Re: [PATCH v5 2/9] iomap/gfs2: Unlock and put folio in page_done handler

["Darrick J. Wong" <djwong@kernel.org>]

On Sat, Dec 31, 2022 at 04:09:12PM +0100, Andreas Gruenbacher wrote:
> When an iomap defines a ->page_done() handler in its page_ops, delegate
> unlocking the folio and putting the folio reference to that handler.
> 
> This allows to fix a race between journaled data writes and folio
> writeback in gfs2: before this change, gfs2_iomap_page_done() was called
> after unlocking the folio, so writeback could start writing back the
> folio's buffers before they could be marked for writing to the journal.
> Also, try_to_free_buffers() could free the buffers before
> gfs2_iomap_page_done() was done adding the buffers to the current
> current transaction.  With this change, gfs2_iomap_page_done() adds the
> buffers to the current transaction while the folio is still locked, so
> the problems described above can no longer occur.
> 
> The only current user of ->page_done() is gfs2, so other filesystems are
> not affected.  To catch out any out-of-tree users, switch from a page to
> a folio in ->page_done().

I really hope there aren't any out of tree users...

> Signed-off-by: Andreas Gruenbacher <agruenba@redhat.com>

Reviewed-by: Darrick J. Wong <djwong@kernel.org>

--D

> ---
>  fs/gfs2/bmap.c         | 15 ++++++++++++---
>  fs/iomap/buffered-io.c |  8 ++++----
>  include/linux/iomap.h  |  7 ++++---
>  3 files changed, 20 insertions(+), 10 deletions(-)
> 
> diff --git a/fs/gfs2/bmap.c b/fs/gfs2/bmap.c
> index e7537fd305dd..46206286ad42 100644
> --- a/fs/gfs2/bmap.c
> +++ b/fs/gfs2/bmap.c
> @@ -968,14 +968,23 @@ static int gfs2_iomap_page_prepare(struct inode *inode, loff_t pos,
>  }
>  
>  static void gfs2_iomap_page_done(struct inode *inode, loff_t pos,
> -				 unsigned copied, struct page *page)
> +				 unsigned copied, struct folio *folio)
>  {
>  	struct gfs2_trans *tr = current->journal_info;
>  	struct gfs2_inode *ip = GFS2_I(inode);
>  	struct gfs2_sbd *sdp = GFS2_SB(inode);
>  
> -	if (page && !gfs2_is_stuffed(ip))
> -		gfs2_page_add_databufs(ip, page, offset_in_page(pos), copied);
> +	if (!folio) {
> +		gfs2_trans_end(sdp);
> +		return;
> +	}
> +
> +	if (!gfs2_is_stuffed(ip))
> +		gfs2_page_add_databufs(ip, &folio->page, offset_in_page(pos),
> +				       copied);
> +
> +	folio_unlock(folio);
> +	folio_put(folio);
>  
>  	if (tr->tr_num_buf_new)
>  		__mark_inode_dirty(inode, I_DIRTY_DATASYNC);
> diff --git a/fs/iomap/buffered-io.c b/fs/iomap/buffered-io.c
> index c30d150a9303..e13d5694e299 100644
> --- a/fs/iomap/buffered-io.c
> +++ b/fs/iomap/buffered-io.c
> @@ -580,12 +580,12 @@ static void iomap_put_folio(struct iomap_iter *iter, loff_t pos, size_t ret,
>  {
>  	const struct iomap_page_ops *page_ops = iter->iomap.page_ops;
>  
> -	if (folio)
> +	if (page_ops && page_ops->page_done) {
> +		page_ops->page_done(iter->inode, pos, ret, folio);
> +	} else if (folio) {
>  		folio_unlock(folio);
> -	if (page_ops && page_ops->page_done)
> -		page_ops->page_done(iter->inode, pos, ret, &folio->page);
> -	if (folio)
>  		folio_put(folio);
> +	}
>  }
>  
>  static int iomap_write_begin_inline(const struct iomap_iter *iter,
> diff --git a/include/linux/iomap.h b/include/linux/iomap.h
> index 0983dfc9a203..743e2a909162 100644
> --- a/include/linux/iomap.h
> +++ b/include/linux/iomap.h
> @@ -131,13 +131,14 @@ static inline bool iomap_inline_data_valid(const struct iomap *iomap)
>   * associated with them.
>   *
>   * When page_prepare succeeds, page_done will always be called to do any
> - * cleanup work necessary.  In that page_done call, @page will be NULL if the
> - * associated page could not be obtained.
> + * cleanup work necessary.  In that page_done call, @folio will be NULL if the
> + * associated folio could not be obtained.  When folio is not NULL, page_done
> + * is responsible for unlocking and putting the folio.
>   */
>  struct iomap_page_ops {
>  	int (*page_prepare)(struct inode *inode, loff_t pos, unsigned len);
>  	void (*page_done)(struct inode *inode, loff_t pos, unsigned copied,
> -			struct page *page);
> +			struct folio *folio);
>  
>  	/*
>  	 * Check that the cached iomap still maps correctly to the filesystem's
> -- 
> 2.38.1
>