On Tue, Jan 27, 2026 at 09:29:29PM +0200, Mike Rapoport wrote: > From: "Mike Rapoport (Microsoft)" <[email protected]> > > 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) <[email protected]>
This patch looks like a real nice cleanup on its own, thanks Mike! I guess I never tried to read into shmem accountings, now after I read some of the codes I don't see any issue with your change. We can also wait for some shmem developers double check those. Comments inline below on something I spot. > > fixup > > Signed-off-by: Mike Rapoport (Microsoft) <[email protected]> [unexpected lines can be removed here] > --- > include/linux/shmem_fs.h | 14 ---- > include/linux/userfaultfd_k.h | 20 +++-- > mm/shmem.c | 148 ++++++++++++---------------------- > mm/userfaultfd.c | 79 +++++++++--------- > 4 files changed, 106 insertions(+), 155 deletions(-) > > diff --git a/include/linux/shmem_fs.h b/include/linux/shmem_fs.h > index e2069b3179c4..754f17e5b53c 100644 > --- a/include/linux/shmem_fs.h > +++ b/include/linux/shmem_fs.h > @@ -223,20 +223,6 @@ 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..75d5b09f2560 100644 > --- a/include/linux/userfaultfd_k.h > +++ b/include/linux/userfaultfd_k.h > @@ -97,6 +97,21 @@ struct vm_uffd_ops { > */ > 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. > + * Returns 0 on success, error code on failre. 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(). > + * The folio should be unlocked, but the reference to it should not be > + * dropped. Might be slightly misleading to explicitly mention this? As page cache also holds references and IIUC they need to be dropped there. But I get your point, on keeping the last refcount due to allocation. IMHO the "should revert the operation of ->filemap_add()" is good enough and accurately describes it. > + */ > + void (*filemap_remove)(struct folio *folio, struct vm_area_struct *vma); > }; > > /* A combined operation mode + behavior flags. */ > @@ -130,11 +145,6 @@ 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 87cd8d2fdb97..6f0485f76cb8 100644 > --- a/mm/shmem.c > +++ b/mm/shmem.c > @@ -3169,118 +3169,73 @@ static inline 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)) { We used to do this early before allocation, IOW, I think we still have an option to leave this to alloc_folio() hook. However I don't see an issue either keeping it in filemap_add(). Maybe this movement should better be spelled out in the commit message anyway on how this decision is made. IIUC it's indeed safe we move this acct_blocks() here, I even see Hugh mentioned such in an older commit 3022fd7af96, but Hugh left uffd alone at that time: Userfaultfd is a foreign country: they do things differently there, and for good reason - to avoid mmap_lock deadlock. Leave ordering in shmem_mfill_atomic_pte() untouched for now, but I would rather like to mesh it better with shmem_get_folio_gfp() in the future. I'm not sure if that's also what you wanted to do - to make userfaultfd code work similarly like what shmem_alloc_and_add_folio() does right now. Maybe you want to mention that too somewhere in the commit log when posting a formal patch. One thing not directly relevant is, shmem_alloc_and_add_folio() also does proper recalc of inode allocation info when acct_blocks() fails here. But if that's a problem, that's pre-existing for userfaultfd, so IIUC we can also leave it alone until someone (maybe quota user) complains about shmem allocation failures on UFFDIO_COPY.. It's just that it looks similar problem here in userfaultfd path. > + err = -ENOMEM; > + goto err_delete_from_cache; > + } > > + folio_add_lru(folio); This change is pretty separate from the work, but looks correct to me: IIUC we moved the lru add earlier now, and it should be safe as long as we're holding folio lock all through the process, and folio_put() (ultimately, __page_cache_release()) will always properly undo the lru change. Please help double check if my understanding is correct. > 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); This looks wrong, or maybe I miss somewhere we did the unacct_blocks()? > - return ret; > } > #endif /* CONFIG_USERFAULTFD */ > > @@ -5317,6 +5272,9 @@ 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 > > diff --git a/mm/userfaultfd.c b/mm/userfaultfd.c > index b3c12630769c..54aa195237ba 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" > @@ -337,10 +336,10 @@ 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; > @@ -384,9 +383,6 @@ 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); > @@ -401,6 +397,9 @@ 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); Nitpick: another small change that looks correct, but IMHO would be nice to either make it a small separate patch, or mention in the commit message. > + > /* No need to invalidate - it was non-present before */ > update_mmu_cache(dst_vma, dst_addr, dst_pte); > ret = 0; > @@ -507,13 +506,22 @@ 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 */ > @@ -526,6 +534,18 @@ 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); > } > > @@ -545,7 +565,8 @@ 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), > @@ -600,11 +621,10 @@ 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: > @@ -827,41 +847,18 @@ 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); > > - /* > - * 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); > - } It's great to merge these otherwise. Thanks! > - > - return err; > + return -EOPNOTSUPP; > } > > static __always_inline ssize_t mfill_atomic(struct userfaultfd_ctx *ctx, > -- > 2.51.0 > -- Peter Xu
