Re: [PATCH v1 05/11] mm: convert VM_PFNMAP tracking to pfnmap_track() + pfnmap_untrack()

Lorenzo Stoakes
2025-04-28T19:47:44Z

+cc Suren, who has worked HEAVILY on VMA field manipulation and such :)

Suren - David is proposing adding a new field. AFAICT this does not add a
new cache line so I think we're all good.

But FYI!

On Fri, Apr 25, 2025 at 10:17:09AM +0200, David Hildenbrand wrote:
> Let's use our new interface. In remap_pfn_range(), we'll now decide
> whether we have to track (full VMA covered) or only sanitize the pgprot
> (partial VMA covered).
>
> Remember what we have to untrack by linking it from the VMA. When
> duplicating VMAs (e.g., splitting, mremap, fork), we'll handle it similar
> to anon VMA names, and use a kref to share the tracking.
>
> Once the last VMA un-refs our tracking data, we'll do the untracking,
> which simplifies things a lot and should sort our various issues we saw
> recently, for example, when partially unmapping/zapping a tracked VMA.
>
> This change implies that we'll keep tracking the original PFN range even
> after splitting + partially unmapping it: not too bad, because it was
> not working reliably before. The only thing that kind-of worked before
> was shrinking such a mapping using mremap(): we managed to adjust the
> reservation in a hacky way, now we won't adjust the reservation but
> leave it around until all involved VMAs are gone.
>
> Signed-off-by: David Hildenbrand <david@redhat.com>
> ---
> include/linux/mm_inline.h | 2 +
> include/linux/mm_types.h | 11 ++++++
> kernel/fork.c | 54 ++++++++++++++++++++++++--
> mm/memory.c | 81 +++++++++++++++++++++++++++++++--------
> mm/mremap.c | 4 --
> 5 files changed, 128 insertions(+), 24 deletions(-)
>
> diff --git a/include/linux/mm_inline.h b/include/linux/mm_inline.h
> index f9157a0c42a5c..89b518ff097e6 100644
> --- a/include/linux/mm_inline.h
> +++ b/include/linux/mm_inline.h
> @@ -447,6 +447,8 @@ static inline bool anon_vma_name_eq(struct anon_vma_name *anon_name1,
>
> #endif /* CONFIG_ANON_VMA_NAME */
>
> +void pfnmap_track_ctx_release(struct kref *ref);
> +
> static inline void init_tlb_flush_pending(struct mm_struct *mm)
> {
> atomic_set(&mm->tlb_flush_pending, 0);
> diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h
> index 56d07edd01f91..91124761cfda8 100644
> --- a/include/linux/mm_types.h
> +++ b/include/linux/mm_types.h
> @@ -764,6 +764,14 @@ struct vma_numab_state {
> int prev_scan_seq;
> };
>
> +#ifdef __HAVE_PFNMAP_TRACKING
> +struct pfnmap_track_ctx {
> + struct kref kref;
> + unsigned long pfn;
> + unsigned long size;
> +};
> +#endif
> +
> /*
> * This struct describes a virtual memory area. There is one of these
> * per VM-area/task. A VM area is any part of the process virtual memory
> @@ -877,6 +885,9 @@ struct vm_area_struct {
> struct anon_vma_name *anon_name;
> #endif
> struct vm_userfaultfd_ctx vm_userfaultfd_ctx;
> +#ifdef __HAVE_PFNMAP_TRACKING
> + struct pfnmap_track_ctx *pfnmap_track_ctx;
> +#endif
> } __randomize_layout;
>
> #ifdef CONFIG_NUMA
> diff --git a/kernel/fork.c b/kernel/fork.c
> index 168681fc4b25a..ae518b8fe752c 100644
> --- a/kernel/fork.c
> +++ b/kernel/fork.c
> @@ -481,7 +481,51 @@ static void vm_area_init_from(const struct vm_area_struct *src,
> #ifdef CONFIG_NUMA
> dest->vm_policy = src->vm_policy;
> #endif
> +#ifdef __HAVE_PFNMAP_TRACKING
> + dest->pfnmap_track_ctx = NULL;
> +#endif
> +}
> +
> +#ifdef __HAVE_PFNMAP_TRACKING
> +static inline int vma_pfnmap_track_ctx_dup(struct vm_area_struct *orig,
> + struct vm_area_struct *new)
> +{
> + struct pfnmap_track_ctx *ctx = orig->pfnmap_track_ctx;
> +
> + if (likely(!ctx))
> + return 0;
> +
> + /*
> + * We don't expect to ever hit this. If ever required, we would have
> + * to duplicate the tracking.
> + */
> + if (unlikely(kref_read(&ctx->kref) >= REFCOUNT_MAX))
> + return -ENOMEM;
> + kref_get(&ctx->kref);
> + new->pfnmap_track_ctx = ctx;
> + return 0;
> +}
> +
> +static inline void vma_pfnmap_track_ctx_release(struct vm_area_struct *vma)
> +{
> + struct pfnmap_track_ctx *ctx = vma->pfnmap_track_ctx;
> +
> + if (likely(!ctx))
> + return;
> +
> + kref_put(&ctx->kref, pfnmap_track_ctx_release);
> + vma->pfnmap_track_ctx = NULL;
> +}
> +#else
> +static inline int vma_pfnmap_track_ctx_dup(struct vm_area_struct *orig,
> + struct vm_area_struct *new)
> +{
> + return 0;
> }
> +static inline void vma_pfnmap_track_ctx_release(struct vm_area_struct *vma)
> +{
> +}
> +#endif
>
> struct vm_area_struct *vm_area_dup(struct vm_area_struct *orig)
> {
> @@ -493,6 +537,11 @@ struct vm_area_struct *vm_area_dup(struct vm_area_struct *orig)
> ASSERT_EXCLUSIVE_WRITER(orig->vm_flags);
> ASSERT_EXCLUSIVE_WRITER(orig->vm_file);
> vm_area_init_from(orig, new);
> +
> + if (vma_pfnmap_track_ctx_dup(orig, new)) {
> + kmem_cache_free(vm_area_cachep, new);
> + return NULL;
> + }
> vma_lock_init(new, true);
> INIT_LIST_HEAD(&new->anon_vma_chain);
> vma_numab_state_init(new);
> @@ -507,6 +556,7 @@ void vm_area_free(struct vm_area_struct *vma)
> vma_assert_detached(vma);
> vma_numab_state_free(vma);
> free_anon_vma_name(vma);
> + vma_pfnmap_track_ctx_release(vma);
> kmem_cache_free(vm_area_cachep, vma);
> }
>
> @@ -669,10 +719,6 @@ static __latent_entropy int dup_mmap(struct mm_struct *mm,
> if (!tmp)
> goto fail_nomem;
>
> - /* track_pfn_copy() will later take care of copying internal state. */
> - if (unlikely(tmp->vm_flags & VM_PFNMAP))
> - untrack_pfn_clear(tmp);
> -
> retval = vma_dup_policy(mpnt, tmp);
> if (retval)
> goto fail_nomem_policy;
> diff --git a/mm/memory.c b/mm/memory.c
> index c737a8625866a..eb2b3f10a97ec 100644
> --- a/mm/memory.c
> +++ b/mm/memory.c
> @@ -1370,7 +1370,7 @@ copy_page_range(struct vm_area_struct *dst_vma, struct vm_area_struct *src_vma)
> struct mm_struct *dst_mm = dst_vma->vm_mm;
> struct mm_struct *src_mm = src_vma->vm_mm;
> struct mmu_notifier_range range;
> - unsigned long next, pfn = 0;
> + unsigned long next;
> bool is_cow;
> int ret;
>
> @@ -1380,12 +1380,6 @@ copy_page_range(struct vm_area_struct *dst_vma, struct vm_area_struct *src_vma)
> if (is_vm_hugetlb_page(src_vma))
> return copy_hugetlb_page_range(dst_mm, src_mm, dst_vma, src_vma);
>
> - if (unlikely(src_vma->vm_flags & VM_PFNMAP)) {
> - ret = track_pfn_copy(dst_vma, src_vma, &pfn);
> - if (ret)
> - return ret;
> - }
> -
> /*
> * We need to invalidate the secondary MMU mappings only when
> * there could be a permission downgrade on the ptes of the
> @@ -1427,8 +1421,6 @@ copy_page_range(struct vm_area_struct *dst_vma, struct vm_area_struct *src_vma)
> raw_write_seqcount_end(&src_mm->write_protect_seq);
> mmu_notifier_invalidate_range_end(&range);
> }
> - if (ret && unlikely(src_vma->vm_flags & VM_PFNMAP))
> - untrack_pfn_copy(dst_vma, pfn);
> return ret;
> }
>
> @@ -1923,9 +1915,6 @@ static void unmap_single_vma(struct mmu_gather *tlb,
> if (vma->vm_file)
> uprobe_munmap(vma, start, end);
>
> - if (unlikely(vma->vm_flags & VM_PFNMAP))
> - untrack_pfn(vma, 0, 0, mm_wr_locked);
> -
> if (start != end) {
> if (unlikely(is_vm_hugetlb_page(vma))) {
> /*
> @@ -2871,6 +2860,36 @@ int remap_pfn_range_notrack(struct vm_area_struct *vma, unsigned long addr,
> return error;
> }
>
> +#ifdef __HAVE_PFNMAP_TRACKING
> +static inline struct pfnmap_track_ctx *pfnmap_track_ctx_alloc(unsigned long pfn,
> + unsigned long size, pgprot_t *prot)
> +{
> + struct pfnmap_track_ctx *ctx;
> +
> + if (pfnmap_track(pfn, size, prot))
> + return ERR_PTR(-EINVAL);
> +
> + ctx = kmalloc(sizeof(*ctx), GFP_KERNEL);
> + if (unlikely(!ctx)) {
> + pfnmap_untrack(pfn, size);
> + return ERR_PTR(-ENOMEM);
> + }
> +
> + ctx->pfn = pfn;
> + ctx->size = size;
> + kref_init(&ctx->kref);
> + return ctx;
> +}
> +
> +void pfnmap_track_ctx_release(struct kref *ref)
> +{
> + struct pfnmap_track_ctx *ctx = container_of(ref, struct pfnmap_track_ctx, kref);
> +
> + pfnmap_untrack(ctx->pfn, ctx->size);
> + kfree(ctx);
> +}
> +#endif /* __HAVE_PFNMAP_TRACKING */
> +
> /**
> * remap_pfn_range - remap kernel memory to userspace
> * @vma: user vma to map to
> @@ -2883,20 +2902,50 @@ int remap_pfn_range_notrack(struct vm_area_struct *vma, unsigned long addr,
> *
> * Return: %0 on success, negative error code otherwise.
> */
> +#ifdef __HAVE_PFNMAP_TRACKING
> int remap_pfn_range(struct vm_area_struct *vma, unsigned long addr,
> unsigned long pfn, unsigned long size, pgprot_t prot)
> {
> + struct pfnmap_track_ctx *ctx = NULL;
> int err;
>
> - err = track_pfn_remap(vma, &prot, pfn, addr, PAGE_ALIGN(size));
> - if (err)
> + size = PAGE_ALIGN(size);
> +
> + /*
> + * If we cover the full VMA, we'll perform actual tracking, and
> + * remember to untrack when the last reference to our tracking
> + * context from a VMA goes away.
> + *
> + * If we only cover parts of the VMA, we'll only sanitize the
> + * pgprot.
> + */
> + if (addr == vma->vm_start && addr + size == vma->vm_end) {
> + if (vma->pfnmap_track_ctx)
> + return -EINVAL;
> + ctx = pfnmap_track_ctx_alloc(pfn, size, &prot);
> + if (IS_ERR(ctx))
> + return PTR_ERR(ctx);
> + } else if (pfnmap_sanitize_pgprot(pfn, size, &prot)) {
> return -EINVAL;
> + }
>
> err = remap_pfn_range_notrack(vma, addr, pfn, size, prot);
> - if (err)
> - untrack_pfn(vma, pfn, PAGE_ALIGN(size), true);
> + if (ctx) {
> + if (err)
> + kref_put(&ctx->kref, pfnmap_track_ctx_release);
> + else
> + vma->pfnmap_track_ctx = ctx;
> + }
> return err;
> }
> +
> +#else
> +int remap_pfn_range(struct vm_area_struct *vma, unsigned long addr,
> + unsigned long pfn, unsigned long size, pgprot_t prot)
> +{
> + return remap_pfn_range_notrack(vma, addr, pfn, size, prot);
> +}
> +#endif
> EXPORT_SYMBOL(remap_pfn_range);
>
> /**
> diff --git a/mm/mremap.c b/mm/mremap.c
> index 7db9da609c84f..6e78e02f74bd3 100644
> --- a/mm/mremap.c
> +++ b/mm/mremap.c
> @@ -1191,10 +1191,6 @@ static int copy_vma_and_data(struct vma_remap_struct *vrm,
> if (is_vm_hugetlb_page(vma))
> clear_vma_resv_huge_pages(vma);
>
> - /* Tell pfnmap has moved from this vma */
> - if (unlikely(vma->vm_flags & VM_PFNMAP))
> - untrack_pfn_clear(vma);
> -
> *new_vma_ptr = new_vma;
> return err;
> }
> --
> 2.49.0
>