Re: [Intel-gfx] [PATCH rdma-next v4 4/4] RDMA/umem: Move to allocate SG table from pages

Maor Gottlieb
2020-09-30T16:53:55Z

On 9/30/2020 2:58 PM, Jason Gunthorpe wrote:
> On Wed, Sep 30, 2020 at 02:53:58PM +0300, Maor Gottlieb wrote:
>> On 9/30/2020 2:45 PM, Jason Gunthorpe wrote:
>>> On Wed, Sep 30, 2020 at 12:53:21PM +0300, Leon Romanovsky wrote:
>>>> On Tue, Sep 29, 2020 at 04:59:29PM -0300, Jason Gunthorpe wrote:
>>>>> On Sun, Sep 27, 2020 at 09:46:47AM +0300, Leon Romanovsky wrote:
>>>>>> @@ -296,11 +223,17 @@ static struct ib_umem *__ib_umem_get(struct ib_device *device,
>>>>>> goto umem_release;
>>>>>>
>>>>>> cur_base += ret * PAGE_SIZE;
>>>>>> - npages -= ret;
>>>>>> -
>>>>>> - sg = ib_umem_add_sg_table(sg, page_list, ret,
>>>>>> - dma_get_max_seg_size(device->dma_device),
>>>>>> - &umem->sg_nents);
>>>>>> + npages -= ret;
>>>>>> + sg = __sg_alloc_table_from_pages(
>>>>>> + &umem->sg_head, page_list, ret, 0, ret << PAGE_SHIFT,
>>>>>> + dma_get_max_seg_size(device->dma_device), sg, npages,
>>>>>> + GFP_KERNEL);
>>>>>> + umem->sg_nents = umem->sg_head.nents;
>>>>>> + if (IS_ERR(sg)) {
>>>>>> + unpin_user_pages_dirty_lock(page_list, ret, 0);
>>>>>> + ret = PTR_ERR(sg);
>>>>>> + goto umem_release;
>>>>>> + }
>>>>>> }
>>>>>>
>>>>>> sg_mark_end(sg);
>>>>> Does it still need the sg_mark_end?
>>>> It is preserved here for correctness, the release logic doesn't rely on
>>>> this marker, but it is better to leave it.
>>> I mean, my read of __sg_alloc_table_from_pages() is that it already
>>> placed it, the final __alloc_table() does it?
>> It marks the last allocated sge, but not the last populated sge (with page).
> Why are those different?
>
> It looks like the last iteration calls __alloc_table() with an exact
> number of sges
>
> + if (!prv) {
> + /* Only the last allocation could be less than the maximum */
> + table_size = left_pages ? SG_MAX_SINGLE_ALLOC : chunks;
> + ret = sg_alloc_table(sgt, table_size, gfp_mask);
> + if (unlikely(ret))
> + return ERR_PTR(ret);
> + }
>
> Jason

This is right only for the last iteration. E.g. in the first iteration
in case that there are more pages (left_pages), then we allocate
SG_MAX_SINGLE_ALLOC.  We don't know how many pages from the second
iteration will be squashed to the SGE from the first iteration.