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mm/vma: refactor vmg_adjust_set_range() for clarity

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Add comments with ASCII diagrams to describe what we're doing, avoid
dubious use of PHYS_PFN(), and use vma_start_pgoff().

The most complicated scenario represented here is vmg->__adjust_next_start
- when this is set, vmg->[start, end] actually indicate the range to be
retained, so take special care to describe this accurately.

No functional change intended.

Link: https://lore.kernel.org/20260710-b4-pre-scalable-cow-v2-15-2a5aa403d977@kernel.org
Signed-off-by: Lorenzo Stoakes <ljs@kernel.org>
Reviewed-by: Pedro Falcato <pfalcato@suse.de>
Reviewed-by: Gregory Price <gourry@gourry.net>
Reviewed-by: Vlastimil Babka (SUSE) <vbabka@kernel.org>
Cc: Ackerley Tng <ackerleytng@google.com>
Cc: David Hildenbrand (Arm) <david@kernel.org>
Cc: Kai Huang <kai.huang@intel.com>
Cc: Marek Szyprowski <m.szyprowski@samsung.com>
Cc: SJ Park <sj@kernel.org>
Cc: Thomas Zimmermann <tzimmermann@suse.de>
Cc: Liam R. Howlett (Oracle) <liam@infradead.org>
Cc: Zi Yan <ziy@nvidia.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
This commit is contained in:
Lorenzo Stoakes 2026-07-10 21:16:56 +01:00 committed by Andrew Morton
parent dfe2b9abf6
commit a30574d073
1 changed files with 47 additions and 4 deletions
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51
mm/vma.c
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@ -704,11 +704,54 @@ static void vmg_adjust_set_range(struct vma_merge_struct *vmg)
pgoff_t pgoff;
if (vmg->__adjust_middle_start) {
adjust = vmg->middle;
pgoff = adjust->vm_pgoff + PHYS_PFN(vmg->end - adjust->vm_start);
/*
* vmg->start vmg->end
* | |
* v merge v
* <------------->
* delta
* <------>
* |------|----------------|
* | prev | middle |
* |------|----------------|
* ^
* |
* middle->vm_start
*/
struct vm_area_struct *middle = vmg->middle;
const unsigned long delta = vmg->end - middle->vm_start;
pgoff = vma_start_pgoff(middle) + (delta >> PAGE_SHIFT);
adjust = middle;
} else if (vmg->__adjust_next_start) {
adjust = vmg->next;
pgoff = adjust->vm_pgoff - PHYS_PFN(adjust->vm_start - vmg->end);
/*
* Originally:
*
* vmg->start vmg->end
* | |
* v merge v
* <------------>
* . .
* merge_existing_range() updates to:
* . .
* vmg->start vmg->end .
* | | .
* v retain v .
* <----------> .
* delta .
* <-----> .
* |----------------|------|
* | middle | next |
* |----------------|------|
* ^
* |
* next->vm_start
*/
struct vm_area_struct *next = vmg->next;
const unsigned long delta = next->vm_start - vmg->end;
pgoff = vma_start_pgoff(next) - (delta >> PAGE_SHIFT);
adjust = next;
} else {
return;
}