RDMA/umem: Prevent small pages from being returned by ib_umem_find_best_pgsz()

rdma_for_each_block() makes assumptions about how the SGL is constructed
that don't work if the block size is below the page size used to to build
the SGL.

The rules for umem SGL construction require that the SG's all be PAGE_SIZE
aligned and we don't encode the actual byte offset of the VA range inside
the SGL using offset and length. So rdma_for_each_block() has no idea
where the actual starting/ending point is to compute the first/last block
boundary if the starting address should be within a SGL.

Fixing the SGL construction turns out to be really hard, and will be the
subject of other patches. For now block smaller pages.

Fixes: 4a353399 ("RDMA/umem: Add API to find best driver supported page size in an MR")
Link: https://lore.kernel.org/r/2-v2-270386b7e60b+28f4-umem_1_jgg@nvidia.comReviewed-by: Leon Romanovsky <leonro@nvidia.com>
Reviewed-by: Shiraz Saleem <shiraz.saleem@intel.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>

drivers/infiniband/core/umem.c

@@ -151,6 +151,12 @@ unsigned long ib_umem_find_best_pgsz(struct ib_umem *umem,
 	dma_addr_t mask;
 	int i;
 
+	/* rdma_for_each_block() has a bug if the page size is smaller than the
+	 * page size used to build the umem. For now prevent smaller page sizes
+	 * from being returned.
+	 */
+	pgsz_bitmap &= GENMASK(BITS_PER_LONG - 1, PAGE_SHIFT);
+
 	/* At minimum, drivers must support PAGE_SIZE or smaller */
 	if (WARN_ON(!(pgsz_bitmap & GENMASK(PAGE_SHIFT, 0))))
 		return 0;