Re: [RFC PATCH] btrfs: defer freeing of subpage private state to free_folio

[Qu Wenruo]

在 2026/1/31 03:40, JP Kobryn 写道:
> On 1/29/26 9:14 PM, Matthew Wilcox wrote:
> > On Fri, Jan 30, 2026 at 01:46:59PM +1030, Qu Wenruo wrote:
> > > Another question is, why only two fses (nfs for dir inode, and 
> > > orangefs) are
> > > utilizing the free_folio() callback.
> >
> > Alas, secretmem and guest_memfd are also using it.  Nevertheless, I'm
> > not a fan of this interface existing, and would prefer to not introduce
> > new users.  Like launder_folio, which btrfs has also mistakenly used.
>
> The part that felt concerning is how the private state is lost. If
> release_folio() frees this state but the folio persists in the cache,
> users of the folio afterward have to recreate the state. Is that the
> expectation on how filesystems should handle this situation?

I believe that's the case.

Just like what we did in btrfs_do_readpage() and prepare_one_folio().

There is no difference between getting a new page and a page that is 
released but not removed from the filemap.

> In the case of the existing btrfs code, when the state is recreated (in
> subpage mode), the bitmap data and lock states are all zeroed.

That's expected.

Thanks,
Qu