linux-kernel - [PATCH v3 4/5] mm: remove unnecessary reset state logic on merge new VMA

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Message-ID: <5d9a59eee6498ae017cc87d89aa723de7179f75d.1729858176.git.lorenzo.stoakes@oracle.com>
Date: Fri, 25 Oct 2024 13:26:26 +0100
From: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
To: Andrew Morton <akpm@...ux-foundation.org>
Cc: "Liam R . Howlett" <Liam.Howlett@...cle.com>,
        Vlastimil Babka <vbabka@...e.cz>, Jann Horn <jannh@...gle.com>,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Peter Xu <peterx@...hat.com>
Subject: [PATCH v3 4/5] mm: remove unnecessary reset state logic on merge new VMA

The only place where this was used was in mmap_region(), which we have now
adjusted to not require this to be performed (we reset ourselves in
effect).

It also created a dangerous assumption that VMG state could be safely
reused after a merge, at which point it may have been mutated in unexpected
ways, leading to subtle bugs.

Note that it was discovered by Wei Yang that there was also an error in
this code - we are comparing vmg->vma with prev after setting it to
NULL.

This however had no impact, as we previously reset VMA iterator state
before attempting merge again, but it was useless effort.

In any case, this patch removes all of the logic so also eliminates this
wasted effort.

Signed-off-by: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
---
 mm/vma.c | 11 -----------
 1 file changed, 11 deletions(-)

diff --git a/mm/vma.c b/mm/vma.c
index b91c947babd6..7c690be67910 100644
--- a/mm/vma.c
+++ b/mm/vma.c
@@ -963,7 +963,6 @@ struct vm_area_struct *vma_merge_new_range(struct vma_merge_struct *vmg)
 	struct vm_area_struct *next = vmg->next;
 	unsigned long start = vmg->start;
 	unsigned long end = vmg->end;
-	pgoff_t pgoff = vmg->pgoff;
 	pgoff_t pglen = PHYS_PFN(end - start);
 	bool can_merge_left, can_merge_right;
 	bool just_expand = vmg->merge_flags & VMG_FLAG_JUST_EXPAND;
@@ -1020,16 +1019,6 @@ struct vm_area_struct *vma_merge_new_range(struct vma_merge_struct *vmg)
 		return vmg->vma;
 	}
 
-	/* If expansion failed, reset state. Allows us to retry merge later. */
-	if (!just_expand) {
-		vmg->vma = NULL;
-		vmg->start = start;
-		vmg->end = end;
-		vmg->pgoff = pgoff;
-		if (vmg->vma == prev)
-			vma_iter_set(vmg->vmi, start);
-	}
-
 	return NULL;
 }
 
-- 
2.47.0