From: Peter Xu <peterx@redhat.com>
To: "Huang, Ying" <ying.huang@intel.com>
Cc: Nadav Amit <nadav.amit@gmail.com>, Linux MM <linux-mm@kvack.org>,
linux-kernel@vger.kernel.org,
Andrea Arcangeli <aarcange@redhat.com>,
David Hildenbrand <david@redhat.com>,
Minchan Kim <minchan@kernel.org>,
Andrew Morton <akpm@linux-foundation.org>,
Vlastimil Babka <vbabka@suse.cz>, Hugh Dickins <hughd@google.com>,
Andi Kleen <andi.kleen@intel.com>,
"Kirill A . Shutemov" <kirill@shutemov.name>
Subject: Re: [PATCH v2 2/2] mm: Remember young/dirty bit for page migrations
Date: Tue, 9 Aug 2022 10:59:23 -0400 [thread overview]
Message-ID: <YvJ2S8X4tAqM4oZ4@xz-m1.local> (raw)
In-Reply-To: <8735e5hkk3.fsf@yhuang6-desk2.ccr.corp.intel.com>
On Tue, Aug 09, 2022 at 04:45:32PM +0800, Huang, Ying wrote:
> Peter Xu <peterx@redhat.com> writes:
>
> > 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:
> >> > +
> >> > 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).
>
> I think it's possible to memorize max_swapfile_size() or
> migration_entry_supports_ad(). Although they are not constant, they are
> not changed after initialized. The challenge is to find a clean way to
> initialize it.
I checked it up today, the conclusion is I think it's safe we initialize ad
bits for migration in swapfile_init() and I'll do that in the next version.
Longer proof material below.
Generic max_swapfile_size() is pretty much a constant except x86. x86 has
two dependency, on (1) X86_BUG_L1TF, and (2) l1tf_mitigation.
The other challenge is the reference to MAX_PHYSMEM_BITS, which is also
only complicated on x86 with 5 level page tables.
Luckily the cpu bits are all setup within early_cpu_init(). The setup of
l1tf_mitigation variable is later but still earlier than most of the init
calls (which swapfile_init belongs to level 4).
A full graph for reference:
- start_kernel
- setup_arch
- early_cpu_init
- get_cpu_cap --> fetch from CPUID (including X86_FEATURE_LA57)
- early_identify_cpu --> setup X86_BUG_L1TF
- early_identify_cpu --> clear X86_FEATURE_LA57 (if early lvl5 not enabled (USE_EARLY_PGTABLE_L5))
- parse_early_param
- l1tf_cmdline --> set l1tf_mitigation
- check_bugs
- l1tf_select_mitigation --> set l1tf_mitigation
- arch_call_rest_init
- rest_init
- kernel_init
- kernel_init_freeable
- do_basic_setup
- do_initcalls --> calls swapfile_init() (initcall level 4)
I'll add one (or maybe >1?) patch on top in next post to optimize the
fetching of these states.
Thanks,
--
Peter Xu
next prev parent reply other threads:[~2022-08-09 14:59 UTC|newest]
Thread overview: 16+ messages / expand[flat|nested] mbox.gz Atom feed top
2022-08-04 20:39 [PATCH v2 0/2] mm: Remember a/d bits for migration entries Peter Xu
2022-08-04 20:39 ` [PATCH v2 1/2] mm/swap: Add swp_offset_pfn() to fetch PFN from swap entry Peter Xu
2022-08-04 20:39 ` [PATCH v2 2/2] mm: Remember young/dirty bit for page migrations Peter Xu
2022-08-04 22:40 ` Nadav Amit
2022-08-05 16:30 ` Peter Xu
2022-08-09 8:45 ` Huang, Ying
2022-08-09 8:47 ` David Hildenbrand
2022-08-09 14:59 ` Peter Xu [this message]
2022-08-05 12:17 ` David Hildenbrand
2022-08-05 16:36 ` Peter Xu
2022-08-09 8:40 ` Huang, Ying
2022-08-09 17:59 ` Peter Xu
2022-08-10 0:53 ` Huang, Ying
2022-08-10 19:21 ` Peter Xu
2022-08-11 5:44 ` Alistair Popple
2022-08-04 22:17 ` [PATCH v2 0/2] mm: Remember a/d bits for migration entries Nadav Amit
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