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From: Yosry Ahmed <[email protected]>
To: Matthew Wilcox <[email protected]>
Cc: Peter Xu <[email protected]>,
	Andrew Morton <[email protected]>,
	Jens Axboe <[email protected]>,
	[email protected], [email protected]
Subject: Re: [PATCH 8/9] mm: Rearrange page flags
Date: Tue, 15 Aug 2023 16:33:19 -0700	[thread overview]
Message-ID: <CAJD7tkbcd=goQ5KH9sT+MWz1f=eo=ObxbzjqzxN79Ek+hKAy1A@mail.gmail.com> (raw)
In-Reply-To: <[email protected]>

On Tue, Aug 15, 2023 at 4:01 PM Matthew Wilcox <[email protected]> wrote:
>
> On Tue, Aug 15, 2023 at 03:31:42PM -0700, Yosry Ahmed wrote:
> > On Tue, Aug 15, 2023 at 1:07 PM Matthew Wilcox <[email protected]> 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.

I should have done the math :)
Never say never though :)

  reply	other threads:[~2023-08-15 23:34 UTC|newest]

Thread overview: 42+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2023-08-15  3:26 [PATCH 0/9] Remove _folio_dtor and _folio_order Matthew Wilcox (Oracle)
2023-08-15  3:26 ` [PATCH 1/9] io_uring: Stop calling free_compound_page() Matthew Wilcox (Oracle)
2023-08-15  7:33   ` David Hildenbrand
2023-08-15 15:00   ` Jens Axboe
2023-08-15 15:36     ` Matthew Wilcox
2023-08-15  3:26 ` [PATCH 2/9] mm: Call the hugetlb destructor directly Matthew Wilcox (Oracle)
2023-08-15  7:36   ` David Hildenbrand
2023-08-15  3:26 ` [PATCH 3/9] mm: Call free_transhuge_folio() directly from destroy_large_folio() Matthew Wilcox (Oracle)
2023-08-15  6:13   ` kernel test robot
2023-08-15  7:40   ` David Hildenbrand
2023-08-15 14:06     ` Matthew Wilcox
2023-08-15  8:09   ` kernel test robot
2023-08-15  3:26 ` [PATCH 4/9] mm: Make free_compound_page() static Matthew Wilcox (Oracle)
2023-08-15  7:47   ` David Hildenbrand
2023-08-15  7:48     ` David Hildenbrand
2023-08-15  3:26 ` [PATCH 5/9] mm: Remove free_compound_page() Matthew Wilcox (Oracle)
2023-08-15  7:48   ` David Hildenbrand
2023-08-15  3:26 ` [PATCH 6/9] mm: Remove HUGETLB_PAGE_DTOR Matthew Wilcox (Oracle)
2023-08-15  7:50   ` David Hildenbrand
2023-08-15  3:26 ` [PATCH 7/9] mm: Add deferred_list page flag Matthew Wilcox (Oracle)
2023-08-15  7:54   ` David Hildenbrand
2023-08-15 15:32     ` Matthew Wilcox
2023-08-15 16:40       ` David Hildenbrand
2023-08-15 17:06         ` Matthew Wilcox
2023-08-15 17:27           ` David Hildenbrand
2023-08-15 19:58             ` Matthew Wilcox
2023-08-16  3:14               ` Matthew Wilcox
2023-08-16 10:12                 ` David Hildenbrand
2023-08-16 12:05                   ` Matthew Wilcox
2023-08-16 12:34                     ` David Hildenbrand
2023-08-16  9:55               ` David Hildenbrand
2023-08-15  3:26 ` [PATCH 8/9] mm: Rearrange page flags Matthew Wilcox (Oracle)
2023-08-15  4:30   ` Yosry Ahmed
2023-08-15 19:24   ` Peter Xu
2023-08-15 20:07     ` Matthew Wilcox
2023-08-15 22:31       ` Yosry Ahmed
2023-08-15 23:01         ` Matthew Wilcox
2023-08-15 23:33           ` Yosry Ahmed [this message]
2023-08-15  3:26 ` [PATCH 9/9] mm: Free up a word in the first tail page Matthew Wilcox (Oracle)
2023-08-15  7:59   ` David Hildenbrand
2023-08-15 11:39     ` Matthew Wilcox
2023-08-15 19:21   ` Peter Xu

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