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Message-ID: <ZBhpPFmAOKyohN0M@vernon-pc>
Date: Mon, 20 Mar 2023 22:25:11 +0800
From: Vernon Yang <vernon2gm@...il.com>
To: Lorenzo Stoakes <lstoakes@...il.com>
Cc: linux-mm@...ck.org, linux-kernel@...r.kernel.org,
Andrew Morton <akpm@...ux-foundation.org>,
David Hildenbrand <david@...hat.com>,
Matthew Wilcox <willy@...radead.org>,
Vlastimil Babka <vbabka@...e.cz>,
"Liam R . Howlett" <Liam.Howlett@...cle.com>,
maple-tree@...ts.infradead.org
Subject: Re: [PATCH 2/4] mm/mmap/vma_merge: set next to NULL if not applicable
On Sat, Mar 18, 2023 at 11:13:19AM +0000, Lorenzo Stoakes wrote:
> We are only interested in next if end == next->vm_start (in which case we
> check to see if we can set merge_next), so perform this check alongside
> checking whether curr should be set.
>
> This groups all of the simple range checks together and establishes the
> invariant that, if prev, curr or next are non-NULL then their positions are
> as expected.
>
> Additionally, use the abstract 'vma' object to look up the possible curr or
> next VMA in order to avoid any confusion as to what these variables
> represent - now curr and next are assigned once and only once.
Hi Lorenzo,
Due to the "vma" variable is used as an intermediate member, I feels a bit
confusing, so cleanup this patch as below.
If this cleanup patch is issue, please let me know, and then ignore it
directly, thanks.
----
>From 7dac3ed8c1b747c2edf2a6c867c4e6342c7447c3 Mon Sep 17 00:00:00 2001
From: Vernon Yang <vernon2gm@...il.com>
Date: Mon, 20 Mar 2023 21:38:09 +0800
Subject: [PATCH] mm/mmap/vma_merge: set next to NULL if not applicable-cleanup
Make code logic simpler and more readable.
Signed-off-by: Vernon Yang <vernon2gm@...il.com>
---
mm/mmap.c | 23 ++++++-----------------
1 file changed, 6 insertions(+), 17 deletions(-)
diff --git a/mm/mmap.c b/mm/mmap.c
index 45bd43225013..78cb96774602 100644
--- a/mm/mmap.c
+++ b/mm/mmap.c
@@ -921,7 +921,7 @@ struct vm_area_struct *vma_merge(struct vma_iterator *vmi, struct mm_struct *mm,
* If there is a previous VMA specified, find the next, otherwise find
* the first.
*/
- vma = find_vma(mm, prev ? prev->vm_end : 0);
+ curr = find_vma(mm, prev ? prev->vm_end : 0);
/*
* Does the input range span an existing VMA? If so, we designate this
@@ -929,21 +929,19 @@ struct vm_area_struct *vma_merge(struct vma_iterator *vmi, struct mm_struct *mm,
*
* Cases 5 - 8.
*/
- if (vma && end > vma->vm_start) {
- curr = vma;
-
+ if (curr && end > curr->vm_start) {
/*
* If the addr - end range spans this VMA entirely, then we
* check to see if another VMA follows it.
*
- * If it is _immediately_ adjacent (checked below), then we
+ * If it is immediately adjacent (checked below), then we
* designate it 'next' (cases 6 - 8).
*/
if (curr->vm_end == end)
- vma = find_vma(mm, curr->vm_end);
+ next = find_vma(mm, curr->vm_end);
else
/* Case 5. */
- vma = NULL;
+ next = NULL;
} else {
/*
* The addr - end range either spans the end of prev or spans no
@@ -952,19 +950,10 @@ struct vm_area_struct *vma_merge(struct vma_iterator *vmi, struct mm_struct *mm,
*
* Cases 1 - 4.
*/
+ next = curr;
curr = NULL;
}
- /*
- * We only actually examine the next VMA if it is immediately adjacent
- * to end which sits either at the end of a hole (cases 1 - 3), PPPP
- * (case 4) or CCCC (cases 6 - 8).
- */
- if (vma && end == vma->vm_start)
- next = vma;
- else
- next = NULL;
-
/*
* By default, we return prev. Cases 3, 4, 8 will instead return next
* and cases 3, 8 will also update vma to point at next.
--
2.34.1
>
> This has no functional impact.
>
> Signed-off-by: Lorenzo Stoakes <lstoakes@...il.com>
> ---
> mm/mmap.c | 61 ++++++++++++++++++++++++++++++++++++++++++++-----------
> 1 file changed, 49 insertions(+), 12 deletions(-)
>
> diff --git a/mm/mmap.c b/mm/mmap.c
> index c9834364ac98..66893fc72e03 100644
> --- a/mm/mmap.c
> +++ b/mm/mmap.c
> @@ -930,15 +930,53 @@ struct vm_area_struct *vma_merge(struct vma_iterator *vmi, struct mm_struct *mm,
> if (vm_flags & VM_SPECIAL)
> return NULL;
>
> - curr = find_vma(mm, prev ? prev->vm_end : 0);
> - if (curr && curr->vm_end == end) /* cases 6, 7, 8 */
> - next = find_vma(mm, curr->vm_end);
> - else
> - next = curr;
> + /*
> + * If there is a previous VMA specified, find the next, otherwise find
> + * the first.
> + */
> + vma = find_vma(mm, prev ? prev->vm_end : 0);
> +
> + /*
> + * Does the input range span an existing VMA? If so, we designate this
> + * VMA 'curr'. The caller will have ensured that curr->vm_start == addr.
> + *
> + * Cases 5 - 8.
> + */
> + if (vma && end > vma->vm_start) {
> + curr = vma;
>
> - /* In cases 1 - 4 there's no CCCC vma */
> - if (curr && end <= curr->vm_start)
> + /*
> + * If the addr - end range spans this VMA entirely, then we
> + * check to see if another VMA follows it.
> + *
> + * If it is _immediately_ adjacent (checked below), then we
> + * designate it 'next' (cases 6 - 8).
> + */
> + if (curr->vm_end == end)
> + vma = find_vma(mm, curr->vm_end);
> + else
> + /* Case 5. */
> + vma = NULL;
> + } else {
> + /*
> + * The addr - end range either spans the end of prev or spans no
> + * VMA at all - in either case we dispense with 'curr' and
> + * maintain only 'prev' and (possibly) 'next'.
> + *
> + * Cases 1 - 4.
> + */
> curr = NULL;
> + }
> +
> + /*
> + * We only actually examine the next VMA if it is immediately adjacent
> + * to end which sits either at the end of a hole (cases 1 - 3), PPPP
> + * (case 4) or CCCC (cases 6 - 8).
> + */
> + if (vma && end == vma->vm_start)
> + next = vma;
> + else
> + next = NULL;
>
> /* verify some invariant that must be enforced by the caller */
> VM_WARN_ON(prev && addr <= prev->vm_start);
> @@ -959,11 +997,10 @@ struct vm_area_struct *vma_merge(struct vma_iterator *vmi, struct mm_struct *mm,
> }
> }
> /* Can we merge the successor? */
> - if (next && end == next->vm_start &&
> - mpol_equal(policy, vma_policy(next)) &&
> - can_vma_merge_before(next, vm_flags,
> - anon_vma, file, pgoff+pglen,
> - vm_userfaultfd_ctx, anon_name)) {
> + if (next && mpol_equal(policy, vma_policy(next)) &&
> + can_vma_merge_before(next, vm_flags,
> + anon_vma, file, pgoff+pglen,
> + vm_userfaultfd_ctx, anon_name)) {
> merge_next = true;
> }
>
>
> --
> 2.39.2
>
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