Messages in this thread | | | From | David Howells <> | Subject | Re: [PATCH v2 2/2] netfs: Fix dodgy maths | Date | Tue, 08 Nov 2022 15:59:08 +0000 |
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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
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