linux-kernel - Re: [PATCH v3 5/5] mm: defer second attempt at merge on mmap()

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Message-ID: <5106696d-e625-4d8a-8545-9d1430301730@lucifer.local>
Date: Sat, 26 Oct 2024 11:34:05 +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: Re: [PATCH v3 5/5] mm: defer second attempt at merge on mmap()

On Fri, Oct 25, 2024 at 01:26:27PM +0100, Lorenzo Stoakes wrote:
> Rather than trying to merge again when ostensibly allocating a new VMA,
> instead defer until the VMA is added and attempt to merge the existing
> range.
>
> This way we have no complicated unwinding logic midway through the process
> of mapping the VMA.
>
> In addition this removes limitations on the VMA not being able to be the
> first in the virtual memory address space which was previously implicitly
> required.
>
> In theory, for this very same reason, we should unconditionally attempt
> merge here, however this is likely to have a performance impact so it is
> better to avoid this given the unlikely outcome of a merge.
>
> Reviewed-by: Vlastimil Babka <vbabka@...e.cz>
> Signed-off-by: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>

Hi Andrew, as per Vlasitmil's review could you squash this fix-patch into
this commit please?  Thanks!

----8<----
>From 51efd2702933919a605eb9a198adbdcf03da9c2f Mon Sep 17 00:00:00 2001
From: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
Date: Sat, 26 Oct 2024 11:33:01 +0100
Subject: [PATCH] mm: remove unnecessary indirection

Now we removed the merge logic from __mmap_new_file_vma() we can simply
pass in the vma direct rather than a pointer to a VMA, as pointed out by
Vlastimil.
---
 mm/vma.c | 5 ++---
 1 file changed, 2 insertions(+), 3 deletions(-)

diff --git a/mm/vma.c b/mm/vma.c
index f22ffb772fe1..68138e8c153e 100644
--- a/mm/vma.c
+++ b/mm/vma.c
@@ -2278,10 +2278,9 @@ static int __mmap_prepare(struct mmap_state *map, struct list_head *uf)


 static int __mmap_new_file_vma(struct mmap_state *map,
-			       struct vm_area_struct **vmap)
+			       struct vm_area_struct *vma)
 {
 	struct vma_iterator *vmi = map->vmi;
-	struct vm_area_struct *vma = *vmap;
 	int error;

 	vma->vm_file = get_file(map->file);
@@ -2349,7 +2348,7 @@ static int __mmap_new_vma(struct mmap_state *map, struct vm_area_struct **vmap)
 	}

 	if (map->file)
-		error = __mmap_new_file_vma(map, &vma);
+		error = __mmap_new_file_vma(map, vma);
 	else if (map->flags & VM_SHARED)
 		error = shmem_zero_setup(vma);
 	else
--
2.47.0