linux-kernel - Re: [PATCH 2/2] mm/dax: Don't enable huge dax mapping by default

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Date:   Thu, 28 Feb 2019 10:40:11 +0100
From:   Jan Kara <jack@...e.cz>
To:     "Aneesh Kumar K.V" <aneesh.kumar@...ux.ibm.com>
Cc:     akpm@...ux-foundation.org,
        "Kirill A . Shutemov" <kirill.shutemov@...ux.intel.com>,
        Jan Kara <jack@...e.cz>, mpe@...erman.id.au,
        Ross Zwisler <zwisler@...nel.org>,
        Oliver O'Halloran <oohall@...il.com>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org,
        Dan Williams <dan.j.williams@...el.com>
Subject: Re: [PATCH 2/2] mm/dax: Don't enable huge dax mapping by default

On Thu 28-02-19 14:05:22, Aneesh Kumar K.V wrote:
> Add a flag to indicate the ability to do huge page dax mapping. On architecture
> like ppc64, the hypervisor can disable huge page support in the guest. In
> such a case, we should not enable huge page dax mapping. This patch adds
> a flag which the architecture code will update to indicate huge page
> dax mapping support.
> 
> Architectures mostly do transparent_hugepage_flag = 0; if they can't
> do hugepages. That also takes care of disabling dax hugepage mapping
> with this change.
> 
> Without this patch we get the below error with kvm on ppc64.
> 
> [  118.849975] lpar: Failed hash pte insert with error -4
> 
> NOTE: The patch also use
> 
> echo never > /sys/kernel/mm/transparent_hugepage/enabled
> to disable dax huge page mapping.
> 
> Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@...ux.ibm.com>

Added Dan to CC for opinion. I kind of fail to see why you don't use
TRANSPARENT_HUGEPAGE_FLAG for this. I know that technically DAX huge pages
and normal THPs are different things but so far we've tried to avoid making
that distinction visible to userspace.

								Honza
> ---
> TODO:
> * Add Fixes: tag
> 
>  include/linux/huge_mm.h | 4 +++-
>  mm/huge_memory.c        | 4 ++++
>  2 files changed, 7 insertions(+), 1 deletion(-)
> 
> diff --git a/include/linux/huge_mm.h b/include/linux/huge_mm.h
> index 381e872bfde0..01ad5258545e 100644
> --- a/include/linux/huge_mm.h
> +++ b/include/linux/huge_mm.h
> @@ -53,6 +53,7 @@ vm_fault_t vmf_insert_pfn_pud(struct vm_area_struct *vma, unsigned long addr,
>  			pud_t *pud, pfn_t pfn, bool write);
>  enum transparent_hugepage_flag {
>  	TRANSPARENT_HUGEPAGE_FLAG,
> +	TRANSPARENT_HUGEPAGE_DAX_FLAG,
>  	TRANSPARENT_HUGEPAGE_REQ_MADV_FLAG,
>  	TRANSPARENT_HUGEPAGE_DEFRAG_DIRECT_FLAG,
>  	TRANSPARENT_HUGEPAGE_DEFRAG_KSWAPD_FLAG,
> @@ -111,7 +112,8 @@ static inline bool __transparent_hugepage_enabled(struct vm_area_struct *vma)
>  	if (transparent_hugepage_flags & (1 << TRANSPARENT_HUGEPAGE_FLAG))
>  		return true;
>  
> -	if (vma_is_dax(vma))
> +	if (vma_is_dax(vma) &&
> +	    (transparent_hugepage_flags & (1 << TRANSPARENT_HUGEPAGE_DAX_FLAG)))
>  		return true;
>  
>  	if (transparent_hugepage_flags &
> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> index faf357eaf0ce..43d742fe0341 100644
> --- a/mm/huge_memory.c
> +++ b/mm/huge_memory.c
> @@ -53,6 +53,7 @@ unsigned long transparent_hugepage_flags __read_mostly =
>  #ifdef CONFIG_TRANSPARENT_HUGEPAGE_MADVISE
>  	(1<<TRANSPARENT_HUGEPAGE_REQ_MADV_FLAG)|
>  #endif
> +	(1 << TRANSPARENT_HUGEPAGE_DAX_FLAG) |
>  	(1<<TRANSPARENT_HUGEPAGE_DEFRAG_REQ_MADV_FLAG)|
>  	(1<<TRANSPARENT_HUGEPAGE_DEFRAG_KHUGEPAGED_FLAG)|
>  	(1<<TRANSPARENT_HUGEPAGE_USE_ZERO_PAGE_FLAG);
> @@ -475,6 +476,8 @@ static int __init setup_transparent_hugepage(char *str)
>  			  &transparent_hugepage_flags);
>  		clear_bit(TRANSPARENT_HUGEPAGE_REQ_MADV_FLAG,
>  			  &transparent_hugepage_flags);
> +		clear_bit(TRANSPARENT_HUGEPAGE_DAX_FLAG,
> +			  &transparent_hugepage_flags);
>  		ret = 1;
>  	}
>  out:
> @@ -753,6 +756,7 @@ static void insert_pfn_pmd(struct vm_area_struct *vma, unsigned long addr,
>  	spinlock_t *ptl;
>  
>  	ptl = pmd_lock(mm, pmd);
> +	/* should we check for none here again? */
>  	entry = pmd_mkhuge(pfn_t_pmd(pfn, prot));
>  	if (pfn_t_devmap(pfn))
>  		entry = pmd_mkdevmap(entry);
> -- 
> 2.20.1
> 
-- 
Jan Kara <jack@...e.com>
SUSE Labs, CR