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

Suren Baghdasaryan
2025-04-28T20:01:13Z

On Mon, Apr 28, 2025 at 12:47 PM Lorenzo Stoakes
<lorenzo.stoakes@oracle.com> wrote:
>
> +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!

Thanks! Yes, there should be some space in the last cacheline after my
last field reshuffling.

>
> 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
> >