linux-kernel - Re: [PATCH v2 04/10] mm: remove duplicated open-coded VMA policy check

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Message-ID: <6dts7wb7wycm53st22aupcj6ilvl4gmsheq2hyhfcjkmbfysb6@wz7p3dretp4u>
Date: Wed, 28 Aug 2024 15:42:18 -0400
From: "Liam R. Howlett" <Liam.Howlett@...cle.com>
To: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
Cc: linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        Andrew Morton <akpm@...ux-foundation.org>,
        Vlastimil Babka <vbabka@...e.cz>
Subject: Re: [PATCH v2 04/10] mm: remove duplicated open-coded VMA policy
 check

* Lorenzo Stoakes <lorenzo.stoakes@...cle.com> [240823 16:07]:
> Both can_vma_merge_before() and can_vma_merge_after() are invoked after
> checking for compatible VMA NUMA policy, we can simply move this to
> is_mergeable_vma() and abstract this altogether.
> 
> In mmap_region() we set vmg->policy to NULL, so the policy comparisons
> checked in can_vma_merge_before() and can_vma_merge_after() are exactly
> equivalent to !vma_policy(vmg.next) and !vma_policy(vmg.prev).
> 
> Equally, in do_brk_flags(), vmg->policy is NULL, so the
> can_vma_merge_after() is checking !vma_policy(vma), as we set vmg.prev to
> vma.
> 
> In vma_merge(), we compare prev and next policies with vmg->policy before
> checking can_vma_merge_after() and can_vma_merge_before() respectively,
> which this patch causes to be checked in precisely the same way.
> 
> This therefore maintains precisely the same logic as before, only now
> abstracted into is_mergeable_vma().
> 
> Signed-off-by: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
> Acked-by: Vlastimil Babka <vbabka@...e.cz>

Reviewed-by: Liam R. Howlett <Liam.Howlett@...cle.com>

> ---
>  mm/mmap.c | 8 +++-----
>  mm/vma.c  | 9 ++++-----
>  2 files changed, 7 insertions(+), 10 deletions(-)
> 
> diff --git a/mm/mmap.c b/mm/mmap.c
> index 4066c0444495..c72f50feb98a 100644
> --- a/mm/mmap.c
> +++ b/mm/mmap.c
> @@ -1423,8 +1423,7 @@ unsigned long mmap_region(struct file *file, unsigned long addr,
>  
>  	/* Attempt to expand an old mapping */
>  	/* Check next */
> -	if (next && next->vm_start == end && !vma_policy(next) &&
> -	    can_vma_merge_before(&vmg)) {
> +	if (next && next->vm_start == end && can_vma_merge_before(&vmg)) {

It occurs to me that we could use the vma iterators index/last to check
the limits prior to needing to move at all.. but I'll leave that for
future work.

>  		merge_end = next->vm_end;
>  		vma = next;
>  		vmg.pgoff = next->vm_pgoff - pglen;
> @@ -1438,8 +1437,7 @@ unsigned long mmap_region(struct file *file, unsigned long addr,
>  	}
>  
>  	/* Check prev */
> -	if (prev && prev->vm_end == addr && !vma_policy(prev) &&
> -	    can_vma_merge_after(&vmg)) {
> +	if (prev && prev->vm_end == addr && can_vma_merge_after(&vmg)) {
>  		merge_start = prev->vm_start;
>  		vma = prev;
>  		vmg.pgoff = prev->vm_pgoff;
> @@ -1778,7 +1776,7 @@ static int do_brk_flags(struct vma_iterator *vmi, struct vm_area_struct *vma,
>  	 * Expand the existing vma if possible; Note that singular lists do not
>  	 * occur after forking, so the expand will only happen on new VMAs.
>  	 */
> -	if (vma && vma->vm_end == addr && !vma_policy(vma)) {
> +	if (vma && vma->vm_end == addr) {
>  		VMG_STATE(vmg, mm, vmi, addr, addr + len, flags, PHYS_PFN(addr));
>  
>  		vmg.prev = vma;
> diff --git a/mm/vma.c b/mm/vma.c
> index 74c627ff0313..b1ec412fac7f 100644
> --- a/mm/vma.c
> +++ b/mm/vma.c
> @@ -19,6 +19,8 @@ static inline bool is_mergeable_vma(struct vma_merge_struct *vmg, bool merge_nex
>  	 */
>  	bool may_remove_vma = merge_next;
>  
> +	if (!mpol_equal(vmg->policy, vma_policy(vma)))
> +		return false;

Should we ensure there is any vma to check here?  Maybe just an assert?

>  	/*
>  	 * VM_SOFTDIRTY should not prevent from VMA merging, if we
>  	 * match the flags but dirty bit -- the caller should mark
> @@ -1058,17 +1060,14 @@ static struct vm_area_struct *vma_merge(struct vma_merge_struct *vmg)
>  		vma_pgoff = prev->vm_pgoff;
>  
>  		/* Can we merge the predecessor? */
> -		if (addr == prev->vm_end && mpol_equal(vma_policy(prev), vmg->policy)
> -		    && can_vma_merge_after(vmg)) {
> -
> +		if (addr == prev->vm_end && can_vma_merge_after(vmg)) {
>  			merge_prev = true;
>  			vma_prev(vmg->vmi);
>  		}
>  	}
>  
>  	/* Can we merge the successor? */
> -	if (next && mpol_equal(vmg->policy, vma_policy(next)) &&
> -	    can_vma_merge_before(vmg)) {
> +	if (next && can_vma_merge_before(vmg)) {
>  		merge_next = true;
>  	}
>  
> -- 
> 2.46.0
>