> + npages = buf->size >> PAGE_SHIFT;
Why round down? usually we use round up.
> + down_read(¤t->mm->mmap_sem);
> +
> + /*
> + * Basically, all the pages from get_user_pages() can not be not only
> + * migrated by CMA but also swapped out.
> + *
> + * The migration issue.
> + * - Pages reserved by CMA for some device using DMA could be used by
> + * kernel and if the device driver wants to use those pages
> + * while being used by kernel then the pages are copied into
> + * other ones allocated to migrate them and then finally,
> + * the device driver can use the pages for itself.
> + * Thus, migrated, the pages being accessed by DMA could be changed
> + * to other so this situation may incur that DMA accesses any pages
> + * it doesn't want.
> + *
> + * But the use of get_user_pages is safe from such magration issue
> + * because all the pages from get_user_pages CAN NOT be not only
> + * migrated, but also swapped out.
> + */
> + get_npages = get_user_pages(current, current->mm, userptr,
> + npages, write, 1, buf->pages, NULL);
Why force=1? It is almostly core-dump specific option. Why don't you return
EFAULT when the page has write permission. IOW, Why your Xorg module
don't map memory w/ PROT_WRITE?
> + up_read(¤t->mm->mmap_sem);
> + if (get_npages != npages)
> + DRM_ERROR("failed to get user_pages.\n");
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