+       copied = ufshpb_fill_ppn_from_page(hpb, srgn->mctx, srgn_offset,
+                                          pre_req->wb.len - offset,
+                                          &addr[offset]);
+
+       if (copied < 0)
+               goto mctx_error;
+
+       offset += copied;
+       srgn_offset += offset;

This seems wrong.
How come the region offset is affected from the offset inside the pages?

+
+       if (srgn_offset == hpb->entries_per_srgn) {
+               srgn_offset = 0;
+
+               if (++srgn_idx == hpb->srgns_per_rgn) {
+                       srgn_idx = 0;
+                       rgn_idx++;
+               }
+       }
+
+       if (offset < pre_req->wb.len)
+               goto next_offset;

If the 512k resides in a single subregion, and span across pages, fill_ppn should take care of that.
If the 512k spans across subregion regions, than it spans across 2 subregions at most,
and maybe you can use it.