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

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Message-ID: <Y8/ZekMEAfi8VeFl@nvidia.com>
Date:   Tue, 24 Jan 2023 09:13:30 -0400
From:   Jason Gunthorpe <jgg@...dia.com>
To:     John Hubbard <jhubbard@...dia.com>
Cc:     David Howells <dhowells@...hat.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

On Mon, Jan 23, 2023 at 07:11:42PM -0800, John Hubbard wrote:
> On 1/23/23 19:08, John Hubbard wrote:
> > On 1/23/23 09:30, David Howells wrote:
> > > Renumber FOLL_PIN and FOLL_GET down to bit 0 and 1 respectively so that
> > > they are coincidentally the same as BIO_PAGE_PINNED and BIO_PAGE_REFFED and
> > > also so that they can be stored in the bottom two bits of a page pointer
> > > (something I'm looking at for zerocopy socket fragments).
> > > 
> > > (Note that BIO_PAGE_REFFED should probably be got rid of at some point,
> > > hence why FOLL_PIN is at 0.)
> > > 
> > > Also renumber down the other FOLL_* flags to close the gaps.
> > 
> > Should we also get these sorted into internal-to-mm and public sets?
> > Because Jason (+Cc) again was about to split them apart into
> > mm/internal.h [1] and that might make that a little cleaner.
> 
> I see that I omitted both Jason's Cc and the reference, so I'll resend.
> Sorry for the extra noise.
> 
> [1] https://lore.kernel.org/all/fcdb3294-3740-a0e0-b115-12842eb0696d@nvidia.com/

Yeah, I already wrote a similar patch, using the 1<< notation, 
splitting the internal/external, and rebasing on the move to
mm_types.. I can certainly drop that patch if we'd rather do this.

Though, I'm not so keen on using FOLL_ internal flags inside the block
layer.. Can you stick with the BIO versions of these?

Jason