linux-kernel - Re: [PATCH] drm/ttm: don't set page->mapping

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Message-ID: <26a62dfb-02e4-1707-c833-a3c8d5cbe828@amd.com>
Date:   Fri, 20 Nov 2020 11:04:27 +0100
From:   Christian König <christian.koenig@....com>
To:     Daniel Vetter <daniel.vetter@...ll.ch>,
        DRI Development <dri-devel@...ts.freedesktop.org>
Cc:     Intel Graphics Development <intel-gfx@...ts.freedesktop.org>,
        linux-mm@...ck.org, linux-xfs@...r.kernel.org,
        linux-fsdevel@...r.kernel.org, LKML <linux-kernel@...r.kernel.org>,
        Thomas Hellstrom <thellstrom@...are.com>,
        Brian Paul <brianp@...are.com>,
        Daniel Vetter <daniel.vetter@...el.com>,
        Huang Rui <ray.huang@....com>
Subject: Re: [PATCH] drm/ttm: don't set page->mapping

Am 20.11.20 um 10:54 schrieb Daniel Vetter:
> Random observation while trying to review Christian's patch series to
> stop looking at struct page for dma-buf imports.
>
> This was originally added in
>
> commit 58aa6622d32af7d2c08d45085f44c54554a16ed7
> Author: Thomas Hellstrom <thellstrom@...are.com>
> Date:   Fri Jan 3 11:47:23 2014 +0100
>
>      drm/ttm: Correctly set page mapping and -index members
>
>      Needed for some vm operations; most notably unmap_mapping_range() with
>      even_cows = 0.
>
>      Signed-off-by: Thomas Hellstrom <thellstrom@...are.com>
>      Reviewed-by: Brian Paul <brianp@...are.com>
>
> but we do not have a single caller of unmap_mapping_range with
> even_cows == 0. And all the gem drivers don't do this, so another
> small thing we could standardize between drm and ttm drivers.
>
> Plus I don't really see a need for unamp_mapping_range where we don't
> want to indiscriminately shoot down all ptes.
>
> Cc: Thomas Hellstrom <thellstrom@...are.com>
> Cc: Brian Paul <brianp@...are.com>
> Signed-off-by: Daniel Vetter <daniel.vetter@...el.com>
> Cc: Christian Koenig <christian.koenig@....com>
> Cc: Huang Rui <ray.huang@....com>

This is still a NAK as long as we can't come up with a better way to 
track TTMs page allocations.

Additional to that page_mapping() is used quite extensively in the mm 
code and I'm not sure if that isn't needed for other stuff as well.

Regards,
Christian.

> ---
>   drivers/gpu/drm/ttm/ttm_tt.c | 12 ------------
>   1 file changed, 12 deletions(-)
>
> diff --git a/drivers/gpu/drm/ttm/ttm_tt.c b/drivers/gpu/drm/ttm/ttm_tt.c
> index da9eeffe0c6d..5b2eb6d58bb7 100644
> --- a/drivers/gpu/drm/ttm/ttm_tt.c
> +++ b/drivers/gpu/drm/ttm/ttm_tt.c
> @@ -284,17 +284,6 @@ int ttm_tt_swapout(struct ttm_bo_device *bdev, struct ttm_tt *ttm)
>   	return ret;
>   }
>   
> -static void ttm_tt_add_mapping(struct ttm_bo_device *bdev, struct ttm_tt *ttm)
> -{
> -	pgoff_t i;
> -
> -	if (ttm->page_flags & TTM_PAGE_FLAG_SG)
> -		return;
> -
> -	for (i = 0; i < ttm->num_pages; ++i)
> -		ttm->pages[i]->mapping = bdev->dev_mapping;
> -}
> -
>   int ttm_tt_populate(struct ttm_bo_device *bdev,
>   		    struct ttm_tt *ttm, struct ttm_operation_ctx *ctx)
>   {
> @@ -313,7 +302,6 @@ int ttm_tt_populate(struct ttm_bo_device *bdev,
>   	if (ret)
>   		return ret;
>   
> -	ttm_tt_add_mapping(bdev, ttm);
>   	ttm->page_flags |= TTM_PAGE_FLAG_PRIV_POPULATED;
>   	if (unlikely(ttm->page_flags & TTM_PAGE_FLAG_SWAPPED)) {
>   		ret = ttm_tt_swapin(ttm);