linux-kernel - Re: [PATCH 1/4] mm: ksm: have KSM VMA checks not require a VMA pointer

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Message-ID: <c5feb16f-d524-42ce-bdf0-f09a9ca4ccda@linux.dev>
Date: Tue, 20 May 2025 11:14:25 +0800
From: Chengming Zhou <chengming.zhou@...ux.dev>
To: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>,
 Andrew Morton <akpm@...ux-foundation.org>
Cc: Alexander Viro <viro@...iv.linux.org.uk>,
 Christian Brauner <brauner@...nel.org>, Jan Kara <jack@...e.cz>,
 "Liam R . Howlett" <Liam.Howlett@...cle.com>,
 Vlastimil Babka <vbabka@...e.cz>, Jann Horn <jannh@...gle.com>,
 Pedro Falcato <pfalcato@...e.de>, David Hildenbrand <david@...hat.com>,
 Xu Xin <xu.xin16@....com.cn>, linux-mm@...ck.org,
 linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org
Subject: Re: [PATCH 1/4] mm: ksm: have KSM VMA checks not require a VMA
 pointer

On 2025/5/19 16:51, Lorenzo Stoakes wrote:
> In subsequent commits we are going to determine KSM eligibility prior to a
> VMA being constructed, at which point we will of course not yet have access
> to a VMA pointer.
> 
> It is trivial to boil down the check logic to be parameterised on
> mm_struct, file and VMA flags, so do so.
> 
> As a part of this change, additionally expose and use file_is_dax() to
> determine whether a file is being mapped under a DAX inode.
> 
> Signed-off-by: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>

Looks good to me.

Reviewed-by: Chengming Zhou <chengming.zhou@...ux.dev>

Thanks!

> ---
>   include/linux/fs.h |  7 ++++++-
>   mm/ksm.c           | 32 ++++++++++++++++++++------------
>   2 files changed, 26 insertions(+), 13 deletions(-)
> 
> diff --git a/include/linux/fs.h b/include/linux/fs.h
> index 09c8495dacdb..e1397e2b55ea 100644
> --- a/include/linux/fs.h
> +++ b/include/linux/fs.h
> @@ -3691,9 +3691,14 @@ void setattr_copy(struct mnt_idmap *, struct inode *inode,
>   
>   extern int file_update_time(struct file *file);
>   
> +static inline bool file_is_dax(const struct file *file)
> +{
> +	return file && IS_DAX(file->f_mapping->host);
> +}
> +
>   static inline bool vma_is_dax(const struct vm_area_struct *vma)
>   {
> -	return vma->vm_file && IS_DAX(vma->vm_file->f_mapping->host);
> +	return file_is_dax(vma->vm_file);
>   }
>   
>   static inline bool vma_is_fsdax(struct vm_area_struct *vma)
> diff --git a/mm/ksm.c b/mm/ksm.c
> index 8583fb91ef13..08d486f188ff 100644
> --- a/mm/ksm.c
> +++ b/mm/ksm.c
> @@ -677,28 +677,33 @@ static int break_ksm(struct vm_area_struct *vma, unsigned long addr, bool lock_v
>   	return (ret & VM_FAULT_OOM) ? -ENOMEM : 0;
>   }
>   
> -static bool vma_ksm_compatible(struct vm_area_struct *vma)
> +static bool ksm_compatible(const struct file *file, vm_flags_t vm_flags)
>   {
> -	if (vma->vm_flags & (VM_SHARED  | VM_MAYSHARE   | VM_PFNMAP  |
> -			     VM_IO      | VM_DONTEXPAND | VM_HUGETLB |
> -			     VM_MIXEDMAP| VM_DROPPABLE))
> +	if (vm_flags & (VM_SHARED   | VM_MAYSHARE   | VM_PFNMAP  |
> +			VM_IO       | VM_DONTEXPAND | VM_HUGETLB |
> +			VM_MIXEDMAP | VM_DROPPABLE))
>   		return false;		/* just ignore the advice */
>   
> -	if (vma_is_dax(vma))
> +	if (file_is_dax(file))
>   		return false;
>   
>   #ifdef VM_SAO
> -	if (vma->vm_flags & VM_SAO)
> +	if (vm_flags & VM_SAO)
>   		return false;
>   #endif
>   #ifdef VM_SPARC_ADI
> -	if (vma->vm_flags & VM_SPARC_ADI)
> +	if (vm_flags & VM_SPARC_ADI)
>   		return false;
>   #endif
>   
>   	return true;
>   }
>   
> +static bool vma_ksm_compatible(struct vm_area_struct *vma)
> +{
> +	return ksm_compatible(vma->vm_file, vma->vm_flags);
> +}
> +
>   static struct vm_area_struct *find_mergeable_vma(struct mm_struct *mm,
>   		unsigned long addr)
>   {
> @@ -2696,14 +2701,17 @@ static int ksm_scan_thread(void *nothing)
>   	return 0;
>   }
>   
> -static void __ksm_add_vma(struct vm_area_struct *vma)
> +static bool __ksm_should_add_vma(const struct file *file, vm_flags_t vm_flags)
>   {
> -	unsigned long vm_flags = vma->vm_flags;
> -
>   	if (vm_flags & VM_MERGEABLE)
> -		return;
> +		return false;
> +
> +	return ksm_compatible(file, vm_flags);
> +}
>   
> -	if (vma_ksm_compatible(vma))
> +static void __ksm_add_vma(struct vm_area_struct *vma)
> +{
> +	if (__ksm_should_add_vma(vma->vm_file, vma->vm_flags))
>   		vm_flags_set(vma, VM_MERGEABLE);
>   }
>