On Wed, Sep 02, 2020 at 02:51:19PM +0300, Leon Romanovsky wrote:
> On Tue, Sep 01, 2020 at 09:43:30PM -0300, Jason Gunthorpe wrote:
> > 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: 4a35339958f1 ("RDMA/umem: Add API to find best driver supported page size in an MR")
> > Signed-off-by: Jason Gunthorpe <jgg@xxxxxxxxxx>
> > drivers/infiniband/core/umem.c | 6 ++++++
> > 1 file changed, 6 insertions(+)
> >
> > diff --git a/drivers/infiniband/core/umem.c b/drivers/infiniband/core/umem.c
> > index 120e98403c345d..7b5bc969e55630 100644
> > +++ b/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);
> > +
>
> Why do we care about such case? Why can't we leave this check forever?
If HW supports only, say 4k page size, and runs on a 64k page size
architecture it should be able to fragment into the native HW page
size.
The whole point of these APIs is to decouple the system and HW page
sizes.
Jason
