mm: eliminate adj_start parameter from commit_merge()

Introduce internal vmg->__adjust_middle_start and vmg->__adjust_next_start
merge flags, enabling us to indicate to commit_merge() that we are
performing a merge which either spans only part of vmg->middle, or part of
vmg->next respectively.

In the former instance, we change the start of vmg->middle to match the
attributes of vmg->prev, without spanning all of vmg->middle.

This implies that vmg->prev->vm_end and vmg->middle->vm_start are both
increased to form the new merged VMA (vmg->prev) and the new subsequent
VMA (vmg->middle).

In the latter case, we change the end of vmg->middle to match the
attributes of vmg->next, without spanning all of vmg->next.

This implies that vmg->middle->vm_end and vmg->next->vm_start are both
decreased to form the new merged VMA (vmg->next) and the new prior VMA
(vmg->middle).

Since we now have a stable set of prev, middle, next VMAs threaded through
vmg and with these flags set know what is happening, we can perform the
calculation in commit_merge() instead.

This allows us to drop the confusing adj_start parameter and instead pass
semantic information to commit_merge().

In the latter case the -(middle->vm_end - start) calculation becomes
-(middle->vm-end - vmg->end), however this is correct as vmg->end is set
to the start parameter.

This is because in this case (rather confusingly), we manipulate
vmg->middle, but ultimately return vmg->next, whose range will be
correctly specified.  At this point vmg->start, end is the new range for
the prior VMA rather than the merged one.

This patch has no change in functional behaviour.

Link: https://lkml.kernel.org/r/bcec0cd980b373a5eb02236cb033034ce1effe42.1738326519.git.lorenzo.stoakes@oracle.com
Signed-off-by: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
Reviewed-by: Vlastimil Babka <vbabka@suse.cz>
Cc: Jann Horn <jannh@google.com>
Cc: Liam Howlett <liam.howlett@oracle.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

1 files changed, 6 insertions, 2 deletions

diff --git a/mm/debug.c b/mm/debug.c
index 60c6f1134383..e1282b85a877 100644
--- a/mm/debug.c
+++ b/mm/debug.c
@@ -267,7 +267,9 @@ void dump_vmg(const struct vma_merge_struct *vmg, const char *reason)
 		"uffd_ctx %px\n"
 		"anon_name %px\n"
 		"state %x\n"
-		"just_expand %d __remove_middle %d __remove_next %d\n",
+		"just_expand %d\n"
+		"__adjust_middle_start %d __adjust_next_start %d\n"
+		"__remove_middle %d __remove_next %d\n",
 		vmg, vmg->mm, vmg->pgoff,
 		vmg->vmi, vmg->vmi ? vma_iter_addr(vmg->vmi) : 0,
 		vmg->vmi ? vma_iter_end(vmg->vmi) : 0,
@@ -281,7 +283,9 @@ void dump_vmg(const struct vma_merge_struct *vmg, const char *reason)
 #endif
 		vmg->anon_name,
 		(int)vmg->state,
-		vmg->just_expand, vmg->__remove_middle, vmg->__remove_next);
+		vmg->just_expand,
+		vmg->__adjust_middle_start, vmg->__adjust_next_start,
+		vmg->__remove_middle, vmg->__remove_next);
 
 	if (vmg->mm) {
 		pr_warn("vmg %px mm:\n", vmg);