[PATCH RFC 1/9] cachefiles: improve FSCACHE_COOKIE_NO_DATA_TO_READ optimization

[Jingbo Xu]

In the following introduced content map feature,
cachefiles_prepare_[read|write] can query if the requested range is
cached through either SEEK_[DATA|HOLE] llseek or a self maintained
bitmap according to object->content_info.

For already existing backing files, content_info can be derived from the
xattr of the backing file. While for newly created tmpfile, content_info
is initialized when the backing file is written for the first time. This
time sequence requires FSCACHE_COOKIE_NO_DATA_TO_READ optimization, so
that llseek will only be called after the first write, i.e. after
content_info has been initializaed.

This patch includes following changes:

1. Enable NO_DATA optimization in cachefiles_prepare_[read|write].

2. Clear FSCACHE_COOKIE_NO_DATA_TO_READ on first write to the backing
   file.

When working in non-on-demand mode, FSCACHE_COOKIE_NO_DATA_TO_READ is
cleared when a_ops->release_folio() called. While for on-demand mode,
there's a retry logic in cachefiles_prepare_read(), i.e. the requested
range will be checked for the second time after the on-demand read, thus
FSCACHE_COOKIE_NO_DATA_TO_READ needs to be cleared for on-demand mode
once write completes.

3. Improve the setting/clearing of FSCACHE_COOKIE_NO_DATA_TO_READ in
on-demand mode.

Since now we rely on NO_DATA optimization when the backing file is
actually tmpfile, the setting of FSCACHE_COOKIE_NO_DATA_TO_READ flag in
on-demand mode is delayed until the size of the backing file is acquired
when copen completes, so that FSCACHE_COOKIE_NO_DATA_TO_READ flag of
tmpfile can be retained.

Signed-off-by: Jingbo Xu <jefflexu@xxxxxxxxxxxxxxxxx>
---
 fs/cachefiles/io.c       | 20 +++++++++++++-------
 fs/cachefiles/ondemand.c |  5 +----
 fs/fscache/cookie.c      |  2 +-
 3 files changed, 15 insertions(+), 12 deletions(-)

diff --git a/fs/cachefiles/io.c b/fs/cachefiles/io.c
index 000a28f46e59..b513d9bf81f1 100644
--- a/fs/cachefiles/io.c
+++ b/fs/cachefiles/io.c
@@ -255,6 +255,7 @@ static void cachefiles_write_complete(struct kiocb *iocb, long ret)
 {
 	struct cachefiles_kiocb *ki = container_of(iocb, struct cachefiles_kiocb, iocb);
 	struct cachefiles_object *object = ki->object;
+	struct fscache_cookie *cookie = object->cookie;
 	struct inode *inode = file_inode(ki->iocb.ki_filp);
 
 	_enter("%ld", ret);
@@ -269,6 +270,9 @@ static void cachefiles_write_complete(struct kiocb *iocb, long ret)
 
 	atomic_long_sub(ki->b_writing, &object->volume->cache->b_writing);
 	set_bit(FSCACHE_COOKIE_HAVE_DATA, &object->cookie->flags);
+	if (cookie->advice & FSCACHE_ADV_WANT_CACHE_SIZE &&
+	    test_bit(FSCACHE_COOKIE_NO_DATA_TO_READ, &cookie->flags))
+		clear_bit(FSCACHE_COOKIE_NO_DATA_TO_READ, &cookie->flags);
 	if (ki->term_func)
 		ki->term_func(ki->term_func_priv, ret, ki->was_async);
 	cachefiles_put_kiocb(ki);
@@ -413,13 +417,6 @@ static enum netfs_io_source cachefiles_prepare_read(struct netfs_io_subrequest *
 		goto out_no_object;
 	}
 
-	if (test_bit(FSCACHE_COOKIE_NO_DATA_TO_READ, &cookie->flags)) {
-		__set_bit(NETFS_SREQ_COPY_TO_CACHE, &subreq->flags);
-		why = cachefiles_trace_read_no_data;
-		if (!test_bit(NETFS_SREQ_ONDEMAND, &subreq->flags))
-			goto out_no_object;
-	}
-
 	/* The object and the file may be being created in the background. */
 	if (!file) {
 		why = cachefiles_trace_read_no_file;
@@ -434,6 +431,11 @@ static enum netfs_io_source cachefiles_prepare_read(struct netfs_io_subrequest *
 	object = cachefiles_cres_object(cres);
 	cache = object->volume->cache;
 	cachefiles_begin_secure(cache, &saved_cred);
+
+	if (test_bit(FSCACHE_COOKIE_NO_DATA_TO_READ, &cookie->flags)) {
+		why = cachefiles_trace_read_no_data;
+		goto download_and_store;
+	}
 retry:
 	off = cachefiles_inject_read_error();
 	if (off == 0)
@@ -510,6 +512,7 @@ int __cachefiles_prepare_write(struct cachefiles_object *object,
 			       bool no_space_allocated_yet)
 {
 	struct cachefiles_cache *cache = object->volume->cache;
+	struct fscache_cookie *cookie = object->cookie;
 	loff_t start = *_start, pos;
 	size_t len = *_len, down;
 	int ret;
@@ -526,6 +529,9 @@ int __cachefiles_prepare_write(struct cachefiles_object *object,
 	if (no_space_allocated_yet)
 		goto check_space;
 
+	if (test_bit(FSCACHE_COOKIE_NO_DATA_TO_READ, &cookie->flags))
+		goto check_space;
+
 	pos = cachefiles_inject_read_error();
 	if (pos == 0)
 		pos = vfs_llseek(file, *_start, SEEK_DATA);
diff --git a/fs/cachefiles/ondemand.c b/fs/cachefiles/ondemand.c
index 1fee702d5529..a317857e2dfd 100644
--- a/fs/cachefiles/ondemand.c
+++ b/fs/cachefiles/ondemand.c
@@ -166,12 +166,9 @@ int cachefiles_ondemand_copen(struct cachefiles_cache *cache, char *args)
 
 	cookie = req->object->cookie;
 	cookie->object_size = size;
-	if (size)
-		clear_bit(FSCACHE_COOKIE_NO_DATA_TO_READ, &cookie->flags);
-	else
+	if (size == 0)
 		set_bit(FSCACHE_COOKIE_NO_DATA_TO_READ, &cookie->flags);
 	trace_cachefiles_ondemand_copen(req->object, id, size);
-
 out:
 	complete(&req->done);
 	return ret;
diff --git a/fs/fscache/cookie.c b/fs/fscache/cookie.c
index 74920826d8f6..49c269c078eb 100644
--- a/fs/fscache/cookie.c
+++ b/fs/fscache/cookie.c
@@ -340,7 +340,7 @@ static struct fscache_cookie *fscache_alloc_cookie(
 	cookie->key_len		= index_key_len;
 	cookie->aux_len		= aux_data_len;
 	cookie->object_size	= object_size;
-	if (object_size == 0)
+	if (object_size == 0 && !(advice & FSCACHE_ADV_WANT_CACHE_SIZE))
 		__set_bit(FSCACHE_COOKIE_NO_DATA_TO_READ, &cookie->flags);
 
 	if (fscache_set_key(cookie, index_key, index_key_len) < 0)
-- 
2.27.0