[PATCH v2 04/11] mm: convert VM_PFNMAP tracking to pfnmap_track() + pfnmap_untrack()
Lorenzo Stoakes
lorenzo.stoakes at oracle.com
Tue May 13 10:16:16 UTC 2025
On Tue, May 13, 2025 at 11:10:45AM +0200, David Hildenbrand wrote:
> On 12.05.25 18:42, Lorenzo Stoakes wrote:
> > On Mon, May 12, 2025 at 02:34:17PM +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 lookup the
> > > cachemode (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.
> > >
> > > If that ever turns out to be an issue, we could hook into VM splitting
> > > code and split the tracking; however, that adds complexity that might
> > > not be required, so we'll keep it simple for now.
> > >
> > > Acked-by: Ingo Molnar <mingo at kernel.org> # x86 bits
> > > Signed-off-by: David Hildenbrand <david at redhat.com>
> >
> > Other than small nit below,
> >
> > Reviewed-by: Lorenzo Stoakes <lorenzo.stoakes at oracle.com>
> >
> > > ---
> > > include/linux/mm_inline.h | 2 +
> > > include/linux/mm_types.h | 11 ++++++
> > > mm/memory.c | 82 +++++++++++++++++++++++++++++++--------
> > > mm/mmap.c | 5 ---
> > > mm/mremap.c | 4 --
> > > mm/vma_init.c | 50 ++++++++++++++++++++++++
> > > 6 files changed, 129 insertions(+), 25 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 15808cad2bc1a..3e934dc6057c4 100644
> > > --- a/include/linux/mm_types.h
> > > +++ b/include/linux/mm_types.h
> > > @@ -763,6 +763,14 @@ 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; /* in bytes */
> > > +};
> > > +#endif
> > > +
> > > /*
> > > * Describes a VMA that is about to be mmap()'ed. Drivers may choose to
> > > * manipulate mutable fields which will cause those fields to be updated in the
> > > @@ -900,6 +908,9 @@ 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/mm/memory.c b/mm/memory.c
> > > index 064fc55d8eab9..4cf4adb0de266 100644
> > > --- a/mm/memory.c
> > > +++ b/mm/memory.c
> > > @@ -1371,7 +1371,7 @@ 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;
> > >
> > > @@ -1381,12 +1381,6 @@ 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
> > > @@ -1428,8 +1422,6 @@ 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;
> > > }
> > >
> > > @@ -1924,9 +1916,6 @@ 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))) {
> > > /*
> > > @@ -2872,6 +2861,36 @@ 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
> > > @@ -2884,20 +2903,51 @@ 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. We'll keep tracking the whole pfn
> > > + * range even during VMA splits and partial unmapping.
> > > + *
> > > + * If we only cover parts of the VMA, we'll only setup the cachemode
> > > + * in the pgprot for the pfn range.
> > > + */
> > > + 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_setup_cachemode(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/mmap.c b/mm/mmap.c
> > > index 50f902c08341a..09c563c951123 100644
> > > --- a/mm/mmap.c
> > > +++ b/mm/mmap.c
> > > @@ -1784,11 +1784,6 @@ __latent_entropy int dup_mmap(struct mm_struct *mm, struct mm_struct *oldmm)
> > > tmp = vm_area_dup(mpnt);
> > > 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/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;
> > > }
> > > diff --git a/mm/vma_init.c b/mm/vma_init.c
> > > index 967ca85179864..8e53c7943561e 100644
> > > --- a/mm/vma_init.c
> > > +++ b/mm/vma_init.c
> > > @@ -71,7 +71,51 @@ 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))
> >
> > How not expected is this? :) maybe use WARN_ON_ONCE() if it really should
> > never happen?
> I guess if we mmap a large PFNMAP and then split it into individual
> PTE-sized chunks, we could get many VMAs per-process referencing that
> tracing.
>
> Combine that with fork() and I assume one could hit this -- when really
> trying hard to achieve it. (probably as privileged user to get a big
> VM_PFNMAP, though, but not sure)
Right ok, yeah I guess so. It'd be good to see if we could trigger it somehow :)
>
> In that case, a WARN_ON_ONCE() would be bad -- because it could be triggered
> by the user.
Ack
>
> We could do a pr_warn_once() instead, stating that this is not supported
> right now?
Hmm, if we truly think it might happen let's avoid printing anything for now.
Maybe just ++todo for experimenting with triggering?
It's not hugely important!
>
> --
> Cheers,
>
> David / dhildenb
>
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