Re: [PATCH 8/9] mm: Rearrange page flags

[Matthew Wilcox <willy@infradead.org>]

On Tue, Aug 15, 2023 at 03:31:42PM -0700, Yosry Ahmed wrote:
> On Tue, Aug 15, 2023 at 1:07 PM Matthew Wilcox <willy@infradead.org> wrote:
> >
> > On Tue, Aug 15, 2023 at 03:24:16PM -0400, Peter Xu wrote:
> > > On Tue, Aug 15, 2023 at 04:26:44AM +0100, Matthew Wilcox (Oracle) wrote:
> > > > +++ b/include/linux/page-flags.h
> > > > @@ -99,13 +99,15 @@
> > > >   */
> > > >  enum pageflags {
> > > >     PG_locked,              /* Page is locked. Don't touch. */
> > > > +   PG_writeback,           /* Page is under writeback */
> > > >     PG_referenced,
> > > >     PG_uptodate,
> > > >     PG_dirty,
> > > >     PG_lru,
> > > > +   PG_head,                /* Must be in bit 6 */
> > >
> > > Could there be some explanation on "must be in bit 6" here?
> >
> > Not on this line, no.  You get 40-50 characters to say something useful.
> > Happy to elaborate further in some other comment or in the commit log,
> > but not on this line.
> >
> > The idea behind all of this is that _folio_order moves into the bottom
> > byte of _flags_1.  Because the order can never be greater than 63 (and
> > in practice I think the largest we're going to see is about 30 -- a 16GB
> > hugetlb page on some architectures), we know that PG_head and PG_waiters
> > will be clear, so we can write (folio->_flags_1 & 0xff) and the compiler
> > will just load a byte; it won't actually load the word and mask it.
> >
> > We can't move PG_head any lower, or we'll risk having a tail page with
> > PG_head set (which can happen with the vmemmmap optimisation, but eugh).
> > And we don't want to move it any higher because then we'll have a flag
> > that cannot be reused in a tail page.  Bit 6 is the perfect spot for it.
> 
> Is there some compile time assertion that the order cannot overflow into bit 6?

An order-64 folio on a machine with a 4kB page size would be 64
zettabytes.  I intend to retire before we're managing memory in chunks
that large.