Re: [RFC PATCH v2 18/20] powerpc/64s: Use contiguous PMD/PUD instead of HUGEPD

Christophe Leroy
2024-05-22T09:32:20Z

Le 22/05/2024 à 03:13, Nicholas Piggin a écrit :
> On Tue May 21, 2024 at 2:43 AM AEST, Christophe Leroy wrote:
>>
>>
>> Le 20/05/2024 à 14:54, Nicholas Piggin a écrit :
>>> On Sat May 18, 2024 at 5:00 AM AEST, Christophe Leroy wrote:
>>>> On book3s/64, the only user of hugepd is hash in 4k mode.
>>>>
>>>> All other setups (hash-64, radix-4, radix-64) use leaf PMD/PUD.
>>>>
>>>> Rework hash-4k to use contiguous PMD and PUD instead.
>>>>
>>>> In that setup there are only two huge page sizes: 16M and 16G.
>>>>
>>>> 16M sits at PMD level and 16G at PUD level.
>>>>
>>>> pte_update doesn't know page size, lets use the same trick as
>>>> hpte_need_flush() to get page size from segment properties. That's
>>>> not the most efficient way but let's do that until callers of
>>>> pte_update() provide page size instead of just a huge flag.
>>>>
>>>> Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
>>>> ---
>>>> arch/powerpc/include/asm/book3s/64/hash-4k.h | 15 --------
>>>> arch/powerpc/include/asm/book3s/64/hash.h | 38 +++++++++++++++----
>>>> arch/powerpc/include/asm/book3s/64/hugetlb.h | 38 -------------------
>>>> .../include/asm/book3s/64/pgtable-4k.h | 34 -----------------
>>>> .../include/asm/book3s/64/pgtable-64k.h | 20 ----------
>>>> arch/powerpc/include/asm/hugetlb.h | 4 ++
>>>> .../include/asm/nohash/32/hugetlb-8xx.h | 4 --
>>>> .../powerpc/include/asm/nohash/hugetlb-e500.h | 4 --
>>>> arch/powerpc/include/asm/page.h | 8 ----
>>>> arch/powerpc/mm/book3s64/hash_utils.c | 11 ++++--
>>>> arch/powerpc/mm/book3s64/pgtable.c | 12 ------
>>>> arch/powerpc/mm/hugetlbpage.c | 19 ----------
>>>> arch/powerpc/mm/pgtable.c | 2 +-
>>>> arch/powerpc/platforms/Kconfig.cputype | 1 -
>>>> 14 files changed, 43 insertions(+), 167 deletions(-)
>>>>
>>>> diff --git a/arch/powerpc/include/asm/book3s/64/hash-4k.h b/arch/powerpc/include/asm/book3s/64/hash-4k.h
>>>> index 6472b08fa1b0..c654c376ef8b 100644
>>>> --- a/arch/powerpc/include/asm/book3s/64/hash-4k.h
>>>> +++ b/arch/powerpc/include/asm/book3s/64/hash-4k.h
>>>> @@ -74,21 +74,6 @@
>>>> #define remap_4k_pfn(vma, addr, pfn, prot) \
>>>> remap_pfn_range((vma), (addr), (pfn), PAGE_SIZE, (prot))
>>>>
>>>> -#ifdef CONFIG_HUGETLB_PAGE
>>>> -static inline int hash__hugepd_ok(hugepd_t hpd)
>>>> -{
>>>> - unsigned long hpdval = hpd_val(hpd);
>>>> - /*
>>>> - * if it is not a pte and have hugepd shift mask
>>>> - * set, then it is a hugepd directory pointer
>>>> - */
>>>> - if (!(hpdval & _PAGE_PTE) && (hpdval & _PAGE_PRESENT) &&
>>>> - ((hpdval & HUGEPD_SHIFT_MASK) != 0))
>>>> - return true;
>>>> - return false;
>>>> -}
>>>> -#endif
>>>> -
>>>> /*
>>>> * 4K PTE format is different from 64K PTE format. Saving the hash_slot is just
>>>> * a matter of returning the PTE bits that need to be modified. On 64K PTE,
>>>> diff --git a/arch/powerpc/include/asm/book3s/64/hash.h b/arch/powerpc/include/asm/book3s/64/hash.h
>>>> index faf3e3b4e4b2..509811ca7695 100644
>>>> --- a/arch/powerpc/include/asm/book3s/64/hash.h
>>>> +++ b/arch/powerpc/include/asm/book3s/64/hash.h
>>>> @@ -4,6 +4,7 @@
>>>> #ifdef __KERNEL__
>>>>
>>>> #include <asm/asm-const.h>
>>>> +#include <asm/book3s/64/slice.h>
>>>>
>>>> /*
>>>> * Common bits between 4K and 64K pages in a linux-style PTE.
>>>> @@ -161,14 +162,10 @@ extern void hpte_need_flush(struct mm_struct *mm, unsigned long addr,
>>>> pte_t *ptep, unsigned long pte, int huge);
>>>> unsigned long htab_convert_pte_flags(unsigned long pteflags, unsigned long flags);
>>>> /* Atomic PTE updates */
>>>> -static inline unsigned long hash__pte_update(struct mm_struct *mm,
>>>> - unsigned long addr,
>>>> - pte_t *ptep, unsigned long clr,
>>>> - unsigned long set,
>>>> - int huge)
>>>> +static inline unsigned long hash__pte_update_one(pte_t *ptep, unsigned long clr,
>>>> + unsigned long set)
>>>> {
>>>> __be64 old_be, tmp_be;
>>>> - unsigned long old;
>>>>
>>>> __asm__ __volatile__(
>>>> "1: ldarx %0,0,%3 # pte_update\n\
>>>> @@ -182,11 +179,38 @@ static inline unsigned long hash__pte_update(struct mm_struct *mm,
>>>> : "r" (ptep), "r" (cpu_to_be64(clr)), "m" (*ptep),
>>>> "r" (cpu_to_be64(H_PAGE_BUSY)), "r" (cpu_to_be64(set))
>>>> : "cc" );
>>>> +
>>>> + return be64_to_cpu(old_be);
>>>> +}
>>>> +
>>>> +static inline unsigned long hash__pte_update(struct mm_struct *mm,
>>>> + unsigned long addr,
>>>> + pte_t *ptep, unsigned long clr,
>>>> + unsigned long set,
>>>> + int huge)
>>>> +{
>>>> + unsigned long old;
>>>> +
>>>> + old = hash__pte_update_one(ptep, clr, set);
>>>> +
>>>> + if (huge && IS_ENABLED(CONFIG_PPC_4K_PAGES)) {
>>>> + unsigned int psize = get_slice_psize(mm, addr);
>>>> + int nb, i;
>>>> +
>>>> + if (psize == MMU_PAGE_16M)
>>>> + nb = SZ_16M / PMD_SIZE;
>>>> + else if (psize == MMU_PAGE_16G)
>>>> + nb = SZ_16G / PUD_SIZE;
>>>> + else
>>>> + nb = 1;
>>>> +
>>>> + for (i = 1; i < nb; i++)
>>>> + hash__pte_update_one(ptep + i, clr, set);
>>>> + }
>>>> /* huge pages use the old page table lock */
>>>> if (!huge)
>>>> assert_pte_locked(mm, addr);
>>>>
>>>> - old = be64_to_cpu(old_be);
>>>> if (old & H_PAGE_HASHPTE)
>>>> hpte_need_flush(mm, addr, ptep, old, huge);
>>>>
>>>
>>> Nice series, I don't know this hugepd code very well but I'll try.
>>> Why do you have to replicate the PTE entry here? The hash table refill
>>> should always be working on the first PTE of the page otherwise we have
>>> bigger problems.
>>
>> I don't know how book3s/64 works exactly, but on nohash, when you get a
>> TLB miss exception the only thing you have is the address and you don't
>> know yes it is a hugepage so you get the PTE as if it was a 4k page and
>> it is only when you read that PTE that you know it is a hugepage.
>>
>> Ok, on book3s/64 the page size seems to be encoded inside the segment so
>> maybe it is a bit different but anyway the TLB miss exception (or DSI ?)
>> can happen at any address.
>
> Right.
>
> If you think of the hash page table as a software loaded TLB (which
> is how Linux kind of thinks of it), then DSI is a TLB miss. hash_page_x
> calls find the Linux pte and load that translation into hash page table.
>
> One of the hard parts is keeping them coherent with low overhead. This
> requires pte bits H_PAGE_BUSY as a lock and H_PAGE_HASHPTE which means
> it might be in the hash table. So Linux PTE and hash PTE have to be
> 1:1 in general.
>
> There are probably cases where we could get away from 1:1, but I would
> much prefer not to. Maybe read-only access would be okay though. But
> the hash_page will have to always operate on the 0th pte, which I think
> we get via segment size masking, same for any set / update / clear of
> the pte.
>
>>>
>>> What paths look at the N > 0 PTEs of a contiguous page entry?
>>>
>>
>> pte_offset_kernel() or pte_offset_map_lock() will land on any contiguous
>> PTE based on the address handed to pte_index(), as if it was a standard
>> (4k or 64k) page.
>>
>> pte_index() doesn't know it is a hugepage, that's the reason why we need
>> to duplicate the entry.
>
> From the mm/ side of things, hugetlb page tables are always walked via
> the huge vma which knows the page size and could align address... I
> guess except for fast gup? Which should be read-only. So okay you do
> need to replicate huge ptes for fast gup at least. Any others?
>
> There's going to need to be a little more to it. __hash_page_huge sets
> PTE accessed and dirty for example, so if we allow any PTE readers to
> check the non-0th pte we would have to do something about that.
>
> How do you deal with dirty/accessed bits for other subarchs?

All nohash bail out of TLB miss handler when accessing a page which
doesn have the ACCESSED bit or writing a page which doesn't have DIRTY
bit, see commit 2c74e2586bb9 ("powerpc/40x: Rework 40x PTE access and
TLB miss") and other commits it refers to.

Same for the 603 which is the nohash version of book3s/32, see commits
f8b58c64eaef ("powerpc/603: let's handle PAGE_DIRTY directly") and
84de6ab0e904 ("powerpc/603: don't handle PAGE_ACCESSED in TLB miss
handlers.").

Only the hash version of book3s/32 still updated PTE in miss handler,
see
https://elixir.bootlin.com/linux/v6.9/source/arch/powerpc/mm/book3s32/hash_low.S#L146
but there are no hugepages on book3s/32

>
> We could just remove the hash_page setting of those bits and just cause
> a fault and require Linux mm to set them. At least for hugepages we
> could do that probably without any real performance worry.
>
> Thanks,
> Nick