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From:   David Howells <dhowells@redhat.com>
In-Reply-To: <Y8/kfPEp1GkZKklc@nvidia.com>
References: <Y8/kfPEp1GkZKklc@nvidia.com> <Y8/hhvfDtVcsgQd6@nvidia.com> <Y8/ZekMEAfi8VeFl@nvidia.com> <20230123173007.325544-1-dhowells@redhat.com> <20230123173007.325544-11-dhowells@redhat.com> <31f7d71d-0eb9-2250-78c0-2e8f31023c66@nvidia.com> <84721e8d-d40e-617c-b75e-ead51c3e1edf@nvidia.com> <852117.1674567983@warthog.procyon.org.uk> <852914.1674568628@warthog.procyon.org.uk>
To:     Jason Gunthorpe <jgg@nvidia.com>
Cc:     dhowells@redhat.com, John Hubbard <jhubbard@nvidia.com>,
        Al Viro <viro@zeniv.linux.org.uk>,
        Christoph Hellwig <hch@infradead.org>,
        Matthew Wilcox <willy@infradead.org>,
        Jens Axboe <axboe@kernel.dk>, Jan Kara <jack@suse.cz>,
        Jeff Layton <jlayton@kernel.org>,
        Logan Gunthorpe <logang@deltatee.com>,
        linux-fsdevel@vger.kernel.org, linux-block@vger.kernel.org,
        linux-kernel@vger.kernel.org, Christoph Hellwig <hch@lst.de>,
        linux-mm@kvack.org
Subject: Re: [PATCH v8 10/10] mm: Renumber FOLL_PIN and FOLL_GET down
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Content-ID: <859141.1674569510.1@warthog.procyon.org.uk>
Date:   Tue, 24 Jan 2023 14:11:50 +0000
Message-ID: <859142.1674569510@warthog.procyon.org.uk>
Precedence: bulk

Jason Gunthorpe <jgg@nvidia.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