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Date: Mon, 9 May 2022 09:19:00 +0800
From: Baolin Wang <baolin.wang@...ux.alibaba.com>
To: nh26223@...com, catalin.marinas@....com, will@...nel.org,
arnd@...db.de, mike.kravetz@...cle.com, akpm@...ux-foundation.org,
sj@...nel.org
Cc: linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
linux-arch@...r.kernel.org, linux-fsdevel@...r.kernel.org,
linux-mm@...ck.org
Subject: Re: [RFC PATCH 1/3] arm64/hugetlb: Introduce new
huge_ptep_get_access_flags() interface
On 5/8/2022 9:14 PM, nh26223@...com wrote:
> On 2022年5月8日星期日 CST 下午4:58:52 Baolin Wang wrote:
>> Now we use huge_ptep_get() to get the pte value of a hugetlb page,
>> however it will only return one specific pte value for the CONT-PTE
>> or CONT-PMD size hugetlb on ARM64 system, which can contain seravel
>> continuous pte or pmd entries with same page table attributes. And it
>> will not take into account the subpages' dirty or young bits of a
>> CONT-PTE/PMD size hugetlb page.
>>
>> So the huge_ptep_get() is inconsistent with huge_ptep_get_and_clear(),
>> which already takes account the dirty or young bits for any subpages
>> in this CONT-PTE/PMD size hugetlb [1]. Meanwhile we can miss dirty or
>> young flags statistics for hugetlb pages with current huge_ptep_get(),
>> such as the gather_hugetlb_stats() function.
>>
>> Thus introduce a new huge_ptep_get_access_flags() interface and define
>> an ARM64 specific implementation, that will take into account any subpages'
>> dirty or young bits for CONT-PTE/PMD size hugetlb page, for those functions
>> that want to check the dirty and young flags of a hugetlb page.
>>
>> [1]
>> https://lore.kernel.org/linux-mm/85bd80b4-b4fd-0d3f-a2e5-149559f2f387@oracl
>> e.com/
>>
>> Signed-off-by: Baolin Wang <baolin.wang@...ux.alibaba.com>
>> ---
>> arch/arm64/include/asm/hugetlb.h | 2 ++
>> arch/arm64/mm/hugetlbpage.c | 24 ++++++++++++++++++++++++
>> include/asm-generic/hugetlb.h | 7 +++++++
>> 3 files changed, 33 insertions(+)
>>
>> diff --git a/arch/arm64/include/asm/hugetlb.h
>> b/arch/arm64/include/asm/hugetlb.h index 616b2ca..a473544 100644
>> --- a/arch/arm64/include/asm/hugetlb.h
>> +++ b/arch/arm64/include/asm/hugetlb.h
>> @@ -44,6 +44,8 @@ extern pte_t huge_ptep_clear_flush(struct vm_area_struct
>> *vma, #define __HAVE_ARCH_HUGE_PTE_CLEAR
>> extern void huge_pte_clear(struct mm_struct *mm, unsigned long addr,
>> pte_t *ptep, unsigned long sz);
>> +#define __HAVE_ARCH_HUGE_PTEP_GET_ACCESS_FLAGS
>> +extern pte_t huge_ptep_get_access_flags(pte_t *ptep, unsigned long sz);
>> extern void set_huge_swap_pte_at(struct mm_struct *mm, unsigned long addr,
>> pte_t *ptep, pte_t pte, unsigned long
> sz);
>> #define set_huge_swap_pte_at set_huge_swap_pte_at
>> diff --git a/arch/arm64/mm/hugetlbpage.c b/arch/arm64/mm/hugetlbpage.c
>> index ca8e65c..ce39699 100644
>> --- a/arch/arm64/mm/hugetlbpage.c
>> +++ b/arch/arm64/mm/hugetlbpage.c
>> @@ -158,6 +158,30 @@ static inline int num_contig_ptes(unsigned long size,
>> size_t *pgsize) return contig_ptes;
>> }
>>
>> +pte_t huge_ptep_get_access_flags(pte_t *ptep, unsigned long sz)
> The function name looks to me that it returns access flags of PTE.
Yes, not a good name. That's why this is a RFC patch set to get more
suggestion :)
Maybe huge_ptep_get_with_access_flags()? or do you have some better idea?
>
>> +{
>> + int ncontig, i;
>> + size_t pgsize;
>> + pte_t orig_pte = ptep_get(ptep);
>> +
>> + if (!pte_cont(orig_pte))
>> + return orig_pte;
>> +
>> + ncontig = num_contig_ptes(sz, &pgsize);
>> +
>> + for (i = 0; i < ncontig; i++, ptep++) {
>> + pte_t pte = ptep_get(ptep);
>> +
>> + if (pte_dirty(pte))
>> + orig_pte = pte_mkdirty(orig_pte);
>> +
>> + if (pte_young(pte))
>> + orig_pte = pte_mkyoung(orig_pte);
>> + }
>> +
>> + return orig_pte;
>> +}
> Not sure whether it's worthy being changed to:
>
> bool dirty = false, young = false;
>
> for (i = 0; i < ncontig; i++, ptep++) {
> pte_t pte = ptep_get(ptep);
>
> if (pte_dirty(pte))
> dirty = true;
>
> if (pte_young(pte))
> young = true;
>
> if (dirty && young)
> break;
> }
>
> if (dirty)
> orig_pte = pte_mkdirty(orig_pte);
>
> if (young)
> orig_pte = pte_mkyoung(orit_pte);
>
> return orig_pte;
I followed the same logics in get_clear_flush(), which is more readable
I think. Yes, your approach can save some cycles, I can change to use it
in next version if arm64 maintainers have no objection.
>> +
>> /*
>> * Changing some bits of contiguous entries requires us to follow a
>> * Break-Before-Make approach, breaking the whole contiguous set
>> diff --git a/include/asm-generic/hugetlb.h b/include/asm-generic/hugetlb.h
>> index a57d667..bb77fb0 100644
>> --- a/include/asm-generic/hugetlb.h
>> +++ b/include/asm-generic/hugetlb.h
>> @@ -150,6 +150,13 @@ static inline pte_t huge_ptep_get(pte_t *ptep)
>> }
>> #endif
>>
>> +#ifndef __HAVE_ARCH_HUGE_PTEP_GET_ACCESS_FLAGS
>> +static inline pte_t huge_ptep_get_access_flags(pte_t *ptep, unsigned long
>> sz) +{
>> + return ptep_get(ptep);
> Should be:
> return huge_ptep_get(ptep) ?
I don't think so. If no ARCH-specific definition, the
huge_ptep_get_access_flags() implementation should be same as
huge_ptep_get(). Thanks for your comments.
#ifndef __HAVE_ARCH_HUGE_PTEP_GET
static inline pte_t huge_ptep_get(pte_t *ptep)
{
return ptep_get(ptep);
}
#endif
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