Re: [Intel-gfx] [RFC 06/17] drm: i915: fix sg_table nents vs. orig_nents misuse

Marek Szyprowski
2020-04-30T14:18:09Z

Hi

On 28.04.2020 16:27, Tvrtko Ursulin wrote:
>
> On 28/04/2020 14:19, Marek Szyprowski wrote:
>> The Documentation/DMA-API-HOWTO.txt states that dma_map_sg returns the
>> numer of the created entries in the DMA address space. However the
>> subsequent calls to dma_sync_sg_for_{device,cpu} and dma_unmap_sg
>> must be
>> called with the original number of entries passed to dma_map_sg. The
>> sg_table->nents in turn holds the result of the dma_map_sg call as
>> stated
>> in include/linux/scatterlist.h. Adapt the code to obey those rules.
>>
>> Signed-off-by: Marek Szyprowski <m.szyprowski@samsung.com>
>> ---
>>   drivers/gpu/drm/i915/gem/i915_gem_dmabuf.c       | 13 +++++++------
>>   drivers/gpu/drm/i915/gem/i915_gem_internal.c     |  4 ++--
>>   drivers/gpu/drm/i915/gem/i915_gem_region.c       |  4 ++--
>>   drivers/gpu/drm/i915/gem/i915_gem_shmem.c        |  5 +++--
>>   drivers/gpu/drm/i915/gem/selftests/huge_pages.c  | 10 +++++-----
>>   drivers/gpu/drm/i915/gem/selftests/mock_dmabuf.c |  5 +++--
>>   drivers/gpu/drm/i915/gt/intel_ggtt.c             | 12 ++++++------
>>   drivers/gpu/drm/i915/i915_gem_gtt.c              | 12 +++++++-----
>>   drivers/gpu/drm/i915/i915_scatterlist.c          |  4 ++--
>>   drivers/gpu/drm/i915/selftests/scatterlist.c     |  8 ++++----
>>   10 files changed, 41 insertions(+), 36 deletions(-)
>>
>> diff --git a/drivers/gpu/drm/i915/gem/i915_gem_dmabuf.c
>> b/drivers/gpu/drm/i915/gem/i915_gem_dmabuf.c
>> index 7db5a79..d829852 100644
>> --- a/drivers/gpu/drm/i915/gem/i915_gem_dmabuf.c
>> +++ b/drivers/gpu/drm/i915/gem/i915_gem_dmabuf.c
>> @@ -36,21 +36,22 @@ static struct sg_table
>> *i915_gem_map_dma_buf(struct dma_buf_attachment *attachme
>>           goto err_unpin_pages;
>>       }
>>   -    ret = sg_alloc_table(st, obj->mm.pages->nents, GFP_KERNEL);
>> +    ret = sg_alloc_table(st, obj->mm.pages->orig_nents, GFP_KERNEL);
>>       if (ret)
>>           goto err_free;
>>         src = obj->mm.pages->sgl;
>>       dst = st->sgl;
>> -    for (i = 0; i < obj->mm.pages->nents; i++) {
>> +    for (i = 0; i < obj->mm.pages->orig_nents; i++) {
>>           sg_set_page(dst, sg_page(src), src->length, 0);
>>           dst = sg_next(dst);
>>           src = sg_next(src);
>>       }
>>   -    if (!dma_map_sg_attrs(attachment->dev,
>> -                  st->sgl, st->nents, dir,
>> -                  DMA_ATTR_SKIP_CPU_SYNC)) {
>> +    st->nents = dma_map_sg_attrs(attachment->dev,
>> +                     st->sgl, st->orig_nents, dir,
>> +                     DMA_ATTR_SKIP_CPU_SYNC);
>> +    if (!st->nents) {
>>           ret = -ENOMEM;
>>           goto err_free_sg;
>>       }
>> @@ -74,7 +75,7 @@ static void i915_gem_unmap_dma_buf(struct
>> dma_buf_attachment *attachment,
>>       struct drm_i915_gem_object *obj =
>> dma_buf_to_obj(attachment->dmabuf);
>>         dma_unmap_sg_attrs(attachment->dev,
>> -               sg->sgl, sg->nents, dir,
>> +               sg->sgl, sg->orig_nents, dir,
>>                  DMA_ATTR_SKIP_CPU_SYNC);
>>       sg_free_table(sg);
>>       kfree(sg);
>> diff --git a/drivers/gpu/drm/i915/gem/i915_gem_internal.c
>> b/drivers/gpu/drm/i915/gem/i915_gem_internal.c
>> index cbbff81..a8ebfdd 100644
>> --- a/drivers/gpu/drm/i915/gem/i915_gem_internal.c
>> +++ b/drivers/gpu/drm/i915/gem/i915_gem_internal.c
>> @@ -73,7 +73,7 @@ static int
>> i915_gem_object_get_pages_internal(struct drm_i915_gem_object *obj)
>>       }
>>         sg = st->sgl;
>> -    st->nents = 0;
>> +    st->nents = st->orig_nents = 0;
>>       sg_page_sizes = 0;
>>         do {
>> @@ -94,7 +94,7 @@ static int
>> i915_gem_object_get_pages_internal(struct drm_i915_gem_object *obj)
>>             sg_set_page(sg, page, PAGE_SIZE << order, 0);
>>           sg_page_sizes |= PAGE_SIZE << order;
>> -        st->nents++;
>> +        st->nents = st->orig_nents = st->nents + 1;
>>             npages -= 1 << order;
>>           if (!npages) {
>> diff --git a/drivers/gpu/drm/i915/gem/i915_gem_region.c
>> b/drivers/gpu/drm/i915/gem/i915_gem_region.c
>> index 1515384..58ca560 100644
>> --- a/drivers/gpu/drm/i915/gem/i915_gem_region.c
>> +++ b/drivers/gpu/drm/i915/gem/i915_gem_region.c
>> @@ -53,7 +53,7 @@
>>       GEM_BUG_ON(list_empty(blocks));
>>         sg = st->sgl;
>> -    st->nents = 0;
>> +    st->nents = st->orig_nents = 0;
>>       sg_page_sizes = 0;
>>       prev_end = (resource_size_t)-1;
>>   @@ -78,7 +78,7 @@
>>                 sg->length = block_size;
>>   -            st->nents++;
>> +            st->nents = st->orig_nents = st->nents + 1;
>>           } else {
>>               sg->length += block_size;
>>               sg_dma_len(sg) += block_size;
>> diff --git a/drivers/gpu/drm/i915/gem/i915_gem_shmem.c
>> b/drivers/gpu/drm/i915/gem/i915_gem_shmem.c
>> index 5d5d7ee..851a732 100644
>> --- a/drivers/gpu/drm/i915/gem/i915_gem_shmem.c
>> +++ b/drivers/gpu/drm/i915/gem/i915_gem_shmem.c
>> @@ -80,7 +80,7 @@ static int shmem_get_pages(struct
>> drm_i915_gem_object *obj)
>>       noreclaim |= __GFP_NORETRY | __GFP_NOWARN;
>>         sg = st->sgl;
>> -    st->nents = 0;
>> +    st->nents = st->orig_nents = 0;
>>       sg_page_sizes = 0;
>>       for (i = 0; i < page_count; i++) {
>>           const unsigned int shrink[] = {
>> @@ -140,7 +140,8 @@ static int shmem_get_pages(struct
>> drm_i915_gem_object *obj)
>>                   sg_page_sizes |= sg->length;
>>                   sg = sg_next(sg);
>>               }
>> -            st->nents++;
>> +            st->nents = st->orig_nents = st->nents + 1;
>
> A bit higher up, not shown in the patch, we have allocated a table via
> sg_alloc_table giving it a pessimistic max nents, sometimes much
> larger than the st->nents this loops will create. But orig_nents has
> been now been overwritten. Will that leak memory come sg_table_free?

Indeed this will leak memory. I'm missed that sg_trim() will adjust
nents and orig_nents.

I will drop those changes as they are only a creative (or hacky) way of
using sg_table and scatterlists.

> As minimum it will nerf our i915_sg_trim optimization a bit lower
> down, also not shown in the diff.
>
> > [...]

Best regards
--
Marek Szyprowski, PhD
Samsung R&D Institute Poland