On 09/01/2026 13:07, Boris Brezillon wrote:
> Defer pages allocation until their first access.
>
> Signed-off-by: Boris Brezillon <[email protected]>
> ---
> drivers/gpu/drm/panthor/panthor_gem.c | 119 ++++++++++++++++----------
> 1 file changed, 75 insertions(+), 44 deletions(-)
>
> diff --git a/drivers/gpu/drm/panthor/panthor_gem.c
> b/drivers/gpu/drm/panthor/panthor_gem.c
> index 0e52c7a07c87..44f05bd957e7 100644
> --- a/drivers/gpu/drm/panthor/panthor_gem.c
> +++ b/drivers/gpu/drm/panthor/panthor_gem.c
> @@ -600,15 +600,6 @@ static int panthor_gem_mmap(struct drm_gem_object *obj,
> struct vm_area_struct *v
> if (is_cow_mapping(vma->vm_flags))
> return -EINVAL;
>
> - dma_resv_lock(obj->resv, NULL);
> - ret = panthor_gem_backing_get_pages_locked(bo);
> - if (!ret)
> - ret = panthor_gem_prep_for_cpu_map_locked(bo);
> - dma_resv_unlock(obj->resv);
> -
> - if (ret)
> - return ret;
> -
> vm_flags_set(vma, VM_PFNMAP | VM_DONTEXPAND | VM_DONTDUMP);
> vma->vm_page_prot = vm_get_page_prot(vma->vm_flags);
> if (should_map_wc(bo))
> @@ -628,82 +619,122 @@ static enum drm_gem_object_status
> panthor_gem_status(struct drm_gem_object *obj)
> return res;
> }
>
> -static bool try_map_pmd(struct vm_fault *vmf, unsigned long addr, struct
> page *page)
> +static vm_fault_t insert_page(struct vm_fault *vmf, struct page *page)
> {
> + struct vm_area_struct *vma = vmf->vma;
> + vm_fault_t ret;
> +
> #ifdef CONFIG_ARCH_SUPPORTS_PMD_PFNMAP
> unsigned long pfn = page_to_pfn(page);
> unsigned long paddr = pfn << PAGE_SHIFT;
> - bool aligned = (addr & ~PMD_MASK) == (paddr & ~PMD_MASK);
> + bool aligned = (vmf->address & ~PMD_MASK) == (paddr & ~PMD_MASK);
>
> if (aligned &&
> pmd_none(*vmf->pmd) &&
> folio_test_pmd_mappable(page_folio(page))) {
> pfn &= PMD_MASK >> PAGE_SHIFT;
> - if (vmf_insert_pfn_pmd(vmf, pfn, false) == VM_FAULT_NOPAGE)
> - return true;
> + ret = vmf_insert_pfn_pmd(vmf, pfn, false);
> + if (ret == VM_FAULT_NOPAGE)
> + return VM_FAULT_NOPAGE;
> }
> #endif
>
> - return false;
> + return vmf_insert_pfn(vma, vmf->address, page_to_pfn(page));
> }
>
> -static vm_fault_t panthor_gem_fault(struct vm_fault *vmf)
> +static vm_fault_t nonblocking_page_setup(struct vm_fault *vmf, pgoff_t
> page_offset)
> {
> struct vm_area_struct *vma = vmf->vma;
> - struct drm_gem_object *obj = vma->vm_private_data;
> struct panthor_gem_object *bo = to_panthor_bo(vma->vm_private_data);
> - loff_t num_pages = obj->size >> PAGE_SHIFT;
> vm_fault_t ret;
> - pgoff_t page_offset;
> - unsigned long pfn;
>
> - /* Offset to faulty address in the VMA. */
> - page_offset = vmf->pgoff - vma->vm_pgoff;
> + if (!dma_resv_trylock(bo->base.resv))
> + return VM_FAULT_RETRY;
>
> - dma_resv_lock(bo->base.resv, NULL);
> + if (bo->backing.pages)
> + ret = insert_page(vmf, bo->backing.pages[page_offset]);
> + else
> + ret = VM_FAULT_RETRY;
>
> - if (page_offset >= num_pages ||
> - drm_WARN_ON_ONCE(obj->dev, !bo->backing.pages)) {
> - ret = VM_FAULT_SIGBUS;
> - goto out;
> + dma_resv_unlock(bo->base.resv);
> + return ret;
> +}
> +
> +static vm_fault_t blocking_page_setup(struct vm_fault *vmf,
> + struct panthor_gem_object *bo,
> + pgoff_t page_offset, bool mmap_lock_held)
> +{
> + vm_fault_t ret;
> + int err;
> +
> + if (vmf->flags & FAULT_FLAG_INTERRUPTIBLE) {
> + err = dma_resv_lock_interruptible(bo->base.resv, NULL);
> + if (err)
> + return mmap_lock_held ? VM_FAULT_NOPAGE :
> VM_FAULT_RETRY;
I'm not sure about this. First FAULT_FLAG_INTERRUPTIBLE is currently
only used by userfaultfd AFAICT. Second returning VM_FAULT_NOPAGE seems
wrong - that's for the case were we've inserted a pte but in this case
we haven't.
Otherwise I couldn't spot any issues staring at the code, but I might
have missed something. mm code is always hard to follow!
Thanks,
Steve
> + } else {
> + dma_resv_lock(bo->base.resv, NULL);
> }
>
> - if (try_map_pmd(vmf, vmf->address, bo->backing.pages[page_offset])) {
> - ret = VM_FAULT_NOPAGE;
> - goto out;
> + err = panthor_gem_backing_get_pages_locked(bo);
> + if (!err)
> + err = panthor_gem_prep_for_cpu_map_locked(bo);
> +
> + if (err) {
> + ret = mmap_lock_held ? VM_FAULT_SIGBUS : VM_FAULT_RETRY;
> + } else {
> + struct page *page = bo->backing.pages[page_offset];
> +
> + if (mmap_lock_held)
> + ret = insert_page(vmf, page);
> + else
> + ret = VM_FAULT_RETRY;
> }
>
> - pfn = page_to_pfn(bo->backing.pages[page_offset]);
> - ret = vmf_insert_pfn(vma, vmf->address, pfn);
> -
> - out:
> dma_resv_unlock(bo->base.resv);
>
> return ret;
> }
>
> -static void panthor_gem_vm_open(struct vm_area_struct *vma)
> +static vm_fault_t panthor_gem_fault(struct vm_fault *vmf)
> {
> + struct vm_area_struct *vma = vmf->vma;
> struct panthor_gem_object *bo = to_panthor_bo(vma->vm_private_data);
> + loff_t num_pages = bo->base.size >> PAGE_SHIFT;
> + pgoff_t page_offset;
> + vm_fault_t ret;
>
> - drm_WARN_ON(bo->base.dev, drm_gem_is_imported(&bo->base));
> + /* We don't use vmf->pgoff since that has the fake offset */
> + page_offset = (vmf->address - vma->vm_start) >> PAGE_SHIFT;
> + if (page_offset >= num_pages)
> + return VM_FAULT_SIGBUS;
>
> - dma_resv_lock(bo->base.resv, NULL);
> + ret = nonblocking_page_setup(vmf, page_offset);
> + if (ret != VM_FAULT_RETRY)
> + return ret;
>
> - /* We should have already pinned the pages when the buffer was first
> - * mmap'd, vm_open() just grabs an additional reference for the new
> - * mm the vma is getting copied into (ie. on fork()).
> - */
> - drm_WARN_ON_ONCE(bo->base.dev, !bo->backing.pages);
> + /* Check if we're allowed to retry. */
> + if (fault_flag_allow_retry_first(vmf->flags)) {
> + /* If we're allowed to retry but not wait here, return
> + * immediately, the wait will be done when the fault
> + * handler is called again, with the mmap_lock held.
> + */
> + if (vmf->flags & FAULT_FLAG_RETRY_NOWAIT)
> + return VM_FAULT_RETRY;
>
> - dma_resv_unlock(bo->base.resv);
> + /* Wait with the mmap lock released, if we're allowed to. */
> + drm_gem_object_get(&bo->base);
> + mmap_read_unlock(vmf->vma->vm_mm);
> + ret = blocking_page_setup(vmf, bo, page_offset, false);
> + drm_gem_object_put(&bo->base);
> + return ret;
> + }
>
> - drm_gem_vm_open(vma);
> + return blocking_page_setup(vmf, bo, page_offset, true);
> }
>
> const struct vm_operations_struct panthor_gem_vm_ops = {
> .fault = panthor_gem_fault,
> - .open = panthor_gem_vm_open,
> + .open = drm_gem_vm_open,
> .close = drm_gem_vm_close,
> };
>
