On Wed, Jan 12, 2022 at 06:37:03PM +0000, Matthew Wilcox wrote:

But let's go further than that (which only brings us to 32 bytes per
range).  For the systems you care about which use an identity mapping,
and have sizeof(dma_addr_t) == sizeof(phys_addr_t), we can simply
point the dma_range pointer to the same memory as the phyr.  We just
have to not free it too early.  That gets us down to 16 bytes per range,
a saving of 33%.

Even without an IOMMU the dma_addr_t can have offsets vs the actual
physical address.  Not on x86 except for a weirdo SOC, but just about
everywhere else.