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

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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.

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