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mm: make vmg->target consistent and further simplify commit_merge()
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It is confusing for vmg->target to sometimes be the target merged VMA and
in one case not.

Fix this by having commit_merge() use its awareness of the
vmg->_adjust_next_start case to know that it is manipulating a separate
vma, abstracted in the 'vma' local variable.

Place removal and adjust VMA determination logic into
init_multi_vma_prep(), as the flags give us enough information to do so,
and since this is the function that sets up the vma_prepare struct it
makes sense to do so here.

Doing this significantly simplifies commit_merge(), allowing us to
eliminate the 'merge_target' handling, initialise the VMA iterator in a
more sensible place and simply return vmg->target consistently.

This also allows us to simplify setting vmg->target in
vma_merge_existing_range() since we are then left only with two cases -
merge left (or both) where the target is vmg->prev or merge right in which
the target is vmg->next.

This makes it easy for somebody reading the code to know what VMA will
actually be the one returned and merged into and removes a great deal of
the confusing 'adjust' nonsense.

This patch has no change in functional behaviour.

Link: https://lkml.kernel.org/r/50f96e31ab1980eaaf1006e34a4f6e6dad9320b8.1738326519.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>
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Lorenzo Stoakes authored and Andrew Morton committed Mar 17, 2025
1 parent fe3e9cf commit 0e5ffe9
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Showing 2 changed files with 62 additions and 63 deletions.
119 changes: 61 additions & 58 deletions mm/vma.c
View file
Original file line number Diff line number Diff line change
Expand Up @@ -106,24 +106,40 @@ static inline bool are_anon_vmas_compatible(struct vm_area_struct *vma1,
* init_multi_vma_prep() - Initializer for struct vma_prepare
* @vp: The vma_prepare struct
* @vma: The vma that will be altered once locked
* @next: The next vma if it is to be adjusted
* @remove: The first vma to be removed
* @remove2: The second vma to be removed
* @vmg: The merge state that will be used to determine adjustment and VMA
* removal.
*/
static void init_multi_vma_prep(struct vma_prepare *vp,
struct vm_area_struct *vma,
struct vm_area_struct *next,
struct vm_area_struct *remove,
struct vm_area_struct *remove2)
struct vma_merge_struct *vmg)
{
struct vm_area_struct *adjust;
struct vm_area_struct **remove = &vp->remove;

memset(vp, 0, sizeof(struct vma_prepare));
vp->vma = vma;
vp->anon_vma = vma->anon_vma;
vp->remove = remove ? remove : remove2;
vp->remove2 = remove ? remove2 : NULL;
vp->adj_next = next;
if (!vp->anon_vma && next)
vp->anon_vma = next->anon_vma;

if (vmg && vmg->__remove_middle) {
*remove = vmg->middle;
remove = &vp->remove2;
}
if (vmg && vmg->__remove_next)
*remove = vmg->next;

if (vmg && vmg->__adjust_middle_start)
adjust = vmg->middle;
else if (vmg && vmg->__adjust_next_start)
adjust = vmg->next;
else
adjust = NULL;

vp->adj_next = adjust;
if (!vp->anon_vma && adjust)
vp->anon_vma = adjust->anon_vma;

VM_WARN_ON(vp->anon_vma && adjust && adjust->anon_vma &&
vp->anon_vma != adjust->anon_vma);

vp->file = vma->vm_file;
if (vp->file)
Expand Down Expand Up @@ -360,7 +376,7 @@ static void vma_complete(struct vma_prepare *vp, struct vma_iterator *vmi,
*/
static void init_vma_prep(struct vma_prepare *vp, struct vm_area_struct *vma)
{
init_multi_vma_prep(vp, vma, NULL, NULL, NULL);
init_multi_vma_prep(vp, vma, NULL);
}

/*
Expand Down Expand Up @@ -634,76 +650,63 @@ void validate_mm(struct mm_struct *mm)
*/
static struct vm_area_struct *commit_merge(struct vma_merge_struct *vmg)
{
struct vm_area_struct *remove = NULL;
struct vm_area_struct *remove2 = NULL;
struct vm_area_struct *vma;
struct vma_prepare vp;
struct vm_area_struct *adjust = NULL;
struct vm_area_struct *adjust;
long adj_start;
bool merge_target;

/*
* If modifying an existing VMA and we don't remove vmg->middle, then we
* shrink the adjacent VMA.
*/
if (vmg->__adjust_middle_start) {
vma = vmg->target;
adjust = vmg->middle;
/* The POSITIVE value by which we offset vmg->middle->vm_start. */
adj_start = vmg->end - vmg->middle->vm_start;
merge_target = true;

/* Note: vma iterator must be pointing to 'start'. */
vma_iter_config(vmg->vmi, vmg->start, vmg->end);
} else if (vmg->__adjust_next_start) {
/*
* In this case alone, the VMA we manipulate is vmg->middle, but
* we ultimately return vmg->next.
*/
vma = vmg->middle;
adjust = vmg->next;
/* The NEGATIVE value by which we offset vmg->next->vm_start. */
adj_start = -(vmg->middle->vm_end - vmg->end);
/*
* In all cases but this - merge right, shrink next - we write
* vmg->target to the maple tree and return this as the merged VMA.
*/
merge_target = false;

vma_iter_config(vmg->vmi, vmg->next->vm_start + adj_start,
vmg->next->vm_end);
} else {
vma = vmg->target;
adjust = NULL;
adj_start = 0;
merge_target = true;
}

if (vmg->__remove_middle)
remove = vmg->middle;
if (vmg->__remove_next)
remove2 = vmg->next;

init_multi_vma_prep(&vp, vmg->target, adjust, remove, remove2);

VM_WARN_ON(vp.anon_vma && adjust && adjust->anon_vma &&
vp.anon_vma != adjust->anon_vma);

if (merge_target) {
/* Note: vma iterator must be pointing to 'start'. */
/* Note: vma iterator must be pointing to 'start'. */
vma_iter_config(vmg->vmi, vmg->start, vmg->end);
} else {
vma_iter_config(vmg->vmi, adjust->vm_start + adj_start,
adjust->vm_end);
}

if (vma_iter_prealloc(vmg->vmi, vmg->target))
init_multi_vma_prep(&vp, vma, vmg);

if (vma_iter_prealloc(vmg->vmi, vma))
return NULL;

vma_prepare(&vp);
vma_adjust_trans_huge(vmg->target, vmg->start, vmg->end, adj_start);
vma_set_range(vmg->target, vmg->start, vmg->end, vmg->pgoff);

if (merge_target)
vma_iter_store(vmg->vmi, vmg->target);
vma_adjust_trans_huge(vma, vmg->start, vmg->end, adj_start);
vma_set_range(vma, vmg->start, vmg->end, vmg->pgoff);

if (adj_start) {
adjust->vm_start += adj_start;
adjust->vm_pgoff += PHYS_PFN(adj_start);

if (!merge_target)
vma_iter_store(vmg->vmi, adjust);
}

vma_complete(&vp, vmg->vmi, vmg->target->vm_mm);
vma_iter_store(vmg->vmi, vmg->target);

vma_complete(&vp, vmg->vmi, vma->vm_mm);

return merge_target ? vmg->target : vmg->next;
return vmg->target;
}

/* We can only remove VMAs when merging if they do not have a close hook. */
Expand Down Expand Up @@ -833,11 +836,15 @@ static __must_check struct vm_area_struct *vma_merge_existing_range(
/* No matter what happens, we will be adjusting middle. */
vma_start_write(middle);

if (merge_left)
vma_start_write(prev);

if (merge_right)
if (merge_right) {
vma_start_write(next);
vmg->target = next;
}

if (merge_left) {
vma_start_write(prev);
vmg->target = prev;
}

if (merge_both) {
/*
Expand All @@ -847,7 +854,6 @@ static __must_check struct vm_area_struct *vma_merge_existing_range(
* extend delete delete
*/

vmg->target = prev;
vmg->start = prev->vm_start;
vmg->end = next->vm_end;
vmg->pgoff = prev->vm_pgoff;
Expand All @@ -868,7 +874,6 @@ static __must_check struct vm_area_struct *vma_merge_existing_range(
* extend shrink/delete
*/

vmg->target = prev;
vmg->start = prev->vm_start;
vmg->pgoff = prev->vm_pgoff;

Expand All @@ -892,7 +897,6 @@ static __must_check struct vm_area_struct *vma_merge_existing_range(
VM_WARN_ON_VMG(vmg->start > middle->vm_start && prev && middle != prev, vmg);

if (vmg->__remove_middle) {
vmg->target = next;
vmg->end = next->vm_end;
vmg->pgoff = next->vm_pgoff - pglen;
} else {
Expand All @@ -903,7 +907,6 @@ static __must_check struct vm_area_struct *vma_merge_existing_range(
* merged VMA is NOT vmg->target, but rather vmg->next.
*/
vmg->__adjust_next_start = true;
vmg->target = middle;
vmg->start = middle->vm_start;
vmg->end = start;
vmg->pgoff = middle->vm_pgoff;
Expand Down
6 changes: 1 addition & 5 deletions mm/vma.h
View file
Original file line number Diff line number Diff line change
Expand Up @@ -82,11 +82,7 @@ struct vma_merge_struct {
struct vm_area_struct *prev;
struct vm_area_struct *middle;
struct vm_area_struct *next;
/*
* This is the VMA we ultimately target to become the merged VMA, except
* for the one exception of merge right, shrink next (for details of
* this scenario see vma_merge_existing_range()).
*/
/* This is the VMA we ultimately target to become the merged VMA. */
struct vm_area_struct *target;
/*
* Initially, the start, end, pgoff fields are provided by the caller
Expand Down

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