Date:   Fri, 27 Jan 2023 08:21:46 -0800
From:   Eric Biggers <ebiggers@kernel.org>
To:     Matthew Wilcox <willy@infradead.org>
Cc:     Theodore Tso <tytso@mit.edu>,
        Andreas Dilger <adilger.kernel@dilger.ca>,
        linux-ext4@vger.kernel.org, linux-fsdevel@vger.kernel.org
Subject: Re: [PATCH 02/31] fscrypt: Add some folio helper functions
Message-ID: <Y9P6GkuXHp9Gh6ai@sol.localdomain>
References: <20230126202415.1682629-1-willy@infradead.org>
 <20230126202415.1682629-3-willy@infradead.org>
 <Y9M+tl5CcNfRScds@sol.localdomain>
 <Y9P4MYXE9NcC8+gv@casper.infradead.org>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <Y9P4MYXE9NcC8+gv@casper.infradead.org>
Precedence: bulk

On Fri, Jan 27, 2023 at 04:13:37PM +0000, Matthew Wilcox wrote:
> On Thu, Jan 26, 2023 at 07:02:14PM -0800, Eric Biggers wrote:
> > On Thu, Jan 26, 2023 at 08:23:46PM +0000, Matthew Wilcox (Oracle) wrote:
> > > fscrypt_is_bounce_folio() is the equivalent of fscrypt_is_bounce_page()
> > > and fscrypt_pagecache_folio() is the equivalent of fscrypt_pagecache_page().
> > > 
> > > Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
> > > ---
> > >  include/linux/fscrypt.h | 21 +++++++++++++++++++++
> > >  1 file changed, 21 insertions(+)
> > > 
> > > diff --git a/include/linux/fscrypt.h b/include/linux/fscrypt.h
> > > index 4f5f8a651213..c2c07d36fb3a 100644
> > > --- a/include/linux/fscrypt.h
> > > +++ b/include/linux/fscrypt.h
> > > @@ -273,6 +273,16 @@ static inline struct page *fscrypt_pagecache_page(struct page *bounce_page)
> > >  	return (struct page *)page_private(bounce_page);
> > >  }
> > >  
> > > +static inline bool fscrypt_is_bounce_folio(struct folio *folio)
> > > +{
> > > +	return folio->mapping == NULL;
> > > +}
> > > +
> > > +static inline struct folio *fscrypt_pagecache_folio(struct folio *bounce_folio)
> > > +{
> > > +	return bounce_folio->private;
> > > +}
> > 
> > ext4_bio_write_folio() is still doing:
> > 
> > 	bounce_page = fscrypt_encrypt_pagecache_blocks(&folio->page, ...);
> > 
> > Should it be creating a "bounce folio" instead, or is that not in the scope of
> > this patchset?
> 
> It's out of scope for _this_ patchset.  I think it's a patchset that
> could come either before or after, and is needed to support large folios
> with ext4.  The biggest problem with doing that conversion is that
> bounce pages are allocated from a mempool which obviously only allocates
> order-0 folios.  I don't know what to do about that.  Have a mempool
> for each order of folio that the filesystem supports?  Try to allocate
> folios without a mempool and then split the folio if allocation fails?
> Have a mempool containing PMD-order pages and split them ourselves if
> we need to allocate from the mempool?
> 
> Nothing's really standing out to me as the perfect answer.  There are
> probably other alternatives.

Would it be possible to keep using bounce *pages* all the time, even when the
pagecache contains large folios?

- Eric