Re: [PATCH for-next v5 6/7] RDMA/rxe: Add support for Send/Recv/Write/Read with ODP

[Jason Gunthorpe]

On Fri, Sep 08, 2023 at 06:35:56AM +0000, Daisuke Matsuda (Fujitsu) wrote:
> > IIRC this also needs to keep track in the xarray on a per page basis
> > if the page is writable.
> 
> An xarray entry can hold a pointer or a value from 0 to LONG_MAX.
> That is not enough to store page address and its permission.

It is, this is a page list so you know the lower 12 bits are not used
and you can encode stuff there.

> If we try to do everything with xarray, we need to allocate a new struct
> for each page that holds a pointer to a page and a value to store r/w permission.
> That is inefficient in terms of memory usage and implementation.

No, just use the lower extra bits.

> I think the xarray can be used to check presence of pages just like we have
> been doing in the non-ODP case. On the other hand, the permission
> should be fetched from umem_odp->pfn_list, which is updated everytime
> page fault is executed.

Definately not

Jason