On Tue, June 13, 2023 1:23 AM Jason Gunthorpe wrote:
> On Thu, May 18, 2023 at 05:21:51PM +0900, Daisuke Matsuda wrote:
>
> > +/* umem mutex must be locked before entering this function. */
> > +static int rxe_odp_map_range(struct rxe_mr *mr, u64 iova, int length, u32 flags)
> > +{
> > + struct ib_umem_odp *umem_odp = to_ib_umem_odp(mr->umem);
> > + const int max_tries = 3;
> > + int cnt = 0;
> > +
> > + int err;
> > + u64 perm;
> > + bool need_fault;
> > +
> > + if (unlikely(length < 1)) {
> > + mutex_unlock(&umem_odp->umem_mutex);
> > + return -EINVAL;
> > + }
> > +
> > + perm = ODP_READ_ALLOWED_BIT;
> > + if (!(flags & RXE_PAGEFAULT_RDONLY))
> > + perm |= ODP_WRITE_ALLOWED_BIT;
> > +
> > + /*
> > + * A successful return from rxe_odp_do_pagefault() does not guarantee
> > + * that all pages in the range became present. Recheck the DMA address
> > + * array, allowing max 3 tries for pagefault.
> > + */
> > + while ((need_fault = rxe_is_pagefault_neccesary(umem_odp,
> > + iova, length, perm))) {
> > + if (cnt >= max_tries)
> > + break;
>
> I don't think this makes sense..
I have posted the new implementation, but it is somewhat different from
your suggestion. The reason is as below.
>
> You need to make this work more like mlx5 does, you take the spinlock
> on the xarray, you search it for your index and whatever is there
> tells what to do. Hold the spinlock while doing the copy. This is
> enough locking for the fast path.
This lock should be umem mutex. Actual page-out is executed in the
kernel after rxe_ib_invalidate_range() is done. It is possible the spinlock
does not block this flow if it is locked while the invalidation is running.
The umem mutex can serialize these accesses.
>
> If there is no index present, or it is not writable and you need
> write, then you unlock the spinlock and prefetch the missing entry and
> try again, this time also holding the mutex so there isn't a race.
>
> You shouldn't probe into parts of the umem to discover information
> already stored in the xarray then do the same lookup into the xarray.
>
> 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.
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.
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.
Thanks,
Daisuke
>
> Jason
