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>

diff --git a/mm/debug.c b/mm/debug.c
index 60c6f1134383dbd30ac4dc725d03ab2d5ccbf34e..e1282b85a8776fdda0585c72e6069c3d9427748c 100644 (file)
--- 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);

diff --git a/mm/vma.c b/mm/vma.c
index dd9fdf5c9429f61a2ebbdcf7028c328bb64db7db..bac0e72ccb6294e1e3a9e8db04bbeccb197df3ff 100644 (file)
--- a/mm/vma.c
+++ b/mm/vma.c
@@ -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;

diff --git a/mm/vma.h b/mm/vma.h
index 7935681a2db855196c94ed231c52691a577ccf4f..e18487797fa44face5e55ada65b8798261d0b201 100644 (file)
--- a/mm/vma.h
+++ b/mm/vma.h
@@ -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.

diff --git a/tools/testing/vma/vma.c b/tools/testing/vma/vma.c
index 7728498b2f7e77665142b767c00753a2f4c95db8..c7ffa71841ca9a8d13e0e4f168acd6829d95dbd3 100644 (file)
--- a/tools/testing/vma/vma.c
+++ b/tools/testing/vma/vma.c
@@ -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;
 }
 
 /*