[PATCH] vmtrunc: restart_addr in truncate_count

Despite its restart_pgoff pretentions, unmap_mapping_range_vma was fatally
unable to distinguish a vma to be restarted from the case where that vma
has been freed, and its vm_area_struct reused for the top part of a
!new_below split of an isomorphic vma yet to be scanned.

The obvious answer is to note restart_vma in the struct address_space, and
cancel it when that vma is freed; but I'm reluctant to enlarge every struct
inode just for this.  Another answer is to flag valid restart in the
vm_area_struct; but vm_flags is protected by down_write of mmap_sem, which
we cannot take within down_write of i_sem.  If we're going to need yet
another field, better to record the restart_addr itself: restart_vma only
recorded the last restart, but a busy tree could well use more.

Actually, we don't need another field: we can neatly (though naughtily)
keep restart_addr in vm_truncate_count, provided mapping->truncate_count
leaps over those values which look like a page-aligned address.  Zero
remains good for forcing a scan (though now interpreted as restart_addr 0),
and it turns out no change is needed to any of the vm_truncate_count
settings in dup_mmap, vma_link, vma_adjust, move_one_page.

Signed-off-by: Hugh Dickins <hugh@veritas.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

0 files changed, 0 insertions, 0 deletions