linux-kernel - Re: [PATCH v2 09/46] mm: add MADV

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Date:   Fri, 24 Feb 2023 15:25:14 -0800
From:   Mike Kravetz <mike.kravetz@...cle.com>
To:     James Houghton <jthoughton@...gle.com>
Cc:     Muchun Song <songmuchun@...edance.com>,
        Peter Xu <peterx@...hat.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        David Hildenbrand <david@...hat.com>,
        David Rientjes <rientjes@...gle.com>,
        Axel Rasmussen <axelrasmussen@...gle.com>,
        Mina Almasry <almasrymina@...gle.com>,
        Zach O'Keefe <zokeefe@...gle.com>,
        Manish Mishra <manish.mishra@...anix.com>,
        Naoya Horiguchi <naoya.horiguchi@....com>,
        "Dr . David Alan Gilbert" <dgilbert@...hat.com>,
        "Matthew Wilcox (Oracle)" <willy@...radead.org>,
        Vlastimil Babka <vbabka@...e.cz>,
        Baolin Wang <baolin.wang@...ux.alibaba.com>,
        Miaohe Lin <linmiaohe@...wei.com>,
        Yang Shi <shy828301@...il.com>,
        Frank van der Linden <fvdl@...gle.com>,
        Jiaqi Yan <jiaqiyan@...gle.com>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 09/46] mm: add MADV_SPLIT to enable HugeTLB HGM

On 02/18/23 00:27, James Houghton wrote:
> Issuing ioctl(MADV_SPLIT) on a HugeTLB address range will enable
> HugeTLB HGM. MADV_SPLIT was chosen for the name so that this API can be
> applied to non-HugeTLB memory in the future, if such an application is
> to arise.
> 
> MADV_SPLIT provides several API changes for some syscalls on HugeTLB
> address ranges:
> 1. UFFDIO_CONTINUE is allowed for MAP_SHARED VMAs at PAGE_SIZE
>    alignment.
> 2. read()ing a page fault event from a userfaultfd will yield a
>    PAGE_SIZE-rounded address, instead of a huge-page-size-rounded
>    address (unless UFFD_FEATURE_EXACT_ADDRESS is used).
> 
> There is no way to disable the API changes that come with issuing
> MADV_SPLIT. MADV_COLLAPSE can be used to collapse high-granularity page
> table mappings that come from the extended functionality that comes with
> using MADV_SPLIT.
> 
> For post-copy live migration, the expected use-case is:
> 1. mmap(MAP_SHARED, some_fd) primary mapping
> 2. mmap(MAP_SHARED, some_fd) alias mapping
> 3. MADV_SPLIT the primary mapping
> 4. UFFDIO_REGISTER/etc. the primary mapping
> 5. Copy memory contents into alias mapping and UFFDIO_CONTINUE the
>    corresponding PAGE_SIZE sections in the primary mapping.
> 
> More API changes may be added in the future.
> 
> Signed-off-by: James Houghton <jthoughton@...gle.com>
> 
> diff --git a/arch/alpha/include/uapi/asm/mman.h b/arch/alpha/include/uapi/asm/mman.h
> index 763929e814e9..7a26f3648b90 100644
> --- a/arch/alpha/include/uapi/asm/mman.h
> +++ b/arch/alpha/include/uapi/asm/mman.h
> @@ -78,6 +78,8 @@
>  
>  #define MADV_COLLAPSE	25		/* Synchronous hugepage collapse */
>  
> +#define MADV_SPLIT	26		/* Enable hugepage high-granularity APIs */
> +
>  /* compatibility flags */
>  #define MAP_FILE	0
>  
> diff --git a/arch/mips/include/uapi/asm/mman.h b/arch/mips/include/uapi/asm/mman.h
> index c6e1fc77c996..f8a74a3a0928 100644
> --- a/arch/mips/include/uapi/asm/mman.h
> +++ b/arch/mips/include/uapi/asm/mman.h
> @@ -105,6 +105,8 @@
>  
>  #define MADV_COLLAPSE	25		/* Synchronous hugepage collapse */
>  
> +#define MADV_SPLIT	26		/* Enable hugepage high-granularity APIs */
> +
>  /* compatibility flags */
>  #define MAP_FILE	0
>  
> diff --git a/arch/parisc/include/uapi/asm/mman.h b/arch/parisc/include/uapi/asm/mman.h
> index 68c44f99bc93..a6dc6a56c941 100644
> --- a/arch/parisc/include/uapi/asm/mman.h
> +++ b/arch/parisc/include/uapi/asm/mman.h
> @@ -72,6 +72,8 @@
>  
>  #define MADV_COLLAPSE	25		/* Synchronous hugepage collapse */
>  
> +#define MADV_SPLIT	74		/* Enable hugepage high-granularity APIs */
> +
>  #define MADV_HWPOISON     100		/* poison a page for testing */
>  #define MADV_SOFT_OFFLINE 101		/* soft offline page for testing */
>  
> diff --git a/arch/xtensa/include/uapi/asm/mman.h b/arch/xtensa/include/uapi/asm/mman.h
> index 1ff0c858544f..f98a77c430a9 100644
> --- a/arch/xtensa/include/uapi/asm/mman.h
> +++ b/arch/xtensa/include/uapi/asm/mman.h
> @@ -113,6 +113,8 @@
>  
>  #define MADV_COLLAPSE	25		/* Synchronous hugepage collapse */
>  
> +#define MADV_SPLIT	26		/* Enable hugepage high-granularity APIs */
> +
>  /* compatibility flags */
>  #define MAP_FILE	0
>  
> diff --git a/include/uapi/asm-generic/mman-common.h b/include/uapi/asm-generic/mman-common.h
> index 6ce1f1ceb432..996e8ded092f 100644
> --- a/include/uapi/asm-generic/mman-common.h
> +++ b/include/uapi/asm-generic/mman-common.h
> @@ -79,6 +79,8 @@
>  
>  #define MADV_COLLAPSE	25		/* Synchronous hugepage collapse */
>  
> +#define MADV_SPLIT	26		/* Enable hugepage high-granularity APIs */
> +
>  /* compatibility flags */
>  #define MAP_FILE	0
>  
> diff --git a/mm/madvise.c b/mm/madvise.c
> index c2202f51e9dd..8c004c678262 100644
> --- a/mm/madvise.c
> +++ b/mm/madvise.c
> @@ -1006,6 +1006,28 @@ static long madvise_remove(struct vm_area_struct *vma,
>  	return error;
>  }
>  
> +static int madvise_split(struct vm_area_struct *vma,
> +			 unsigned long *new_flags)
> +{
> +#ifdef CONFIG_HUGETLB_HIGH_GRANULARITY_MAPPING
> +	if (!is_vm_hugetlb_page(vma) || !hugetlb_hgm_eligible(vma))
> +		return -EINVAL;
> +
> +	/*
> +	 * PMD sharing doesn't work with HGM. If this MADV_SPLIT is on part
> +	 * of a VMA, then we will split the VMA. Here, we're unsharing before
> +	 * splitting because it's simpler, although we may be unsharing more
> +	 * than we need.
> +	 */
> +	hugetlb_unshare_all_pmds(vma);

I think we should just unshare the (appropriately aligned) range within the
vma that is the target of MADV_SPLIT.  No need to unshare the entire vma.

> +
> +	*new_flags |= VM_HUGETLB_HGM;
> +	return 0;
> +#else
> +	return -EINVAL;
> +#endif
> +}
> +
>  /*
>   * Apply an madvise behavior to a region of a vma.  madvise_update_vma
>   * will handle splitting a vm area into separate areas, each area with its own
> @@ -1084,6 +1106,11 @@ static int madvise_vma_behavior(struct vm_area_struct *vma,
>  		break;
>  	case MADV_COLLAPSE:
>  		return madvise_collapse(vma, prev, start, end);
> +	case MADV_SPLIT:
> +		error = madvise_split(vma, &new_flags);
> +		if (error)
> +			goto out;

Not a huge deal, but if one passes an invalid range (such as not huge page
size aligned) to MADV_SPLIT, then we will not notice the error until
later in madvise_update_vma() when the vma split fails.  By then, we will
have unshared all pmds in the entire vma (or just the range if you agree
with my suggestion above).

-- 
Mike Kravetz

> +		break;
>  	}
>  
>  	anon_name = anon_vma_name(vma);
> @@ -1178,6 +1205,9 @@ madvise_behavior_valid(int behavior)
>  	case MADV_HUGEPAGE:
>  	case MADV_NOHUGEPAGE:
>  	case MADV_COLLAPSE:
> +#endif
> +#ifdef CONFIG_HUGETLB_HIGH_GRANULARITY_MAPPING
> +	case MADV_SPLIT:
>  #endif
>  	case MADV_DONTDUMP:
>  	case MADV_DODUMP:
> @@ -1368,6 +1398,8 @@ int madvise_set_anon_name(struct mm_struct *mm, unsigned long start,
>   *		transparent huge pages so the existing pages will not be
>   *		coalesced into THP and new pages will not be allocated as THP.
>   *  MADV_COLLAPSE - synchronously coalesce pages into new THP.
> + *  MADV_SPLIT - allow HugeTLB pages to be mapped at PAGE_SIZE. This allows
> + *		UFFDIO_CONTINUE to accept PAGE_SIZE-aligned regions.
>   *  MADV_DONTDUMP - the application wants to prevent pages in the given range
>   *		from being included in its core dump.
>   *  MADV_DODUMP - cancel MADV_DONTDUMP: no longer exclude from core dump.
> -- 
> 2.39.2.637.g21b0678d19-goog
>