linux-kernel - Re: [PATCH 24/28] mm, swap: implement helpers for reserving data in swap table

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Message-ID: <CAMgjq7DJvHZ-0VztN5axwy4X3z-dKRtp9xHSP4RrE=nQ4G_siA@mail.gmail.com>
Date: Fri, 16 May 2025 10:35:11 +0800
From: Kairui Song <ryncsn@...il.com>
To: Klara Modin <klarasmodin@...il.com>
Cc: linux-mm@...ck.org, Andrew Morton <akpm@...ux-foundation.org>, 
	Matthew Wilcox <willy@...radead.org>, Hugh Dickins <hughd@...gle.com>, Chris Li <chrisl@...nel.org>, 
	David Hildenbrand <david@...hat.com>, Yosry Ahmed <yosryahmed@...gle.com>, 
	"Huang, Ying" <ying.huang@...ux.alibaba.com>, Nhat Pham <nphamcs@...il.com>, 
	Johannes Weiner <hannes@...xchg.org>, Baolin Wang <baolin.wang@...ux.alibaba.com>, 
	Baoquan He <bhe@...hat.com>, Barry Song <baohua@...nel.org>, 
	Kalesh Singh <kaleshsingh@...gle.com>, Kemeng Shi <shikemeng@...weicloud.com>, 
	Tim Chen <tim.c.chen@...ux.intel.com>, Ryan Roberts <ryan.roberts@....com>, 
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 24/28] mm, swap: implement helpers for reserving data in
 swap table

On Thu, May 15, 2025 at 5:42 PM Klara Modin <klarasmodin@...il.com> wrote:
>
> Hi,
>
> On 2025-05-15 04:17:24 +0800, Kairui Song wrote:
> > From: Kairui Song <kasong@...cent.com>
> >
> > To prepare for using swap table as the unified swap layer, introduce
> > macro and helpers for storing multiple kind of data in an swap table
> > entry.
> >
> > Signed-off-by: Kairui Song <kasong@...cent.com>
> > ---
> >  mm/swap_table.h | 130 ++++++++++++++++++++++++++++++++++++++++++++----
> >  1 file changed, 119 insertions(+), 11 deletions(-)
> >
> > diff --git a/mm/swap_table.h b/mm/swap_table.h
> > index 69a074339444..9356004d211a 100644
> > --- a/mm/swap_table.h
> > +++ b/mm/swap_table.h
> > @@ -5,9 +5,41 @@
> >  #include "swap.h"
> >
> >  /*
> > - * Swap table entry could be a pointer (folio), a XA_VALUE (shadow), or NULL.
> > + * Swap table entry type and bit layouts:
> > + *
> > + * NULL:     | ------------    0   -------------|
> > + * Shadow:   | SWAP_COUNT |---- SHADOW_VAL ---|1|
> > + * Folio:    | SWAP_COUNT |------ PFN -------|10|
> > + * Pointer:  |----------- Pointer ----------|100|
> > + *
> > + * Usage:
> > + * - NULL: Swap Entry is unused.
> > + *
> > + * - Shadow: Swap Entry is used and not cached (swapped out).
> > + *   It's reusing XA_VALUE format to be compatible with workingset
> > + *   shadows. SHADOW_VAL part could be all 0.
> > + *
> > + * - Folio: Swap Entry is in cache.
> > + *
> > + * - Pointer: Unused yet. Because only the last three bit of a pointer
> > + *   is usable so now `100` is reserved for potential pointer use.
> >   */
> >
> > +#define ENTRY_COUNT_BITS     BITS_PER_BYTE
> > +#define ENTRY_SHADOW_MARK    0b1UL
> > +#define ENTRY_PFN_MARK               0b10UL
> > +#define ENTRY_PFN_LOW_MASK   0b11UL
> > +#define ENTRY_PFN_SHIFT              2
> > +#define ENTRY_PFN_MASK               ((~0UL) >> ENTRY_COUNT_BITS)
> > +#define ENTRY_COUNT_MASK     (~((~0UL) >> ENTRY_COUNT_BITS))
> > +#define ENTRY_COUNT_SHIFT    (BITS_PER_LONG - BITS_PER_BYTE)
> > +#define ENTRY_COUNT_MAX              ((1 << ENTRY_COUNT_BITS) - 2)
> > +#define ENTRY_COUNT_BAD              ((1 << ENTRY_COUNT_BITS) - 1) /* ENTRY_BAD */
> > +#define ENTRY_BAD            (~0UL)
> > +
> > +/* For shadow offset calculation */
> > +#define SWAP_COUNT_SHIFT     ENTRY_COUNT_BITS
> > +
> >  /*
> >   * Helpers for casting one type of info into a swap table entry.
> >   */
> > @@ -19,17 +51,27 @@ static inline swp_te_t null_swp_te(void)
> >
> >  static inline swp_te_t folio_swp_te(struct folio *folio)
> >  {
> > -     BUILD_BUG_ON(sizeof(swp_te_t) != sizeof(void *));
> > -     swp_te_t swp_te = { .counter = (unsigned long)folio };
>
> > +     BUILD_BUG_ON((MAX_POSSIBLE_PHYSMEM_BITS - PAGE_SHIFT) >
> > +                  (BITS_PER_LONG - ENTRY_PFN_SHIFT - ENTRY_COUNT_BITS));
>
> MAX_POSSIBLE_PHYSMEM_BITS does not seem to be available on all
> arches/configs.
>
> E.g. zsmalloc seems to set it to MAX_PHYSMEM_BITS when this is the case
> but I don't know if that applies here.
>

Thanks, I think I'll just copy the snip from zsmalloc, it's basically
doing the same check to ensure there are still enough bits left after
embedding a PFN in a LONG.