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).
