Re: [PATCH v2 2/2] mm: Remember young/dirty bit for page migrations

[Peter Xu <peterx@redhat.com>]

On Fri, Aug 05, 2022 at 02:17:55PM +0200, David Hildenbrand wrote:
> On 04.08.22 22:39, Peter Xu wrote:
> > When page migration happens, we always ignore the young/dirty bit settings
> > in the old pgtable, and marking the page as old in the new page table using
> > either pte_mkold() or pmd_mkold(), and keeping the pte clean.
> > 
> > That's fine from functional-wise, but that's not friendly to page reclaim
> > because the moving page can be actively accessed within the procedure.  Not
> > to mention hardware setting the young bit can bring quite some overhead on
> > some systems, e.g. x86_64 needs a few hundreds nanoseconds to set the bit.
> > The same slowdown problem to dirty bits when the memory is first written
> > after page migration happened.
> > 
> > Actually we can easily remember the A/D bit configuration and recover the
> > information after the page is migrated.  To achieve it, define a new set of
> > bits in the migration swap offset field to cache the A/D bits for old pte.
> > Then when removing/recovering the migration entry, we can recover the A/D
> > bits even if the page changed.
> > 
> > One thing to mention is that here we used max_swapfile_size() to detect how
> > many swp offset bits we have, and we'll only enable this feature if we know
> > the swp offset can be big enough to store both the PFN value and the young
> > bit.  Otherwise the A/D bits are dropped like before.
> > 
> > Signed-off-by: Peter Xu <peterx@redhat.com>
> > ---
> >  include/linux/swapops.h | 91 +++++++++++++++++++++++++++++++++++++++++
> >  mm/huge_memory.c        | 26 +++++++++++-
> >  mm/migrate.c            |  6 ++-
> >  mm/migrate_device.c     |  4 ++
> >  mm/rmap.c               |  5 ++-
> >  5 files changed, 128 insertions(+), 4 deletions(-)
> > 
> > diff --git a/include/linux/swapops.h b/include/linux/swapops.h
> > index 1d17e4bb3d2f..34aa448ac6ee 100644
> > --- a/include/linux/swapops.h
> > +++ b/include/linux/swapops.h
> > @@ -8,6 +8,8 @@
> >  
> >  #ifdef CONFIG_MMU
> >  
> > +#include <linux/swapfile.h>
> > +
> >  /*
> >   * swapcache pages are stored in the swapper_space radix tree.  We want to
> >   * get good packing density in that tree, so the index should be dense in
> > @@ -35,6 +37,24 @@
> >  #endif
> >  #define SWP_PFN_MASK			((1UL << SWP_PFN_BITS) - 1)
> >  
> > +/**
> > + * Migration swap entry specific bitfield definitions.
> > + *
> > + * @SWP_MIG_YOUNG_BIT: Whether the page used to have young bit set
> > + * @SWP_MIG_DIRTY_BIT: Whether the page used to have dirty bit set
> > + *
> > + * Note: these bits will be stored in migration entries iff there're enough
> > + * free bits in arch specific swp offset.  By default we'll ignore A/D bits
> > + * when migrating a page.  Please refer to migration_entry_supports_ad()
> > + * for more information.
> > + */
> > +#define SWP_MIG_YOUNG_BIT		(SWP_PFN_BITS)
> > +#define SWP_MIG_DIRTY_BIT		(SWP_PFN_BITS + 1)
> > +#define SWP_MIG_TOTAL_BITS		(SWP_PFN_BITS + 2)
> > +
> > +#define SWP_MIG_YOUNG			(1UL << SWP_MIG_YOUNG_BIT)
> > +#define SWP_MIG_DIRTY			(1UL << SWP_MIG_DIRTY_BIT)
> > +
> >  static inline bool is_pfn_swap_entry(swp_entry_t entry);
> >  
> >  /* Clear all flags but only keep swp_entry_t related information */
> > @@ -265,6 +285,57 @@ static inline swp_entry_t make_writable_migration_entry(pgoff_t offset)
> >  	return swp_entry(SWP_MIGRATION_WRITE, offset);
> >  }
> >  
> > +/*
> > + * Returns whether the host has large enough swap offset field to support
> > + * carrying over pgtable A/D bits for page migrations.  The result is
> > + * pretty much arch specific.
> > + */
> > +static inline bool migration_entry_supports_ad(void)
> > +{
> > +	/*
> > +	 * max_swapfile_size() returns the max supported swp-offset plus 1.
> > +	 * We can support the migration A/D bits iff the pfn swap entry has
> > +	 * the offset large enough to cover all of them (PFN, A & D bits).
> > +	 */
> > +#ifdef CONFIG_SWAP
> > +	return max_swapfile_size() >= (1UL << SWP_MIG_TOTAL_BITS);
> > +#else
> > +	return false;
> > +#endif
> > +}
> 
> 
> This looks much cleaner to me. It might be helpful to draw an ascii
> picture where exatcly these bits reside isnide the offset.

Yes that'll be helpful especially when more bits are defined.  Not sure how
much it'll help for now but I can definitely do that.

> 
> > +
> > +static inline swp_entry_t make_migration_entry_young(swp_entry_t entry)
> > +{
> > +	if (migration_entry_supports_ad())
> 
> Do we maybe want to turn that into a static key and enable it once and
> for all? As Nadav says, the repeated max_swapfile_size() calls/checks
> might be worth optimizing out.

Since there're a few arch related issues to answer (as replied to Nadav -
both max_swapfile_size and SWP_MIG_TOTAL_BITS may not be constant), my
current plan is to first attach the const attribute, then leave the other
optimizations for later.

If this is a super hot path, I probably need to do this in reversed order,
but hopefully it's fine to this case.

Thanks,

-- 
Peter Xu