Date:   Fri, 27 Jan 2023 16:37:59 +0000
From:   Matthew Wilcox <willy@infradead.org>
To:     Eric Biggers <ebiggers@kernel.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: <Y9P959mJherU8Yru@casper.infradead.org>
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 <Y9P6GkuXHp9Gh6ai@sol.localdomain>
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On Fri, Jan 27, 2023 at 08:21:46AM -0800, Eric Biggers wrote:
> 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?

I _think_ so.  Probably the best solution is to attempt to allocate an
order-N folio (with GFP_NOWAIT?) and then fall back to allocating 2^N
pages from the mempool.  It'll require some surgery to ext4_finish_bio()
as well as ext4_bio_write_folio(), fscrypt_encrypt_pagecache_blocks()
and fscrypt_free_bounce_page(), but I think it's doable.  I'll try
to whip something up.