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mm/mmap/vma_merge: set mid to NULL if not applicable 

There are several places where we test if 'mid' is really the area NNNN in
the diagram and the tests have two variants and are non-obvious to follow.
Instead, set 'mid' to NULL up-front if it's not the NNNN area, and
simplify the tests.

Also update the description in comment accordingly.

[vbabka@suse.cz: adjust/add comments as suggested by Lorenzo]
  Link: https://lkml.kernel.org/r/def43190-53f7-a607-d1b0-b657565f4288@suse.cz
Link: https://lkml.kernel.org/r/20230309111258.24079-7-vbabka@suse.cz
Signed-off-by: Vlastimil Babka <vbabka@suse.cz>
Reviewed-by: Liam R. Howlett <Liam.Howlett@oracle.com>
Cc: Lorenzo Stoakes <lstoakes@gmail.com>
Cc: Matthew Wilcox (Oracle) <willy@infradead.org>
Cc: Suren Baghdasaryan <surenb@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
This commit is contained in:
Vlastimil Babka 2023-03-09 12:12:54 +01:00 committed by Andrew Morton
parent 5cd70b96de
commit 9e8a39d2a9
1 changed files with 15 additions and 8 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

23
mm/mmap.c
View File

@ -848,10 +848,12 @@ can_vma_merge_after(struct vm_area_struct *vma, unsigned long vm_flags,
*
* The following mprotect cases have to be considered, where AAAA is
* the area passed down from mprotect_fixup, never extending beyond one
* vma, PPPPPP is the prev vma specified, and NNNNNN the next vma after:
* vma, PPPP is the previous vma, NNNN is a vma that starts at the same
* address as AAAA and is of the same or larger span, and XXXX the next
* vma after AAAA:
*
* AAAA AAAA AAAA
* PPPPPPNNNNNN PPPPPPXXXXXX PPPPPPNNNNNN
* PPPPPPXXXXXX PPPPPPXXXXXX PPPPPPNNNNNN
* cannot merge might become might become
* PPXXXXXXXXXX PPPPPPPPPPNN
* mmap, brk or case 4 below case 5 below
@ -879,9 +881,10 @@ can_vma_merge_after(struct vm_area_struct *vma, unsigned long vm_flags,
*
* In the code below:
* PPPP is represented by *prev
* NNNN is represented by *mid (and possibly equal to *next)
* XXXX is represented by *next or not represented at all.
* AAAA is not represented - it will be merged or the function will return NULL
* NNNN is represented by *mid or not represented at all (NULL)
* XXXX is represented by *next or not represented at all (NULL)
* AAAA is not represented - it will be merged and the vma containing the
* area is returned, or the function will return NULL
*/
struct vm_area_struct *vma_merge(struct vma_iterator *vmi, struct mm_struct *mm,
struct vm_area_struct *prev, unsigned long addr,
@ -918,6 +921,10 @@ struct vm_area_struct *vma_merge(struct vma_iterator *vmi, struct mm_struct *mm,
else
next = mid;
/* In cases 1 - 4 there's no NNNN vma */
if (mid && end <= mid->vm_start)
mid = NULL;
/* verify some invariant that must be enforced by the caller */
VM_WARN_ON(prev && addr <= prev->vm_start);
VM_WARN_ON(mid && end > mid->vm_end);
@ -952,7 +959,7 @@ struct vm_area_struct *vma_merge(struct vma_iterator *vmi, struct mm_struct *mm,
remove = next; /* case 1 */
vma_end = next->vm_end;
err = dup_anon_vma(prev, next);
if (mid != next) { /* case 6 */
if (mid) { /* case 6 */
remove = mid;
remove2 = next;
if (!next->anon_vma)
@ -960,7 +967,7 @@ struct vm_area_struct *vma_merge(struct vma_iterator *vmi, struct mm_struct *mm,
}
} else if (merge_prev) {
err = 0; /* case 2 */
if (mid && end > mid->vm_start) {
if (mid) {
err = dup_anon_vma(prev, mid);
if (end == mid->vm_end) { /* case 7 */
remove = mid;
@ -982,7 +989,7 @@ struct vm_area_struct *vma_merge(struct vma_iterator *vmi, struct mm_struct *mm,
vma_end = next->vm_end;
vma_pgoff = next->vm_pgoff;
err = 0;
if (mid != next) { /* case 8 */
if (mid) { /* case 8 */
vma_pgoff = mid->vm_pgoff;
remove = mid;
err = dup_anon_vma(next, mid);