[PATCH] drm/shmem-helper: Don't remove the offset in vm_area_struct pgoff
Thomas Zimmermann
tzimmermann at suse.de
Mon Feb 22 14:24:17 UTC 2021
Hi
Am 17.02.21 um 17:59 schrieb Neil Roberts:
> When mmapping the shmem, it would previously adjust the pgoff in the
> vm_area_struct to remove the fake offset that is added to be able to
> identify the buffer. This patch removes the adjustment and makes the
> fault handler use the vm_fault address to calculate the page offset
> instead. Although using this address is apparently discouraged, several
> DRM drivers seem to be doing it anyway.
>
> The problem with removing the pgoff is that it prevents
> drm_vma_node_unmap from working because that searches the mapping tree
> by address. That doesn't work because all of the mappings are at offset
> 0. drm_vma_node_unmap is being used by the shmem helpers when purging
> the buffer.
I just want to ask if this is how the mmap callbacks are supposed to
work now?
I have a number of patches in here that convert several drivers to the
GEM object's mmap callback. They might not be affected by the vm_pgoff
bug, but I'd like to submit something that could work in the longer term.
Best regards
Thomas
>
> It looks like panfrost is using drm_gem_shmem_purge so this might fix a
> potential bug there.
>
> Signed-off-by: Neil Roberts <nroberts at igalia.com>
> ---
> drivers/gpu/drm/drm_gem_shmem_helper.c | 12 +++++++-----
> 1 file changed, 7 insertions(+), 5 deletions(-)
>
> diff --git a/drivers/gpu/drm/drm_gem_shmem_helper.c b/drivers/gpu/drm/drm_gem_shmem_helper.c
> index 9825c378dfa6..4b14157f1962 100644
> --- a/drivers/gpu/drm/drm_gem_shmem_helper.c
> +++ b/drivers/gpu/drm/drm_gem_shmem_helper.c
> @@ -526,11 +526,16 @@ static vm_fault_t drm_gem_shmem_fault(struct vm_fault *vmf)
> struct drm_gem_shmem_object *shmem = to_drm_gem_shmem_obj(obj);
> loff_t num_pages = obj->size >> PAGE_SHIFT;
> struct page *page;
> + pgoff_t page_offset;
>
> - if (vmf->pgoff >= num_pages || WARN_ON_ONCE(!shmem->pages))
> + /* We don't use vmf->pgoff since that has the fake offset */
> + page_offset = (vmf->address - vma->vm_start) >> PAGE_SHIFT;
> +
> + if (page_offset < 0 || page_offset >= num_pages ||
> + WARN_ON_ONCE(!shmem->pages))
> return VM_FAULT_SIGBUS;
>
> - page = shmem->pages[vmf->pgoff];
> + page = shmem->pages[page_offset];
>
> return vmf_insert_page(vma, vmf->address, page);
> }
> @@ -581,9 +586,6 @@ int drm_gem_shmem_mmap(struct drm_gem_object *obj, struct vm_area_struct *vma)
> struct drm_gem_shmem_object *shmem;
> int ret;
>
> - /* Remove the fake offset */
> - vma->vm_pgoff -= drm_vma_node_start(&obj->vma_node);
> -
> if (obj->import_attach) {
> /* Drop the reference drm_gem_mmap_obj() acquired.*/
> drm_gem_object_put(obj);
>
--
Thomas Zimmermann
Graphics Driver Developer
SUSE Software Solutions Germany GmbH
Maxfeldstr. 5, 90409 Nürnberg, Germany
(HRB 36809, AG Nürnberg)
Geschäftsführer: Felix Imendörffer
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