On Mon, Feb 02, 2026 at 04:23:16PM -0500, Peter Xu wrote: > Hi, Mike, > > On Tue, Jan 27, 2026 at 09:29:24PM +0200, Mike Rapoport wrote: > > From: "Mike Rapoport (Microsoft)" <[email protected]> > > > > Implementation of UFFDIO_COPY for anonymous memory might fail to copy > > data data from userspace buffer when the destination VMA is locked > > (either with mm_lock or with per-VMA lock). > > > > In that case, mfill_atomic() releases the locks, retries copying the > > data with locks dropped and then re-locks the destination VMA and > > re-establishes PMD. > > > > Since this retry-reget dance is only relevant for UFFDIO_COPY and it > > never happens for other UFFDIO_ operations, make it a part of > > mfill_atomic_pte_copy() that actually implements UFFDIO_COPY for > > anonymous memory. > > > > shmem implementation will be updated later and the loop in > > mfill_atomic() will be adjusted afterwards. > > Thanks for the refactoring. Looks good to me in general, only some > nitpicks inline. > > > > > Signed-off-by: Mike Rapoport (Microsoft) <[email protected]> > > --- > > mm/userfaultfd.c | 70 +++++++++++++++++++++++++++++++----------------- > > 1 file changed, 46 insertions(+), 24 deletions(-) > > > > diff --git a/mm/userfaultfd.c b/mm/userfaultfd.c > > index 45d8f04aaf4f..01a2b898fa40 100644 > > --- a/mm/userfaultfd.c > > +++ b/mm/userfaultfd.c > > @@ -404,35 +404,57 @@ static int mfill_copy_folio_locked(struct folio > > *folio, unsigned long src_addr) > > return ret; > > } > > > > +static int mfill_copy_folio_retry(struct mfill_state *state, struct folio > > *folio) > > +{ > > + unsigned long src_addr = state->src_addr; > > + void *kaddr; > > + int err; > > + > > + /* retry copying with mm_lock dropped */ > > + mfill_put_vma(state); > > + > > + kaddr = kmap_local_folio(folio, 0); > > + err = copy_from_user(kaddr, (const void __user *) src_addr, PAGE_SIZE); > > + kunmap_local(kaddr); > > + if (unlikely(err)) > > + return -EFAULT; > > + > > + flush_dcache_folio(folio); > > + > > + /* reget VMA and PMD, they could change underneath us */ > > + err = mfill_get_vma(state); > > + if (err) > > + return err; > > + > > + err = mfill_get_pmd(state); > > + if (err) > > + return err; > > + > > + return 0; > > +} > > + > > static int mfill_atomic_pte_copy(struct mfill_state *state) > > { > > - struct vm_area_struct *dst_vma = state->vma; > > unsigned long dst_addr = state->dst_addr; > > unsigned long src_addr = state->src_addr; > > uffd_flags_t flags = state->flags; > > - pmd_t *dst_pmd = state->pmd; > > struct folio *folio; > > int ret; > > > > - if (!state->folio) { > > - ret = -ENOMEM; > > - folio = vma_alloc_folio(GFP_HIGHUSER_MOVABLE, 0, dst_vma, > > - dst_addr); > > - if (!folio) > > - goto out; > > + folio = vma_alloc_folio(GFP_HIGHUSER_MOVABLE, 0, state->vma, dst_addr); > > + if (!folio) > > + return -ENOMEM; > > > > - ret = mfill_copy_folio_locked(folio, src_addr); > > + ret = -ENOMEM; > > + if (mem_cgroup_charge(folio, state->vma->vm_mm, GFP_KERNEL)) > > + goto out_release; > > > > + ret = mfill_copy_folio_locked(folio, src_addr); > > + if (unlikely(ret)) { > > /* fallback to copy_from_user outside mmap_lock */ > > - if (unlikely(ret)) { > > - ret = -ENOENT; > > - state->folio = folio; > > - /* don't free the page */ > > - goto out; > > - } > > - } else { > > - folio = state->folio; > > - state->folio = NULL; > > + ret = mfill_copy_folio_retry(state, folio); > > Yes, I agree this should work and should avoid the previous ENOENT > processing that might be hard to follow. It'll move the complexity into > mfill_state though (e.g., now it's unknown on the vma lock state after this > function returns..), but I guess it's fine.
When this function returns success VMA is locked. If the function fails it does not matter if the VMA is locked. I'll add some comments. > > + if (ret) > > + goto out_release; > > } > > > > /* > > @@ -442,17 +464,16 @@ static int mfill_atomic_pte_copy(struct mfill_state > > *state) > > */ > > __folio_mark_uptodate(folio); > > Since success path should make sure vma lock held when reaching here, but > now with mfill_copy_folio_retry()'s presence it's not as clear as before, > maybe we add an assertion for that here before installing ptes? No strong > feelings. I'll add comments. > > > > - ret = -ENOMEM; > > - if (mem_cgroup_charge(folio, dst_vma->vm_mm, GFP_KERNEL)) > > - goto out_release; > > - > > - ret = mfill_atomic_install_pte(dst_pmd, dst_vma, dst_addr, > > + ret = mfill_atomic_install_pte(state->pmd, state->vma, dst_addr, > > &folio->page, true, flags); > > if (ret) > > goto out_release; > > out: > > return ret; > > out_release: > > + /* Don't return -ENOENT so that our caller won't retry */ > > + if (ret == -ENOENT) > > + ret = -EFAULT; > > I recall the code removed is the only path that can return ENOENT? Then > maybe this line isn't needed? I didn't want to audit all potential errors and this is a temporal safety measure to avoid breaking biscection. This is anyway removed in the following patches. > > folio_put(folio); > > goto out; > > } > > @@ -907,7 +928,8 @@ static __always_inline ssize_t mfill_atomic(struct > > userfaultfd_ctx *ctx, > > break; > > } > > > > - mfill_put_vma(&state); > > + if (state.vma) > > I wonder if we should move this check into mfill_put_vma() directly, it > might be overlooked if we'll put_vma in other paths otherwise. Yeah, I'll check this. -- Sincerely yours, Mike.
