Re: [PATCH] drm/ttm: fix ttm tt init fail when size exceeds kmalloc limit

["Wang, Yang(Kevin)" <KevinYang.Wang@xxxxxxx>]

[AMD Official Use Only]

Hi Chris,

you misunderstood background about this case.

although we expect this test case to fail, it should fail at the location where the Bo actual memory is actually allocated. now the code logic will cause the failure to allocate memory to store DMA address.

e.g: the case is failed in 2TB system ram machine, it should be allocated successful, but it is failed.

allocate 1TB BO, the ttm should allocate 1TB/4k * 8 buffer to store allocate result (page address), this should not be failed usually.

There is a similar fix in upstream kernel 5.18, before this fix entered the TTM code, this problem existed in TTM.

kernel/git/torvalds/linux.git - Linux kernel source tree

mm: allow !GFP_KERNEL allocations for kvmalloc

Best Regards,

Kevin

---

From: Koenig, Christian <Christian.Koenig@xxxxxxx>
Sent: Wednesday, April 20, 2022 6:53 PM
To: Wang, Yang(Kevin) <KevinYang.Wang@xxxxxxx>; dri-devel@xxxxxxxxxxxxxxxxxxxxx <dri-devel@xxxxxxxxxxxxxxxxxxxxx>; amd-gfx@xxxxxxxxxxxxxxxxxxxxx <amd-gfx@xxxxxxxxxxxxxxxxxxxxx>
Subject: Re: [PATCH] drm/ttm: fix ttm tt init fail when size exceeds kmalloc limit

 

Am 20.04.22 um 11:07 schrieb Wang, Yang(Kevin):

[AMD Official Use Only]

---

From: Koenig, Christian <Christian.Koenig@xxxxxxx>
Sent: Wednesday, April 20, 2022 5:00 PM
To: Wang, Yang(Kevin) <KevinYang.Wang@xxxxxxx>; dri-devel@xxxxxxxxxxxxxxxxxxxxx <dri-devel@xxxxxxxxxxxxxxxxxxxxx>; amd-gfx@xxxxxxxxxxxxxxxxxxxxx <amd-gfx@xxxxxxxxxxxxxxxxxxxxx>
Subject: Re: [PATCH] drm/ttm: fix ttm tt init fail when size exceeds kmalloc limit

 

Am 20.04.22 um 10:56 schrieb Yang Wang:
> if the __GFP_ZERO is set, the kvmalloc() can't fallback to use vmalloc()

Hui what? Why should kvmalloc() not be able to fallback to vmalloc()
when __GFP_ZERO is set?

And even that is really the case then that sounds like a bug in kvmalloc().

Regards,
Christian.

[kevin]:

it is really test case from libdrm amdgpu test, which try to allocate a big BO which will cause ttm tt init fail.

LOL! Guys, this test case is intended to fail!

The test consists of allocating a buffer so ridiculous large that it should never succeed and be rejected by the kernel driver.

This patch here is a really clear NAK.

Regards,
Christian.

it may be a kvmalloc() bug, and this patch can as a workaround in ttm before this issue fix.

void *kvmalloc_node(size_t size, gfp_t flags, int node) 

{

...

        if ((flags & GFP_KERNEL) != GFP_KERNEL)

                return kmalloc_node(size, flags, node);

...

}

Best Regards,

Kevin

> to allocate memory, when request size is exceeds kmalloc limit, it will
> cause allocate memory fail.
>
> e.g: when ttm want to create a BO with 1TB size, it maybe fail.
>
> Signed-off-by: Yang Wang <KevinYang.Wang@xxxxxxx>
> ---
>   drivers/gpu/drm/ttm/ttm_tt.c | 14 +++++++++++---
>   1 file changed, 11 insertions(+), 3 deletions(-)
>
> diff --git a/drivers/gpu/drm/ttm/ttm_tt.c b/drivers/gpu/drm/ttm/ttm_tt.c
> index 79c870a3bef8..9f2f3e576b8d 100644
> --- a/drivers/gpu/drm/ttm/ttm_tt.c
> +++ b/drivers/gpu/drm/ttm/ttm_tt.c
> @@ -97,9 +97,12 @@ int ttm_tt_create(struct ttm_buffer_object *bo, bool zero_alloc)
>   static int ttm_tt_alloc_page_directory(struct ttm_tt *ttm)
>   {
>        ttm->pages = kvmalloc_array(ttm->num_pages, sizeof(void*),
> -                     GFP_KERNEL | __GFP_ZERO);
> +                                 GFP_KERNEL);
>        if (!ttm->pages)
>                return -ENOMEM;
> +
> +     memset(ttm->pages, 0, ttm->num_pages * sizeof(void *));
> +
>        return 0;
>   }
>  
> @@ -108,10 +111,12 @@ static int ttm_dma_tt_alloc_page_directory(struct ttm_tt *ttm)
>        ttm->pages = kvmalloc_array(ttm->num_pages,
>                                    sizeof(*ttm->pages) +
>                                    sizeof(*ttm->dma_address),
> -                                 GFP_KERNEL | __GFP_ZERO);
> +                                 GFP_KERNEL);
>        if (!ttm->pages)
>                return -ENOMEM;
>  
> +     memset(ttm->pages, 0, ttm->num_pages * (sizeof(*ttm->pages) + sizeof(*ttm->dma_address)));
> +
>        ttm->dma_address = (void *)(ttm->pages + ttm->num_pages);
>        return 0;
>   }
> @@ -120,9 +125,12 @@ static int ttm_sg_tt_alloc_page_directory(struct ttm_tt *ttm)
>   {
>        ttm->dma_address = kvmalloc_array(ttm->num_pages,
>                                          sizeof(*ttm->dma_address),
> -                                       GFP_KERNEL | __GFP_ZERO);
> +                                       GFP_KERNEL);
>        if (!ttm->dma_address)
>                return -ENOMEM;
> +
> +     memset(ttm->dma_address, 0, ttm->num_pages * sizeof(*ttm->dma_address));
> +
>        return 0;
>   }
>