linux-kernel - Re: [PATCH v8 10/10] mm: Renumber FOLL_PIN and FOLL

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Message-ID: <859142.1674569510@warthog.procyon.org.uk>
Date:   Tue, 24 Jan 2023 14:11:50 +0000
From:   David Howells <dhowells@...hat.com>
To:     Jason Gunthorpe <jgg@...dia.com>
Cc:     dhowells@...hat.com, John Hubbard <jhubbard@...dia.com>,
        Al Viro <viro@...iv.linux.org.uk>,
        Christoph Hellwig <hch@...radead.org>,
        Matthew Wilcox <willy@...radead.org>,
        Jens Axboe <axboe@...nel.dk>, Jan Kara <jack@...e.cz>,
        Jeff Layton <jlayton@...nel.org>,
        Logan Gunthorpe <logang@...tatee.com>,
        linux-fsdevel@...r.kernel.org, linux-block@...r.kernel.org,
        linux-kernel@...r.kernel.org, Christoph Hellwig <hch@....de>,
        linux-mm@...ck.org
Subject: Re: [PATCH v8 10/10] mm: Renumber FOLL_PIN and FOLL_GET down

Jason Gunthorpe <jgg@...dia.com> wrote:

> if (cleanup_flags & PAGE_CLEANUP_UNPIN)
>    gup_put_folio(folio, 1, true);
> else if (cleanup_flags & PAGE_CLEANUP_PUT)
>    gup_put_folio(folio, 1, false);

gup_put_folio() doesn't take a bool - it takes FOLL_PIN and FOLL_GET:

static void gup_put_folio(struct folio *folio, int refs, unsigned int flags)

though I could do:

	if (cleanup_flags & PAGE_CLEANUP_UNPIN)
		gup_put_folio(folio, 1, FOLL_PIN);
	else if (cleanup_flags & PAGE_CLEANUP_PUT)
		gup_put_folio(folio, 1, FOLL_GET);

But the reason I wrote it like this:

 		gup_flags |= cleanup_flags & PAGE_CLEANUP_UNPIN ? FOLL_PIN : 0;
 		gup_flags |= cleanup_flags & PAGE_CLEANUP_PUT ? FOLL_GET : 0;

is that if PAGE_CLEANUP_UNPIN == FOLL_PIN and PAGE_CLEANUP_PUT == FOLL_GET,
the C compiler optimiser should be able to turn all of that into a single AND
instruction.

David