[PATCH v7 03/14] ceph: add BLOG page-fragment allocator

[Alex Markuze <amarkuze@redhat.com>]

Add blog_pagefrag.c: lockless page-fragment buffer allocator for
per-task BLOG contexts.

Signed-off-by: Alex Markuze <amarkuze@redhat.com>
---
 fs/ceph/blog_pagefrag.c | 62 +++++++++++++++++++++++++++++++++++++++++
 1 file changed, 62 insertions(+)
 create mode 100644 fs/ceph/blog_pagefrag.c

diff --git a/fs/ceph/blog_pagefrag.c b/fs/ceph/blog_pagefrag.c
new file mode 100644
index 000000000000..20f57082fdab
--- /dev/null
+++ b/fs/ceph/blog_pagefrag.c
@@ -0,0 +1,62 @@
+// SPDX-License-Identifier: GPL-2.0
+/*
+ * BLOG page-fragment allocator.
+ */
+
+#include "blog_pagefrag.h"
+
+/**
+ * blog_pagefrag_reserve - reserve space without publishing it
+ * @pf: pagefrag allocator
+ * @n: number of bytes
+ *
+ * Returns the current head offset without advancing. Caller writes, then
+ * blog_pagefrag_publish(). Single-writer (per-task).
+ */
+int blog_pagefrag_reserve(struct blog_pagefrag *pf, unsigned int n)
+{
+	if (n > pf->capacity - pf->head)
+		return -ENOMEM;
+	return pf->head;
+}
+
+/**
+ * blog_pagefrag_publish - make reserved bytes visible to readers
+ * @pf: pagefrag allocator
+ * @publish_head: new head (offset + bytes_written)
+ *
+ * Store-release so a reader that observes the new head (load-acquire or
+ * under pf->lock) sees the preceding entry writes. Single-writer.
+ */
+void blog_pagefrag_publish(struct blog_pagefrag *pf, unsigned int publish_head)
+{
+	/* Release-store pairs with the acquire in readers (pf->lock). */
+	smp_store_release(&pf->head, publish_head);
+}
+
+void *blog_pagefrag_get_ptr(struct blog_pagefrag *pf, u64 val)
+{
+	char *base = (char *)pf->buffer;
+	void *rc = base + val;
+
+	if (unlikely(rc < pf->buffer ||
+		     rc >= (void *)(base + pf->capacity))) {
+		WARN_ON_ONCE(1);
+		return NULL;
+	}
+	return rc;
+}
+
+/**
+ * blog_pagefrag_reset - discard stored entries
+ * @pf: pagefrag allocator to reset
+ *
+ * Callers must ensure no writer is mid-reservation. The pagefrag is
+ * single-writer and writers do not take pf->lock.
+ */
+void blog_pagefrag_reset(struct blog_pagefrag *pf)
+{
+	spin_lock(&pf->lock);
+	pf->head = 0;
+	spin_unlock(&pf->lock);
+}
-- 
2.34.1