Re: [PATCH v3 07/14] drm/panthor: Add the MMU/VM logical block

[Boris Brezillon <boris.brezillon@collabora.com>]

On Mon,  4 Dec 2023 18:33:00 +0100
Boris Brezillon <boris.brezillon@collabora.com> wrote:

> +static int panthor_vm_prepare_unmap_op_ctx(struct panthor_vm_op_ctx *op_ctx,
> +					   struct panthor_vm *vm,
> +					   u64 va, u64 size)
> +{
> +	u32 pt_count = 0;
> +	int ret;
> +
> +	memset(op_ctx, 0, sizeof(*op_ctx));
> +	INIT_LIST_HEAD(&op_ctx->returned_vmas);
> +	op_ctx->va.range = size;
> +	op_ctx->va.addr = va;
> +	op_ctx->flags = DRM_PANTHOR_VM_BIND_OP_TYPE_UNMAP;
> +
> +	/* Pre-allocate L3 page tables to account for the split-2M-block
> +	 * situation on unmap.
> +	 */
> +	if (va != ALIGN(va, SZ_2M))
> +		pt_count++;
> +
> +	if (va + size != ALIGN(va + size, SZ_2M) &&
> +	    ALIGN(va + size, SZ_2M) != ALIGN(va, SZ_2M))
> +		pt_count++;
> +
> +	if (pt_count) {
> +		op_ctx->rsvd_page_tables.pages = kcalloc(pt_count,
> +							 sizeof(*op_ctx->rsvd_page_tables.pages),
> +							 GFP_KERNEL);
> +		if (!op_ctx->rsvd_page_tables.pages)
> +			goto err_cleanup;
> +
> +		ret = kmem_cache_alloc_bulk(pt_cache, GFP_KERNEL, pt_count,
> +					    op_ctx->rsvd_page_tables.pages);
> +		if (ret != pt_count) {
> +			ret = -ENOMEM;
> +			goto err_cleanup;
> +		}
> +		op_ctx->rsvd_page_tables.count = pt_count;
> +	}

We also need to allocate {prev,next}_vma objects, because partial
unmaps will call sm_step_remap(), which leads to a NULL deref if we
don't pre-allocate these objects. I'm reworking the code to have a
3-entry array of preallocated VMAs instead of having
->map.{prev,new,next}_vma fields. We can then populate this array when
preparing an op_ctx, and pick from this preallocated pool in
sm_step_[re]map().

> +
> +	return 0;
> +
> +err_cleanup:
> +	panthor_vm_cleanup_op_ctx(op_ctx, vm);
> +	return ret;
> +}