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

Peter Xu
2022-08-05T16:30:15Z

On Thu, Aug 04, 2022 at 03:40:57PM -0700, Nadav Amit wrote:
> On Aug 4, 2022, at 1:39 PM, Peter Xu <peterx@redhat.com> 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>
>
> Shouldn’t the ifdef go into linux/swapfile.h if that’s the right thing to do
> to prevent others from mistakenly including it?

swapfile.h is for max_swapfile_size() that's referenced in the new code. If
!CONFIG_MMU then there shouldn't be a reference to max_swapfile_size() even
if swapops.h included, then logically it should be after CONFIG_MMU?

>
> > +
> > /*
> > * 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)
>
> Any reason not to use BIT(x) ?

Yeh why not..

>
> > +
> > 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);
>
> This is an actual a function call (unless LTO has some trick). A bit of a
> shame it cannot be at least memoized.
>
> Or at least mark max_swapfile_size() as __attribute_const__ so it would not
> be called twice for make_migration_entry_young() and
> make_migration_entry_dirty().

I didn't take too much effort on this one since we're on swap path and I
assumed that's not a super hot path. But __attribute_const__ sounds good
and easy to get, thanks.

Perhaps I should mark it on migration_entry_supports_ad() as a whole? Note
that unfortunately SWP_MIG_TOTAL_BITS may not be a const either (see how
that define roots back to MAX_PHYSMEM_BITS, where on x86_64 it needs to
check 5-lvl).

>
> > +#else
> > + return false;
> > +#endif
> > +}
> > +
> > +static inline swp_entry_t make_migration_entry_young(swp_entry_t entry)
> > +{
> > + if (migration_entry_supports_ad())
> > + return swp_entry(swp_type(entry),
> > + swp_offset(entry) | SWP_MIG_YOUNG);
> > + return entry;
> > +}
> > +
> > +static inline bool is_migration_entry_young(swp_entry_t entry)
> > +{
> > + if (migration_entry_supports_ad())
> > + return swp_offset(entry) & SWP_MIG_YOUNG;
> > + /* Keep the old behavior of aging page after migration */
> > + return false;
> > +}
> > +
> > +static inline swp_entry_t make_migration_entry_dirty(swp_entry_t entry)
> > +{
> > + if (migration_entry_supports_ad())
> > + return swp_entry(swp_type(entry),
> > + swp_offset(entry) | SWP_MIG_DIRTY);
> > + return entry;
> > +}
> > +
> > +static inline bool is_migration_entry_dirty(swp_entry_t entry)
> > +{
> > + if (migration_entry_supports_ad())
> > + return swp_offset(entry) & SWP_MIG_YOUNG_BIT;
>
> Shouldn’t it be SWP_MIG_DIRTY ?

Oops, yes.

>
> > + /* Keep the old behavior of clean page after migration */
> > + return false;
> > +}
> > +
> > extern void __migration_entry_wait(struct mm_struct *mm, pte_t *ptep,
> > spinlock_t *ptl);
> > extern void migration_entry_wait(struct mm_struct *mm, pmd_t *pmd,
> > @@ -311,6 +382,26 @@ static inline int is_readable_migration_entry(swp_entry_t entry)
> > return 0;
> > }
> >
> > +static inline swp_entry_t make_migration_entry_young(swp_entry_t entry)
> > +{
> > + return entry;
> > +}
> > +
> > +static inline bool is_migration_entry_young(swp_entry_t entry)
> > +{
> > + return false;
> > +}
> > +
> > +static inline swp_entry_t make_migration_entry_dirty(swp_entry_t entry)
> > +{
> > + return entry;
> > +}
> > +
> > +static inline bool is_migration_entry_dirty(swp_entry_t entry)
> > +{
> > + return false;
> > +}
> > +
> > #endif
>
> While at it, can you change to:
>
> #endif /* CONFIG_MIGRATION */
>
> [ these ifdefs burn my eyes ]

Ok, but there're a bunch of "ifdef"s in this header that are missing those.
Maybe I'll add a separate patch for all of them just to do the commenting.

>
> Other than that looks good.

Thanks,

--
Peter Xu