Re: [PATCH v1 06/11] mm: support GUP-triggered unsharing via FAULT_FLAG_UNSHARE (!hugetlb)

[Matthew Wilcox]

On Sun, Dec 19, 2021 at 02:12:04PM -0800, Linus Torvalds wrote:
> On Sun, Dec 19, 2021 at 2:02 PM Matthew Wilcox <willy@xxxxxxxxxxxxx> wrote:
> >
> > I'd like to get rid of ->mapcount for file pages too.  And those are
> > definitely never mapped in the majority of cases.
> 
> Fair enough.
> 
> You'd probably be better off checking "is this mapping mapped" though.
> Because otherwise you have to get the page lock to serialize each
> page.

Truncate already has the page locked, eg
truncate_inode_pages_range()
  find_lock_entries()
  truncate_cleanup_page()
    if (page_mapped(page))
      unmap_mapping_page(page)

I think anyone calling unmap_mapping_page() really ought to have the
page lock.  Oh, we actually have an assert already to that effect ;-)
        VM_BUG_ON(!PageLocked(page));