Re: [PATCH v2] iomap: Move page_done callback under the folio lock

Andreas Gruenbacher
2022-12-15T20:14:55Z

On Wed, Dec 14, 2022 at 11:24 AM Andreas Gruenbacher
<agruenba@redhat.com> wrote:
>
> Move the ->page_done() call in iomap_write_end() under the folio lock.
> This closes a race between journaled data writes and the shrinker in
> gfs2. What's happening is that gfs2_iomap_page_done() is called after
> the page has been unlocked, so try_to_free_buffers() can come in and
> free the buffers while gfs2_iomap_page_done() is trying to add them to
> the current transaction. The folio lock prevents that from happening.
>
> The only current user of ->page_done() is gfs2, so other filesystems are
> not affected. Still, to catch out any new users, switch from page to
> folio in ->page_done().
>
> Signed-off-by: Andreas Gruenbacher <agruenba@redhat.com>
> ---
> fs/gfs2/bmap.c | 7 ++++---
> fs/iomap/buffered-io.c | 4 ++--
> include/linux/iomap.h | 10 +++++-----
> 3 files changed, 11 insertions(+), 10 deletions(-)
>
> diff --git a/fs/gfs2/bmap.c b/fs/gfs2/bmap.c
> index e7537fd305dd..c4ee47f8e499 100644
> --- a/fs/gfs2/bmap.c
> +++ b/fs/gfs2/bmap.c
> @@ -968,14 +968,15 @@ 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_is_stuffed(ip))
> + gfs2_page_add_databufs(ip, &folio->page, offset_in_page(pos),
> + copied);
>
> if (tr->tr_num_buf_new)
> __mark_inode_dirty(inode, I_DIRTY_DATASYNC);

This is still screwed up. We really need to unlock the page before
calling into __mark_inode_dirty() and ending the transaction. The
current page_done() hook would force us to then re-lock the page just
so that the caller can unlock it again. This just doesn't make sense,
particularly since the page_prepare and page_done hooks only exist to
allow gfs2 to do data journaling via iomap. I'll follow up with a more
useful approach ...

> diff --git a/fs/iomap/buffered-io.c b/fs/iomap/buffered-io.c
> index 91ee0b308e13..d988c1bedf70 100644
> --- a/fs/iomap/buffered-io.c
> +++ b/fs/iomap/buffered-io.c
> @@ -714,12 +714,12 @@ static size_t iomap_write_end(struct iomap_iter *iter, loff_t pos, size_t len,
> i_size_write(iter->inode, pos + ret);
> iter->iomap.flags |= IOMAP_F_SIZE_CHANGED;
> }
> + if (page_ops && page_ops->page_done)
> + page_ops->page_done(iter->inode, pos, ret, folio);
> folio_unlock(folio);
>
> if (old_size < pos)
> pagecache_isize_extended(iter->inode, old_size, pos);
> - if (page_ops && page_ops->page_done)
> - page_ops->page_done(iter->inode, pos, ret, &folio->page);
> folio_put(folio);
>
> if (ret < len)
> diff --git a/include/linux/iomap.h b/include/linux/iomap.h
> index 238a03087e17..bd6d80453726 100644
> --- a/include/linux/iomap.h
> +++ b/include/linux/iomap.h
> @@ -116,18 +116,18 @@ static inline bool iomap_inline_data_valid(const struct iomap *iomap)
>
> /*
> * When a filesystem sets page_ops in an iomap mapping it returns, page_prepare
> - * and page_done will be called for each page written to. This only applies to
> - * buffered writes as unbuffered writes will not typically have pages
> + * and page_done will be called for each folio written to. This only applies
> + * to buffered writes as unbuffered writes will not typically have folios
> * 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.
> */
> 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);
> };
>
> /*
> --
> 2.38.1
>