(struct file *file, struct vm_area_struct *vma)
vm_flags |= VM_NORESERVE;

if (hugetlb_reserve_pages(inode,
- vma->vm_pgoff >> huge_page_order(h),
- len >> huge_page_shift(h), vma,
- vm_flags) < 0)
+ vma->vm_pgoff >> huge_page_order(h),
+ len >> huge_page_shift(h), vma,
+ vm_flags) < 0) {
+ ret = -ENOMEM;
goto out;
+ }

- ret = 0;
if (vma->vm_flags & VM_WRITE && inode->i_size < len)
i_size_write(inode, len);
+
out:
inode_unlock(inode);
-
return ret;
}

+static int hugetlbfs_file_mmap_prepare(struct vm_area_desc *desc)
+{
+ struct file *file = desc->file;
+ struct hstate *h = hstate_file(file);
+
+ /*
+ * vma address alignment (but not the pgoff alignment) has
+ * already been checked by prepare_hugepage_range. If you add
+ * any error returns here, do so after setting VM_HUGETLB, so
+ * is_vm_hugetlb_page tests below unmap_region go the right
+ * way when do_mmap unwinds (may be important on powerpc
+ * and ia64).
+ */
+ desc->vm_flags |= VM_HUGETLB | VM_DONTEXPAND;
+ desc->vm_ops = &hugetlb_vm_ops;
+
+ /*
+ * page based offset in vm_pgoff could be sufficiently large to
+ * overflow a loff_t when converted to byte offset. This can
+ * only happen on architectures where sizeof(loff_t) ==
+ * sizeof(unsigned long). So, only check in those instances.
+ */
+ if (sizeof(unsigned long) == sizeof(loff_t)) {
+ if (desc->pgoff & PGOFF_LOFFT_MAX)
+ return -EINVAL;
+ }
+
+ /* must be huge page aligned */
+ if (desc->pgoff & (~huge_page_mask(h) >> PAGE_SHIFT))
+ return -EINVAL;
+
+ return 0;
+}
+
/*
* Called under mmap_write_lock(mm).
*/
@@ -1219,7 +1226,8 @@ static void init_once(void *foo)

static const struct file_operations hugetlbfs_file_operations = {
.read_iter = hugetlbfs_read_iter,
- .mmap = hugetlbfs_file_mmap,
+ .mmap_prepare = hugetlbfs_file_mmap_prepare,
+ .mmap_complete = hugetlb_file_mmap_complete,
.fsync = noop_fsync,
.get_unmapped_area = hugetlb_get_unmapped_area,
.llseek = default_llseek,
--
2.51.0