linux-kernel - Re: [RFC PATCH] btrfs: defer freeing of subpage private state to free

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Message-ID: <00d098da-0d01-43f9-9efb-c18b6e8a771e@gmail.com>
Date: Fri, 30 Jan 2026 09:10:11 -0800
From: JP Kobryn <inwardvessel@...il.com>
To: Matthew Wilcox <willy@...radead.org>, Qu Wenruo <quwenruo.btrfs@....com>
Cc: boris@....io, clm@...com, dsterba@...e.com, linux-btrfs@...r.kernel.org,
 linux-kernel@...r.kernel.org, kernel-team@...a.com,
 "linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>
Subject: Re: [RFC PATCH] btrfs: defer freeing of subpage private state to
 free_folio

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?

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.