linux-kernel - Re: [PATCH v2 07/10] drm/msm/gem: Rework vma lookup and pin

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Message-ID: <9de85a5a-0b61-0b20-d8b2-d412fc081647@collabora.com>
Date:   Thu, 31 Mar 2022 21:28:36 +0300
From:   Dmitry Osipenko <dmitry.osipenko@...labora.com>
To:     Rob Clark <robdclark@...il.com>, dri-devel@...ts.freedesktop.org
Cc:     freedreno@...ts.freedesktop.org, linux-arm-msm@...r.kernel.org,
        Dmitry Baryshkov <dmitry.baryshkov@...aro.org>,
        Rob Clark <robdclark@...omium.org>,
        Sean Paul <sean@...rly.run>,
        Abhinav Kumar <quic_abhinavk@...cinc.com>,
        David Airlie <airlied@...ux.ie>,
        Daniel Vetter <daniel@...ll.ch>,
        open list <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2 07/10] drm/msm/gem: Rework vma lookup and pin

On 3/31/22 21:27, Dmitry Osipenko wrote:
> On 3/30/22 23:47, Rob Clark wrote:
>> From: Rob Clark <robdclark@...omium.org>
>>
>> Combines duplicate vma lookup in the get_and_pin path.
>>
>> Signed-off-by: Rob Clark <robdclark@...omium.org>
>> ---
>>  drivers/gpu/drm/msm/msm_gem.c | 50 ++++++++++++++++++-----------------
>>  1 file changed, 26 insertions(+), 24 deletions(-)
>>
>> diff --git a/drivers/gpu/drm/msm/msm_gem.c b/drivers/gpu/drm/msm/msm_gem.c
>> index deafae6feaa8..218744a490a4 100644
>> --- a/drivers/gpu/drm/msm/msm_gem.c
>> +++ b/drivers/gpu/drm/msm/msm_gem.c
>> @@ -376,39 +376,40 @@ put_iova_vmas(struct drm_gem_object *obj)
>>  	}
>>  }
>>  
>> -static int get_iova_locked(struct drm_gem_object *obj,
>> -		struct msm_gem_address_space *aspace, uint64_t *iova,
>> +static struct msm_gem_vma *get_vma_locked(struct drm_gem_object *obj,
>> +		struct msm_gem_address_space *aspace,
>>  		u64 range_start, u64 range_end)
>>  {
>>  	struct msm_gem_vma *vma;
>> -	int ret = 0;
>>  
>>  	GEM_WARN_ON(!msm_gem_is_locked(obj));
>>  
>>  	vma = lookup_vma(obj, aspace);
>>  
>>  	if (!vma) {
>> +		int ret;
>> +
>>  		vma = add_vma(obj, aspace);
>>  		if (IS_ERR(vma))
>> -			return PTR_ERR(vma);
>> +			return vma;
>>  
>>  		ret = msm_gem_init_vma(aspace, vma, obj->size,
>>  			range_start, range_end);
>>  		if (ret) {
> You're allocation range_start -> range_end

*allocating

> 
>>  			del_vma(vma);
>> -			return ret;
>> +			return ERR_PTR(ret);
>>  		}
>> +	} else {
>> +		GEM_WARN_ON(vma->iova < range_start);
>> +		GEM_WARN_ON((vma->iova + obj->size) > range_end);
> 
> and then comparing range_start -> range_start + obj->size, hence you're
> assuming that range_end always equals to obj->size during the allocation.
> 
> I'm not sure what is the idea here.. this looks inconsistent. I think
> you wanted to write:
> 
> 		GEM_WARN_ON(vma->iova < range_start);
> 		GEM_WARN_ON(vma->iova + (vma->node.size << PAGE_SHIFT) > range_end);
> 
> But is it really useful to check whether the new range is inside of the
> old range? Shouldn't it be always a error to change the IOVA range
> without reallocating vma?
> 
> I'd expect to see:
> 
> 		GEM_WARN_ON(vma->iova != range_start);
> 		GEM_WARN_ON(vma->iova + (vma->node.size << PAGE_SHIFT) != range_end);
> 
> and then error out if range mismatches.