Re: [PATCH v2 2/2] netfs: Fix dodgy maths

[David Howells <dhowells@redhat.com>]

JeffleXu <jefflexu@linux.alibaba.com> wrote:

> > Fix the dodgy maths in netfs_rreq_unlock_folios().  start_page could be
> > inside the folio, in which case the calculation of pgpos will be come up
> > with a negative number (though for the moment rreq->start is rounded down
> > earlier and folios would have to get merged whilst locked)
> 
> Hi, the patch itself seems fine. Just some questions about the scenario.
> 
> 1. "start_page could be inside the folio" Is that because
> .expand_readahead() called from netfs_readahead()? Since otherwise,
> req-start is always aligned to the folio boundary.

At the moment, rreq->start is always coincident with the start of the first
folio in the collection because we always read whole folios - however, it
might be best to assume that this might not always hold true if it's simple to
fix the maths to get rid of the assumption.

> 2. If start_page is indeed inside the folio, then only the trailing part
> of the first folio can be covered by the request, and this folio will be
> marked with uptodate, though the beginning part of the folio may have
> not been read from the cache. Is that expected? Or correct me if I'm wrong.

For the moment there's no scenario where this arises; I think we need to wait
until we have a scenario and then see how we'll need to juggle the flags.

David