mm: make vmg->target consistent and further simplify commit_merge()

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>

diff --git a/mm/vma.c b/mm/vma.c
index bac0e72ccb6294e1e3a9e8db04bbeccb197df3ff..b10625e8fc998e4a558e1809dfc8fb7333d7cf7e 100644 (file)
--- a/mm/vma.c
+++ b/mm/vma.c
@@ -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)
@@ -360,7 +376,7 @@ again:
  */
 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);
 }
 
 /*
@@ -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. */
@@ -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) {
                /*
@@ -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;
@@ -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;
 
@@ -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 {
@@ -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;

diff --git a/mm/vma.h b/mm/vma.h
index e18487797fa44face5e55ada65b8798261d0b201..e55e68abfbe3a8ca7780b5cb1057bcccaa5c561f 100644 (file)
--- a/mm/vma.h
+++ b/mm/vma.h
@@ -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