Re: [PATCH V3 8/9] iommufd: optimize file mapping

[Jason Gunthorpe <jgg@nvidia.com>]

On Fri, Oct 18, 2024 at 10:34:39AM -0400, Steven Sistare wrote:
> > Ie instead of things like:
> > 
> > 		batch_from_folios_huge(&pfns->batch, &user->ufolios_next,
> > 				       &user->ufolios_offset, npages);
> > 
> > Where npages is the total number of pages inside the folio list, it
> > would be nfolios.
> 
> The problem is that a batch may reach capacity after accepting only part
> of a folio, a condition I forced in testing by reducing array_size and
> MAX_NPFNS. We need to track which folio is partial and what is the next index
> to use in the folio.  That is ufolios_next and ufolios_offset.

But why is that such a problem? The ufolios_offset already handles a
partial folio.

If the batch fills you leave ufolios_next alone and adjust
ufolios_offset. The next iteration does exactly the same as the first
iteration and takes the partial folio.

The refcount has to be handled carefully as the the portion that has
been put in the batch needs to be refcounted at 4k and the remainder
remain a single ref, but that just means adjusting the refcount after
putting it in the batch.

It seems like it just works naturally and avoids the double loop

Jason