关于线性区合并函数vma_merge中case4的疑问
case4的情况是当新的线性区不能与前面合并，但可以与后面合并，且新线性区的开始地址add < prev->vm_end,如下图：这里case4合并后，按意思来理解，应该是将prev缩减x，然后next与new_area合并，但是vma_adjust实际代码却只是将next的空间向前扩展x，而不是到new_vm_area的vm_start(addr)
vma_merge
mm/mmap.c(linux kernel 2.6.11.7)
代码如下：
struct vm_area_struct *vma_merge(struct mm_struct *mm,
struct vm_area_struct *prev, unsigned long addr,
unsigned long end, unsigned long vm_flags,
struct anon_vma *anon_vma, struct file *file,
pgoff_t pgoff, struct mempolicy *policy)
{
pgoff_t pglen = (end - addr) >> PAGE_SHIFT;
struct vm_area_struct *area, *next;
if (vm_flags & VM_SPECIAL)
return NULL;
if (prev)
next = prev->vm_next;
else
next = mm->mmap;
area = next;
if (next && next->vm_end == end) /* cases 6, 7, 8 */
next = next->vm_next;
if (prev && prev->vm_end == addr &&
mpol_equal(vma_policy(prev), policy) &&
can_vma_merge_after(prev, vm_flags,
anon_vma, file, pgoff)) {
if (next && end == next->vm_start &&
mpol_equal(policy, vma_policy(next)) &&
can_vma_merge_before(next, vm_flags,
anon_vma, file, pgoff+pglen) &&
is_mergeable_anon_vma(prev->anon_vma,
next->anon_vma)) {
/* cases 1, 6 */
vma_adjust(prev, prev->vm_start,
next->vm_end, prev->vm_pgoff, NULL);
} else /* cases 2, 5, 7 */
vma_adjust(prev, prev->vm_start,
end, prev->vm_pgoff, NULL);
return prev;
}
if (next && end == next->vm_start &&
mpol_equal(policy, vma_policy(next)) &&
can_vma_merge_before(next, vm_flags,
anon_vma, file, pgoff+pglen)) {
if (prev && addr < prev->vm_end) /* case 4 */
vma_adjust(prev, prev->vm_start,
addr, prev->vm_pgoff, NULL);
else /* cases 3, 8 */
vma_adjust(area, addr, next->vm_end,
next->vm_pgoff - pglen, NULL);
return area;
}
return NULL;
}
vma_adjust
mm/mmap.c(linux kernel 2.6.11.7)
代码如下：
void vma_adjust(struct vm_area_struct *vma, unsigned long start,
unsigned long end, pgoff_t pgoff, struct vm_area_struct *insert)
{
struct mm_struct *mm = vma->vm_mm;
struct vm_area_struct *next = vma->vm_next;
struct vm_area_struct *importer = NULL;
struct address_space *mapping = NULL;
struct prio_tree_root *root = NULL;
struct file *file = vma->vm_file;
struct anon_vma *anon_vma = NULL;
long adjust_next = 0;
int remove_next = 0;
/*insert为NULL 查看vma和next合并*/
if (next && !insert) {
if (end >= next->vm_end) {
/*
* vma expands, overlapping all the next, and
* perhaps the one after too (mprotect case 6).
*/
/* case 1,6*/
again: remove_next = 1 + (end > next->vm_end);
end = next->vm_end;
anon_vma = next->anon_vma;
importer = vma;
} else if (end > next->vm_start) {
/*
* vma expands, overlapping part of the next:
* mprotect case 5 shifting the boundary up.
*/
/*case 5*/
adjust_next = (end - next->vm_start) >> PAGE_SHIFT;
anon_vma = next->anon_vma;
importer = vma;
} else if (end < vma->vm_end) {
/*
* vma shrinks, and !insert tells it's not
* split_vma inserting another: so it must be
* mprotect case 4 shifting the boundary down.
*/
adjust_next = - ((vma->vm_end - end) >> PAGE_SHIFT); /*case4就是这里adjust_next*/
anon_vma = next->anon_vma;
importer = next;
}
}
if (file) {
mapping = file->f_mapping;
if (!(vma->vm_flags & VM_NONLINEAR))
root = &mapping->i_mmap;
spin_lock(&mapping->i_mmap_lock);
if (importer &&
vma->vm_truncate_count != next->vm_truncate_count) {
/*
* unmap_mapping_range might be in progress:
* ensure that the expanding vma is rescanned.
*/
importer->vm_truncate_count = 0;
}
if (insert) {
insert->vm_truncate_count = vma->vm_truncate_count;
/*
* Put into prio_tree now, so instantiated pages
* are visible to arm/parisc __flush_dcache_page
* throughout; but we cannot insert into address
* space until vma start or end is updated.
*/
__vma_link_file(insert);
}
}
/*
* When changing only vma->vm_end, we don't really need
* anon_vma lock: but is that case worth optimizing out?
*/
if (vma->anon_vma)
anon_vma = vma->anon_vma;
if (anon_vma) {
spin_lock(&anon_vma->lock);
/*
* Easily overlooked: when mprotect shifts the boundary,
* make sure the expanding vma has anon_vma set if the
* shrinking vma had, to cover any anon pages imported.
*/
if (importer && !importer->anon_vma) {
importer->anon_vma = anon_vma;
__anon_vma_link(importer);
}
}
if (root) {
flush_dcache_mmap_lock(mapping);
vma_prio_tree_remove(vma, root);
if (adjust_next)
vma_prio_tree_remove(next, root);
}
vma->vm_start = start;
vma->vm_end = end;
vma->vm_pgoff = pgoff;
if (adjust_next) {/*case4就是这里adjust_next调整next->vm_start 仅仅扩展了上图的x而已*/
next->vm_start += adjust_next << PAGE_SHIFT;
next->vm_pgoff += adjust_next;
}
if (root) {
if (adjust_next)
vma_prio_tree_insert(next, root);
vma_prio_tree_insert(vma, root);
flush_dcache_mmap_unlock(mapping);
}
if (remove_next) {
/*
* vma_merge has merged next into vma, and needs
* us to remove next before dropping the locks.
*/
/*从红黑树和链表中删除*/
__vma_unlink(mm, next, vma);
if (file)
__remove_shared_vm_struct(next, file, mapping);
if (next->anon_vma)
__anon_vma_merge(vma, next);
} else if (insert) {
/*
* split_vma has split insert from vma, and needs
* us to insert it before dropping the locks
* (it may either follow vma or precede it).
*/
__insert_vm_struct(mm, insert);
}
if (anon_vma)
spin_unlock(&anon_vma->lock);
if (mapping)
spin_unlock(&mapping->i_mmap_lock);
if (remove_next) {
if (file)
fput(file);
mm->map_count--;
mpol_free(vma_policy(next));
kmem_cache_free(vm_area_cachep, next);
/*
* In mprotect's case 6 (see comments on vma_merge),
* we must remove another next too. It would clutter
* up the code too much to do both in one go.
*/
if (remove_next == 2) {
next = vma->vm_next;
goto again;
}
}
validate_mm(mm);
}
本帖最后由 rdlkbr 于 2016-05-20 21:09 编辑
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