[PATCH v3 23/59] netfs: Prep to use folio->private for write grouping and streaming write

David Howells
2023-12-07T21:23:41Z

Prepare to use folio->private to hold information write grouping and
streaming write. These are implemented in the same commit as they both
make use of folio->private and will be both checked at the same time in
several places.

"Write grouping" involves ordering the writeback of groups of writes, such
as is needed for ceph snaps. A group is represented by a
filesystem-supplied object which must contain a netfs_group struct. This
contains just a refcount and a pointer to a destructor.

"Streaming write" is the storage of data in folios that are marked dirty,
but not uptodate, to avoid unnecessary reads of data. This is represented
by a netfs_folio struct. This contains the offset and length of the
modified region plus the otherwise displaced write grouping pointer.

The way folio->private is multiplexed is:

(1) If private is NULL then neither is in operation on a dirty folio.

(2) If private is set, with bit 0 clear, then this points to a group.

(3) If private is set, with bit 0 set, then this points to a netfs_folio
struct (with bit 0 AND'ed out).

Signed-off-by: David Howells <dhowells@redhat.com>
cc: Jeff Layton <jlayton@kernel.org>
cc: linux-cachefs@redhat.com
cc: linux-fsdevel@vger.kernel.org
cc: linux-mm@kvack.org
---
fs/netfs/internal.h | 28 ++++++++++++++++++++++++++
fs/netfs/misc.c | 46 +++++++++++++++++++++++++++++++++++++++++++
include/linux/netfs.h | 41 ++++++++++++++++++++++++++++++++++++++
3 files changed, 115 insertions(+)

diff --git a/fs/netfs/internal.h b/fs/netfs/internal.h
index 2f5a83f4b13a..2b0c087c6fc3 100644
--- a/fs/netfs/internal.h
+++ b/fs/netfs/internal.h
@@ -149,6 +149,34 @@ static inline bool netfs_is_cache_enabled(struct netfs_inode *ctx)
#endif
}

+/*
+ * Get a ref on a netfs group attached to a dirty page (e.g. a ceph snap).
+ */
+static inline struct netfs_group *netfs_get_group(struct netfs_group *netfs_group)
+{
+ if (netfs_group)
+ refcount_inc(&netfs_group->ref);
+ return netfs_group;
+}
+
+/*
+ * Dispose of a netfs group attached to a dirty page (e.g. a ceph snap).
+ */
+static inline void netfs_put_group(struct netfs_group *netfs_group)
+{
+ if (netfs_group && refcount_dec_and_test(&netfs_group->ref))
+ netfs_group->free(netfs_group);
+}
+
+/*
+ * Dispose of a netfs group attached to a dirty page (e.g. a ceph snap).
+ */
+static inline void netfs_put_group_many(struct netfs_group *netfs_group, int nr)
+{
+ if (netfs_group && refcount_sub_and_test(nr, &netfs_group->ref))
+ netfs_group->free(netfs_group);
+}
+
/*
* fscache-cache.c
*/
diff --git a/fs/netfs/misc.c b/fs/netfs/misc.c
index 96014c3d1683..40421ced4cd3 100644
--- a/fs/netfs/misc.c
+++ b/fs/netfs/misc.c
@@ -177,9 +177,55 @@ EXPORT_SYMBOL(netfs_clear_inode_writeback);
*/
void netfs_invalidate_folio(struct folio *folio, size_t offset, size_t length)
{
+ struct netfs_folio *finfo = NULL;
+ size_t flen = folio_size(folio);
+
_enter("{%lx},%zx,%zx", folio_index(folio), offset, length);

folio_wait_fscache(folio);
+
+ if (!folio_test_private(folio))
+ return;
+
+ finfo = netfs_folio_info(folio);
+
+ if (offset == 0 && length >= flen)
+ goto erase_completely;
+
+ if (finfo) {
+ /* We have a partially uptodate page from a streaming write. */
+ unsigned int fstart = finfo->dirty_offset;
+ unsigned int fend = fstart + finfo->dirty_len;
+ unsigned int end = offset + length;
+
+ if (offset >= fend)
+ return;
+ if (end <= fstart)
+ return;
+ if (offset <= fstart && end >= fend)
+ goto erase_completely;
+ if (offset <= fstart && end > fstart)
+ goto reduce_len;
+ if (offset > fstart && end >= fend)
+ goto move_start;
+ /* A partial write was split. The caller has already zeroed
+ * it, so just absorb the hole.
+ */
+ }
+ return;
+
+erase_completely:
+ netfs_put_group(netfs_folio_group(folio));
+ folio_detach_private(folio);
+ folio_clear_uptodate(folio);
+ kfree(finfo);
+ return;
+reduce_len:
+ finfo->dirty_len = offset + length - finfo->dirty_offset;
+ return;
+move_start:
+ finfo->dirty_len -= offset - finfo->dirty_offset;
+ finfo->dirty_offset = offset;
}
EXPORT_SYMBOL(netfs_invalidate_folio);

diff --git a/include/linux/netfs.h b/include/linux/netfs.h
index a14af93efccf..2406e11b41bd 100644
--- a/include/linux/netfs.h
+++ b/include/linux/netfs.h
@@ -140,6 +140,47 @@ struct netfs_inode {
#define NETFS_ICTX_ODIRECT 0 /* The file has DIO in progress */
};

+/*
+ * A netfs group - for instance a ceph snap. This is marked on dirty pages and
+ * pages marked with a group must be flushed before they can be written under
+ * the domain of another group.
+ */
+struct netfs_group {
+ refcount_t ref;
+ void (*free)(struct netfs_group *netfs_group);
+};
+
+/*
+ * Information about a dirty page (attached only if necessary).
+ * folio->private
+ */
+struct netfs_folio {
+ struct netfs_group *netfs_group; /* Filesystem's grouping marker (or NULL). */
+ unsigned int dirty_offset; /* Write-streaming dirty data offset */
+ unsigned int dirty_len; /* Write-streaming dirty data length */
+};
+#define NETFS_FOLIO_INFO 0x1UL /* OR'd with folio->private. */
+
+static inline struct netfs_folio *netfs_folio_info(struct folio *folio)
+{
+ void *priv = folio_get_private(folio);
+
+ if ((unsigned long)priv & NETFS_FOLIO_INFO)
+ return (struct netfs_folio *)((unsigned long)priv & ~NETFS_FOLIO_INFO);
+ return NULL;
+}
+
+static inline struct netfs_group *netfs_folio_group(struct folio *folio)
+{
+ struct netfs_folio *finfo;
+ void *priv = folio_get_private(folio);
+
+ finfo = netfs_folio_info(folio);
+ if (finfo)
+ return finfo->netfs_group;
+ return priv;
+}
+
/*
* Resources required to do operations on a cache.
*/