linux-kernel - Re: [PATCH v5 4/9] iov_iter: Add a function to extract a page list from an iterator

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Message-ID: <Y8B9bvh3Mjed8EBc@ZenIV>
Date:   Thu, 12 Jan 2023 21:36:46 +0000
From:   Al Viro <viro@...iv.linux.org.uk>
To:     David Howells <dhowells@...hat.com>
Cc:     Christoph Hellwig <hch@....de>, John Hubbard <jhubbard@...dia.com>,
        Matthew Wilcox <willy@...radead.org>,
        linux-fsdevel@...r.kernel.org, linux-mm@...ck.org,
        Christoph Hellwig <hch@...radead.org>,
        Jens Axboe <axboe@...nel.dk>, Jan Kara <jack@...e.cz>,
        Jeff Layton <jlayton@...nel.org>,
        Logan Gunthorpe <logang@...tatee.com>,
        linux-block@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v5 4/9] iov_iter: Add a function to extract a page list
 from an iterator

On Thu, Jan 12, 2023 at 09:15:50PM +0000, Al Viro wrote:
> On Wed, Jan 11, 2023 at 02:28:05PM +0000, David Howells wrote:
> 
> > +ssize_t iov_iter_extract_pages(struct iov_iter *i,
> > +			       struct page ***pages,
> > +			       size_t maxsize,
> > +			       unsigned int maxpages,
> > +			       unsigned int gup_flags,
> > +			       size_t *offset0,
> > +			       unsigned int *cleanup_mode)
> 
> This cleanup_mode thing is wrong.  It's literally a trivial
> function of ->user_backed and ->data_source - we don't
> even need to look at the ->type.
> 
> Separate it into an inline helper and be done with that;
> don't carry it all over the place.
> 
> It's really "if not user-backed => 0, otherwise it's FOLL_PIN or FOLL_GET,
> depending upon the direction".

Seriously, it would be easier to follow that way; if you really insist upon
keeping these calling conventions, at least put the calculation in one place -
don't make readers to chase down into every sodding helper to check if they
do what you'd expect them to do.