Re: [PATCH v2 10/15] shmem, userfaultfd: implement shmem uffd operations using vm_uffd_ops

James Houghton
2026-03-27T01:14:20Z

On Fri, Mar 6, 2026 at 9:19 AM Mike Rapoport <rppt@kernel.org> wrote:
>
> From: "Mike Rapoport (Microsoft)" <rppt@kernel.org>
>
> Add filemap_add() and filemap_remove() methods to vm_uffd_ops and use
> them in __mfill_atomic_pte() to add shmem folios to page cache and
> remove them in case of error.
>
> Implement these methods in shmem along with vm_uffd_ops->alloc_folio()
> and drop shmem_mfill_atomic_pte().
>
> Since userfaultfd now does not reference any functions from shmem, drop
> include if linux/shmem_fs.h from mm/userfaultfd.c
>
> mfill_atomic_install_pte() is not used anywhere outside of
> mm/userfaultfd, make it static.
>
> Signed-off-by: Mike Rapoport (Microsoft) <rppt@kernel.org>
> ---
> include/linux/shmem_fs.h | 14 ----
> include/linux/userfaultfd_k.h | 21 +++--
> mm/shmem.c | 148 ++++++++++++----------------------
> mm/userfaultfd.c | 79 +++++++++---------
> 4 files changed, 106 insertions(+), 156 deletions(-)
>
> diff --git a/include/linux/shmem_fs.h b/include/linux/shmem_fs.h
> index a8273b32e041..1a345142af7d 100644
> --- a/include/linux/shmem_fs.h
> +++ b/include/linux/shmem_fs.h
> @@ -221,20 +221,6 @@ static inline pgoff_t shmem_fallocend(struct inode *inode, pgoff_t eof)
>
> extern bool shmem_charge(struct inode *inode, long pages);
>
> -#ifdef CONFIG_USERFAULTFD
> -#ifdef CONFIG_SHMEM
> -extern int shmem_mfill_atomic_pte(pmd_t *dst_pmd,
> - struct vm_area_struct *dst_vma,
> - unsigned long dst_addr,
> - unsigned long src_addr,
> - uffd_flags_t flags,
> - struct folio **foliop);
> -#else /* !CONFIG_SHMEM */
> -#define shmem_mfill_atomic_pte(dst_pmd, dst_vma, dst_addr, \
> - src_addr, flags, foliop) ({ BUG(); 0; })
> -#endif /* CONFIG_SHMEM */
> -#endif /* CONFIG_USERFAULTFD */
> -
> /*
> * Used space is stored as unsigned 64-bit value in bytes but
> * quota core supports only signed 64-bit values so use that
> diff --git a/include/linux/userfaultfd_k.h b/include/linux/userfaultfd_k.h
> index 4d8b879eed91..bf4e595ac914 100644
> --- a/include/linux/userfaultfd_k.h
> +++ b/include/linux/userfaultfd_k.h
> @@ -93,10 +93,24 @@ struct vm_uffd_ops {
> struct folio *(*get_folio_noalloc)(struct inode *inode, pgoff_t pgoff);
> /*
> * Called during resolution of UFFDIO_COPY request.
> - * Should return allocate a and return folio or NULL if allocation fails.
> + * Should allocate and return a folio or NULL if allocation
> + * fails.
> */
> struct folio *(*alloc_folio)(struct vm_area_struct *vma,
> unsigned long addr);
> + /*
> + * Called during resolution of UFFDIO_COPY request.
> + * Should lock the folio and add it to VMA's page cache.

I don't think "should lock the folio" is accurate. That sounds like
"it will call folio_lock()" but it actually calls
__folio_set_locked(). Maybe this is better:

"Should only be called with a folio returned by alloc_folio() above.
The folio will set to locked."

> + * Returns 0 on success, error code on failure.
> + */
> + int (*filemap_add)(struct folio *folio, struct vm_area_struct *vma,
> + unsigned long addr);
> + /*
> + * Called during resolution of UFFDIO_COPY request on the error
> + * handling path.
> + * Should revert the operation of ->filemap_add().
> + */
> + void (*filemap_remove)(struct folio *folio, struct vm_area_struct *vma);
> };
>
> /* A combined operation mode + behavior flags. */
> @@ -130,11 +144,6 @@ static inline uffd_flags_t uffd_flags_set_mode(uffd_flags_t flags, enum mfill_at
> /* Flags controlling behavior. These behavior changes are mode-independent. */
> #define MFILL_ATOMIC_WP MFILL_ATOMIC_FLAG(0)
>
> -extern int mfill_atomic_install_pte(pmd_t *dst_pmd,
> - struct vm_area_struct *dst_vma,
> - unsigned long dst_addr, struct page *page,
> - bool newly_allocated, uffd_flags_t flags);
> -
> extern ssize_t mfill_atomic_copy(struct userfaultfd_ctx *ctx, unsigned long dst_start,
> unsigned long src_start, unsigned long len,
> uffd_flags_t flags);
> diff --git a/mm/shmem.c b/mm/shmem.c
> index 7bd887b64f62..68620caaf75f 100644
> --- a/mm/shmem.c
> +++ b/mm/shmem.c
> @@ -3181,118 +3181,73 @@ static struct inode *shmem_get_inode(struct mnt_idmap *idmap,
> #endif /* CONFIG_TMPFS_QUOTA */
>
> #ifdef CONFIG_USERFAULTFD
> -int shmem_mfill_atomic_pte(pmd_t *dst_pmd,
> - struct vm_area_struct *dst_vma,
> - unsigned long dst_addr,
> - unsigned long src_addr,
> - uffd_flags_t flags,
> - struct folio **foliop)
> -{
> - struct inode *inode = file_inode(dst_vma->vm_file);
> - struct shmem_inode_info *info = SHMEM_I(inode);
> +static struct folio *shmem_mfill_folio_alloc(struct vm_area_struct *vma,
> + unsigned long addr)
> +{
> + struct inode *inode = file_inode(vma->vm_file);
> struct address_space *mapping = inode->i_mapping;
> + struct shmem_inode_info *info = SHMEM_I(inode);
> + pgoff_t pgoff = linear_page_index(vma, addr);
> gfp_t gfp = mapping_gfp_mask(mapping);
> - pgoff_t pgoff = linear_page_index(dst_vma, dst_addr);
> - void *page_kaddr;
> struct folio *folio;
> - int ret;
> - pgoff_t max_off;
> -
> - if (shmem_inode_acct_blocks(inode, 1)) {
> - /*
> - * We may have got a page, returned -ENOENT triggering a retry,
> - * and now we find ourselves with -ENOMEM. Release the page, to
> - * avoid a BUG_ON in our caller.
> - */
> - if (unlikely(*foliop)) {
> - folio_put(*foliop);
> - *foliop = NULL;
> - }
> - return -ENOMEM;
> - }
>
> - if (!*foliop) {
> - ret = -ENOMEM;
> - folio = shmem_alloc_folio(gfp, 0, info, pgoff);
> - if (!folio)
> - goto out_unacct_blocks;
> + if (unlikely(pgoff >= DIV_ROUND_UP(i_size_read(inode), PAGE_SIZE)))
> + return NULL;
>
> - if (uffd_flags_mode_is(flags, MFILL_ATOMIC_COPY)) {
> - page_kaddr = kmap_local_folio(folio, 0);
> - /*
> - * The read mmap_lock is held here. Despite the
> - * mmap_lock being read recursive a deadlock is still
> - * possible if a writer has taken a lock. For example:
> - *
> - * process A thread 1 takes read lock on own mmap_lock
> - * process A thread 2 calls mmap, blocks taking write lock
> - * process B thread 1 takes page fault, read lock on own mmap lock
> - * process B thread 2 calls mmap, blocks taking write lock
> - * process A thread 1 blocks taking read lock on process B
> - * process B thread 1 blocks taking read lock on process A
> - *
> - * Disable page faults to prevent potential deadlock
> - * and retry the copy outside the mmap_lock.
> - */
> - pagefault_disable();
> - ret = copy_from_user(page_kaddr,
> - (const void __user *)src_addr,
> - PAGE_SIZE);
> - pagefault_enable();
> - kunmap_local(page_kaddr);
> -
> - /* fallback to copy_from_user outside mmap_lock */
> - if (unlikely(ret)) {
> - *foliop = folio;
> - ret = -ENOENT;
> - /* don't free the page */
> - goto out_unacct_blocks;
> - }
> + folio = shmem_alloc_folio(gfp, 0, info, pgoff);
> + if (!folio)
> + return NULL;
>
> - flush_dcache_folio(folio);
> - } else { /* ZEROPAGE */
> - clear_user_highpage(&folio->page, dst_addr);
> - }
> - } else {
> - folio = *foliop;
> - VM_BUG_ON_FOLIO(folio_test_large(folio), folio);
> - *foliop = NULL;
> + if (mem_cgroup_charge(folio, vma->vm_mm, GFP_KERNEL)) {
> + folio_put(folio);
> + return NULL;
> }
>
> - VM_BUG_ON(folio_test_locked(folio));
> - VM_BUG_ON(folio_test_swapbacked(folio));
> + return folio;
> +}
> +
> +static int shmem_mfill_filemap_add(struct folio *folio,
> + struct vm_area_struct *vma,
> + unsigned long addr)
> +{
> + struct inode *inode = file_inode(vma->vm_file);
> + struct address_space *mapping = inode->i_mapping;
> + pgoff_t pgoff = linear_page_index(vma, addr);
> + gfp_t gfp = mapping_gfp_mask(mapping);
> + int err;
> +
> __folio_set_locked(folio);
> __folio_set_swapbacked(folio);
> - __folio_mark_uptodate(folio);
> -
> - ret = -EFAULT;
> - max_off = DIV_ROUND_UP(i_size_read(inode), PAGE_SIZE);
> - if (unlikely(pgoff >= max_off))
> - goto out_release;
>
> - ret = mem_cgroup_charge(folio, dst_vma->vm_mm, gfp);
> - if (ret)
> - goto out_release;
> - ret = shmem_add_to_page_cache(folio, mapping, pgoff, NULL, gfp);
> - if (ret)
> - goto out_release;
> + err = shmem_add_to_page_cache(folio, mapping, pgoff, NULL, gfp);
> + if (err)
> + goto err_unlock;
>
> - ret = mfill_atomic_install_pte(dst_pmd, dst_vma, dst_addr,
> - &folio->page, true, flags);
> - if (ret)
> - goto out_delete_from_cache;
> + if (shmem_inode_acct_blocks(inode, 1)) {
> + err = -ENOMEM;
> + goto err_delete_from_cache;
> + }
>
> + folio_add_lru(folio);
> shmem_recalc_inode(inode, 1, 0);
> - folio_unlock(folio);
> +
> return 0;
> -out_delete_from_cache:
> +
> +err_delete_from_cache:
> filemap_remove_folio(folio);
> -out_release:
> +err_unlock:
> + folio_unlock(folio);
> + return err;
> +}
> +
> +static void shmem_mfill_filemap_remove(struct folio *folio,
> + struct vm_area_struct *vma)
> +{
> + struct inode *inode = file_inode(vma->vm_file);
> +
> + filemap_remove_folio(folio);
> + shmem_recalc_inode(inode, 0, 0);
> folio_unlock(folio);
> - folio_put(folio);
> -out_unacct_blocks:
> - shmem_inode_unacct_blocks(inode, 1);
> - return ret;
> }
>
> static struct folio *shmem_get_folio_noalloc(struct inode *inode, pgoff_t pgoff)
> @@ -3315,6 +3270,9 @@ static bool shmem_can_userfault(struct vm_area_struct *vma, vm_flags_t vm_flags)
> static const struct vm_uffd_ops shmem_uffd_ops = {
> .can_userfault = shmem_can_userfault,
> .get_folio_noalloc = shmem_get_folio_noalloc,
> + .alloc_folio = shmem_mfill_folio_alloc,
> + .filemap_add = shmem_mfill_filemap_add,
> + .filemap_remove = shmem_mfill_filemap_remove,
> };
> #endif /* CONFIG_USERFAULTFD */
>
> diff --git a/mm/userfaultfd.c b/mm/userfaultfd.c
> index 31f3ab6a73e2..a0f8e67006d6 100644
> --- a/mm/userfaultfd.c
> +++ b/mm/userfaultfd.c
> @@ -14,7 +14,6 @@
> #include <linux/userfaultfd_k.h>
> #include <linux/mmu_notifier.h>
> #include <linux/hugetlb.h>
> -#include <linux/shmem_fs.h>
> #include <asm/tlbflush.h>
> #include <asm/tlb.h>
> #include "internal.h"
> @@ -340,10 +339,10 @@ static bool mfill_file_over_size(struct vm_area_struct *dst_vma,
> * This function handles both MCOPY_ATOMIC_NORMAL and _CONTINUE for both shmem
> * and anon, and for both shared and private VMAs.
> */
> -int mfill_atomic_install_pte(pmd_t *dst_pmd,
> - struct vm_area_struct *dst_vma,
> - unsigned long dst_addr, struct page *page,
> - bool newly_allocated, uffd_flags_t flags)
> +static int mfill_atomic_install_pte(pmd_t *dst_pmd,
> + struct vm_area_struct *dst_vma,
> + unsigned long dst_addr, struct page *page,
> + uffd_flags_t flags)
> {
> int ret;
> struct mm_struct *dst_mm = dst_vma->vm_mm;
> @@ -387,9 +386,6 @@ int mfill_atomic_install_pte(pmd_t *dst_pmd,
> goto out_unlock;
>
> if (page_in_cache) {
> - /* Usually, cache pages are already added to LRU */
> - if (newly_allocated)
> - folio_add_lru(folio);
> folio_add_file_rmap_pte(folio, page, dst_vma);
> } else {
> folio_add_new_anon_rmap(folio, dst_vma, dst_addr, RMAP_EXCLUSIVE);
> @@ -404,6 +400,9 @@ int mfill_atomic_install_pte(pmd_t *dst_pmd,
>
> set_pte_at(dst_mm, dst_addr, dst_pte, _dst_pte);
>
> + if (page_in_cache)
> + folio_unlock(folio);

I don't really like doing the folio_unlock() *here*, I think it's
clearer if the callers (mfill_atomic_pte_continue() and
__mfill_atomic_pte()) unlocked the folio themselves. But that's just
my opinion.

> +
> /* No need to invalidate - it was non-present before */
> update_mmu_cache(dst_vma, dst_addr, dst_pte);
> ret = 0;
> @@ -516,13 +515,22 @@ static int __mfill_atomic_pte(struct mfill_state *state,
> */
> __folio_mark_uptodate(folio);
>
> + if (ops->filemap_add) {
> + ret = ops->filemap_add(folio, state->vma, state->dst_addr);
> + if (ret)
> + goto err_folio_put;
> + }
> +
> ret = mfill_atomic_install_pte(state->pmd, state->vma, dst_addr,
> - &folio->page, true, flags);
> + &folio->page, flags);
> if (ret)
> - goto err_folio_put;
> + goto err_filemap_remove;
>
> return 0;
>
> +err_filemap_remove:
> + if (ops->filemap_remove)
> + ops->filemap_remove(folio, state->vma);
> err_folio_put:
> folio_put(folio);
> /* Don't return -ENOENT so that our caller won't retry */
> @@ -535,6 +543,18 @@ static int mfill_atomic_pte_copy(struct mfill_state *state)
> {
> const struct vm_uffd_ops *ops = vma_uffd_ops(state->vma);
>
> + /*
> + * The normal page fault path for a MAP_PRIVATE mapping in a
> + * file-backed VMA will invoke the fault, fill the hole in the file and
> + * COW it right away. The result generates plain anonymous memory.
> + * So when we are asked to fill a hole in a MAP_PRIVATE mapping, we'll
> + * generate anonymous memory directly without actually filling the
> + * hole. For the MAP_PRIVATE case the robustness check only happens in
> + * the pagetable (to verify it's still none) and not in the page cache.
> + */
> + if (!(state->vma->vm_flags & VM_SHARED))
> + ops = &anon_uffd_ops;
> +
> return __mfill_atomic_pte(state, ops);
> }
>
> @@ -554,7 +574,8 @@ static int mfill_atomic_pte_zeropage(struct mfill_state *state)
> spinlock_t *ptl;
> int ret;
>
> - if (mm_forbids_zeropage(dst_vma->vm_mm))
> + if (mm_forbids_zeropage(dst_vma->vm_mm) ||
> + (dst_vma->vm_flags & VM_SHARED))
> return mfill_atomic_pte_zeroed_folio(state);
>
> _dst_pte = pte_mkspecial(pfn_pte(my_zero_pfn(dst_addr),
> @@ -609,11 +630,10 @@ static int mfill_atomic_pte_continue(struct mfill_state *state)
> }
>
> ret = mfill_atomic_install_pte(dst_pmd, dst_vma, dst_addr,
> - page, false, flags);
> + page, flags);
> if (ret)
> goto out_release;
>
> - folio_unlock(folio);
> return 0;
>
> out_release:
> @@ -836,41 +856,18 @@ extern ssize_t mfill_atomic_hugetlb(struct userfaultfd_ctx *ctx,
>
> static __always_inline ssize_t mfill_atomic_pte(struct mfill_state *state)
> {
> - struct vm_area_struct *dst_vma = state->vma;
> - unsigned long src_addr = state->src_addr;
> - unsigned long dst_addr = state->dst_addr;
> - struct folio **foliop = &state->folio;
> uffd_flags_t flags = state->flags;
> - pmd_t *dst_pmd = state->pmd;
> - ssize_t err;
>
> if (uffd_flags_mode_is(flags, MFILL_ATOMIC_CONTINUE))
> return mfill_atomic_pte_continue(state);
> if (uffd_flags_mode_is(flags, MFILL_ATOMIC_POISON))
> return mfill_atomic_pte_poison(state);
> + if (uffd_flags_mode_is(flags, MFILL_ATOMIC_COPY))
> + return mfill_atomic_pte_copy(state);
> + if (uffd_flags_mode_is(flags, MFILL_ATOMIC_ZEROPAGE))
> + return mfill_atomic_pte_zeropage(state);

Thanks for this cleanup. :)

>
> - /*
> - * The normal page fault path for a shmem will invoke the
> - * fault, fill the hole in the file and COW it right away. The
> - * result generates plain anonymous memory. So when we are
> - * asked to fill an hole in a MAP_PRIVATE shmem mapping, we'll
> - * generate anonymous memory directly without actually filling
> - * the hole. For the MAP_PRIVATE case the robustness check
> - * only happens in the pagetable (to verify it's still none)
> - * and not in the radix tree.
> - */
> - if (!(dst_vma->vm_flags & VM_SHARED)) {
> - if (uffd_flags_mode_is(flags, MFILL_ATOMIC_COPY))
> - err = mfill_atomic_pte_copy(state);
> - else
> - err = mfill_atomic_pte_zeropage(state);
> - } else {
> - err = shmem_mfill_atomic_pte(dst_pmd, dst_vma,
> - dst_addr, src_addr,
> - flags, foliop);
> - }
> -
> - return err;
> + return -EOPNOTSUPP;

WARN_ONCE() here I think.

Feel free to add:

Reviewed-by: James Houghton <jthoughton@google.com>