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mm: eliminate adj_start parameter from commit_merge()

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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>
This commit is contained in:
Lorenzo Stoakes 2025-01-31 12:31:51 +00:00 • committed by Andrew Morton
parent 6ab2d9c7c6
commit fe3e9cf0d7
4 changed files with 48 additions and 22 deletions
Download patch file Download diff file Expand all files Collapse all files

8
mm/debug.c
View file

@ -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);

50
mm/vma.c
View file

@ -632,29 +632,44 @@ void validate_mm(struct mm_struct *mm)
*
* On success, returns the merged VMA. Otherwise returns NULL.
*/
static struct vm_area_struct *commit_merge(struct vma_merge_struct *vmg,
long adj_start)
static struct vm_area_struct *commit_merge(struct vma_merge_struct *vmg)
{
struct vma_prepare vp;
struct vm_area_struct *remove = NULL;
struct vm_area_struct *remove2 = NULL;
struct vma_prepare vp;
struct vm_area_struct *adjust = NULL;
long adj_start;
bool merge_target;
/*
* In all cases but that of merge right, shrink next, we write
* vmg->target to the maple tree and return this as the merged VMA.
* If modifying an existing VMA and we don't remove vmg->middle, then we
* shrink the adjacent VMA.
*/
bool merge_target = adj_start >= 0;
if (vmg->__adjust_middle_start) {
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;
} else if (vmg->__adjust_next_start) {
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;
} else {
adjust = NULL;
adj_start = 0;
merge_target = true;
}
if (vmg->__remove_middle)
remove = vmg->middle;
if (vmg->__remove_next)
remove2 = vmg->next;
if (adj_start > 0)
adjust = vmg->middle;
else if (adj_start < 0)
adjust = vmg->next;
init_multi_vma_prep(&vp, vmg->target, adjust, remove, remove2);
VM_WARN_ON(vp.anon_vma && adjust && adjust->anon_vma &&
@ -738,7 +753,6 @@ static __must_check struct vm_area_struct *vma_merge_existing_range(
bool left_side = middle && start == middle->vm_start;
bool right_side = middle && end == middle->vm_end;
int err = 0;
long adj_start = 0;
bool merge_left, merge_right, merge_both;
mmap_assert_write_locked(vmg->mm);
@ -858,11 +872,8 @@ static __must_check struct vm_area_struct *vma_merge_existing_range(
vmg->start = prev->vm_start;
vmg->pgoff = prev->vm_pgoff;
/*
* We both expand prev and shrink middle.
*/
if (!vmg->__remove_middle)
adj_start = vmg->end - middle->vm_start;
vmg->__adjust_middle_start = true;
err = dup_anon_vma(prev, middle, &anon_dup);
} else { /* merge_right */
@ -891,12 +902,11 @@ static __must_check struct vm_area_struct *vma_merge_existing_range(
* IMPORTANT: This is the ONLY case where the final
* 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;
adj_start = -(middle->vm_end - start);
}
err = dup_anon_vma(next, middle, &anon_dup);
@ -905,7 +915,7 @@ static __must_check struct vm_area_struct *vma_merge_existing_range(
if (err)
goto abort;
res = commit_merge(vmg, adj_start);
res = commit_merge(vmg);
if (!res) {
if (anon_dup)
unlink_anon_vmas(anon_dup);
@ -1079,7 +1089,7 @@ int vma_expand(struct vma_merge_struct *vmg)
if (remove_next)
vmg->__remove_next = true;
if (!commit_merge(vmg, 0))
if (!commit_merge(vmg))
goto nomem;
return 0;

10
mm/vma.h
View file

@ -120,6 +120,16 @@ struct vma_merge_struct {
/* Internal flags set during merge process: */
/*
* Internal flag indicating the merge increases vmg->middle->vm_start
* (and thereby, vmg->prev->vm_end).
*/
bool __adjust_middle_start :1;
/*
* Internal flag indicating the merge decreases vmg->next->vm_start
* (and thereby, vmg->middle->vm_end).
*/
bool __adjust_next_start :1;
/*
* Internal flag used during the merge operation to indicate we will
* remove vmg->middle.

2
tools/testing/vma/vma.c
View file

@ -159,6 +159,8 @@ static void vmg_set_range(struct vma_merge_struct *vmg, unsigned long start,
vmg->just_expand = false;
vmg->__remove_middle = false;
vmg->__remove_next = false;
vmg->__adjust_middle_start = false;
vmg->__adjust_next_start = false;
}
/*