mm: slab: reduce the kmalloc() minimum alignment if DMA bouncing possible

If an architecture opted in to DMA bouncing of unaligned kmalloc() buffers
(ARCH_WANT_KMALLOC_DMA_BOUNCE), reduce the minimum kmalloc() cache
alignment below cache-line size to ARCH_KMALLOC_MINALIGN.

Link: https://lkml.kernel.org/r/[email protected]
Signed-off-by: Catalin Marinas <[email protected]>
Reviewed-by: Vlastimil Babka <[email protected]>
Tested-by: Isaac J. Manjarres <[email protected]>
Cc: Christoph Hellwig <[email protected]>
Cc: Robin Murphy <[email protected]>
Cc: Alasdair Kergon <[email protected]>
Cc: Ard Biesheuvel <[email protected]>
Cc: Arnd Bergmann <[email protected]>
Cc: Daniel Vetter <[email protected]>
Cc: Greg Kroah-Hartman <[email protected]>
Cc: Herbert Xu <[email protected]>
Cc: Jerry Snitselaar <[email protected]>
Cc: Joerg Roedel <[email protected]>
Cc: Jonathan Cameron <[email protected]>
Cc: Jonathan Cameron <[email protected]>
Cc: Lars-Peter Clausen <[email protected]>
Cc: Logan Gunthorpe <[email protected]>
Cc: Marc Zyngier <[email protected]>
Cc: Mark Brown <[email protected]>
Cc: Mike Snitzer <[email protected]>
Cc: "Rafael J. Wysocki" <[email protected]>
Cc: Saravana Kannan <[email protected]>
Cc: Will Deacon <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>

mm/slab_common.c

@@ -18,6 +18,7 @@
 #include <linux/uaccess.h>
 #include <linux/seq_file.h>
 #include <linux/dma-mapping.h>
+#include <linux/swiotlb.h>
 #include <linux/proc_fs.h>
 #include <linux/debugfs.h>
 #include <linux/kasan.h>
@@ -865,6 +866,10 @@ void __init setup_kmalloc_cache_index_table(void)
 
 static unsigned int __kmalloc_minalign(void)
 {
+#ifdef CONFIG_DMA_BOUNCE_UNALIGNED_KMALLOC
+	if (io_tlb_default_mem.nslabs)
+		return ARCH_KMALLOC_MINALIGN;
+#endif
 	return dma_get_cache_alignment();
 }