Re: Mixed page compact code and (higher order) folios for filemap

[Matthew Wilcox <willy@infradead.org>]

On Thu, Nov 16, 2023 at 02:11:00PM +1030, Qu Wenruo wrote:
> E.g. if I allocated a folio with order 2, attached some private data to
> the folio, then call filemap_add_folio().
> 
> Later some one called find_lock_page() and hit the 2nd page of that folio.
> 
> I believe the regular IO is totally fine, but what would happen for the
> page->private of that folio?
> Would them all share the same value of the folio_attach_private()? Or
> some different values?

Well, there's no magic ...

If you call find_lock_page(), you get back the precise page.  If you
call page_folio() on that page, you get back the folio that you stored.
If you then dereference folio->private, you get the pointer that you
passed to folio_attach_private().

If you dereference page->private, *that is a bug*.  You might get
NULL, you might get garbage.  Just like dereferencing page->index or
page->mapping on tail pages.  page_private() will also do the wrong thing
(we could fix that to embed a call to page_folio() ... it hasn't been
necessary before now, but if it'll help convert btrfs, then let's do it).