Date:   Fri, 9 Dec 2022 15:23:22 +0100
From:   Sergei Shtepa <sergei.shtepa@...am.com>
To:     <axboe@...nel.dk>, <corbet@....net>
CC:     <linux-block@...r.kernel.org>, <linux-doc@...r.kernel.org>,
        <linux-kernel@...r.kernel.org>,
        Sergei Shtepa <sergei.shtepa@...am.com>
Subject: [PATCH v2 12/21] block, blksnap: buffer in memory for the minimum data storage unit

The struct diff_buffer describes a buffer in memory for the minimum data
storage block of the original block device (struct chunk).
Buffer allocation and release functions allow to reduce the number of
allocations and releases of a large number of memory pages.

Signed-off-by: Sergei Shtepa <sergei.shtepa@...am.com>
---
 drivers/block/blksnap/diff_buffer.c | 133 ++++++++++++++++++++++++++++
 drivers/block/blksnap/diff_buffer.h |  75 ++++++++++++++++
 2 files changed, 208 insertions(+)
 create mode 100644 drivers/block/blksnap/diff_buffer.c
 create mode 100644 drivers/block/blksnap/diff_buffer.h

diff --git a/drivers/block/blksnap/diff_buffer.c b/drivers/block/blksnap/diff_buffer.c
new file mode 100644
index 000000000000..40ae949b99d1
--- /dev/null
+++ b/drivers/block/blksnap/diff_buffer.c
@@ -0,0 +1,133 @@
+// SPDX-License-Identifier: GPL-2.0
+#define pr_fmt(fmt) KBUILD_MODNAME "-diff-buffer: " fmt
+
+#include "params.h"
+#include "diff_buffer.h"
+#include "diff_area.h"
+
+static void diff_buffer_free(struct diff_buffer *diff_buffer)
+{
+	size_t inx = 0;
+
+	if (unlikely(!diff_buffer))
+		return;
+
+	for (inx = 0; inx < diff_buffer->page_count; inx++) {
+		struct page *page = diff_buffer->pages[inx];
+
+		if (page)
+			__free_page(page);
+	}
+
+	kfree(diff_buffer);
+}
+
+static struct diff_buffer *
+diff_buffer_new(size_t page_count, size_t buffer_size, gfp_t gfp_mask)
+{
+	struct diff_buffer *diff_buffer;
+	size_t inx = 0;
+	struct page *page;
+
+	if (unlikely(page_count <= 0))
+		return NULL;
+
+	/*
+	 * In case of overflow, it is better to get a null pointer
+	 * than a pointer to some memory area. Therefore + 1.
+	 */
+	diff_buffer = kzalloc(sizeof(struct diff_buffer) +
+				      (page_count + 1) * sizeof(struct page *),
+			      gfp_mask);
+	if (!diff_buffer)
+		return NULL;
+
+	INIT_LIST_HEAD(&diff_buffer->link);
+	diff_buffer->size = buffer_size;
+	diff_buffer->page_count = page_count;
+
+	for (inx = 0; inx < page_count; inx++) {
+		page = alloc_page(gfp_mask);
+		if (!page)
+			goto fail;
+
+		diff_buffer->pages[inx] = page;
+	}
+	return diff_buffer;
+fail:
+	diff_buffer_free(diff_buffer);
+	return NULL;
+}
+
+struct diff_buffer *diff_buffer_take(struct diff_area *diff_area,
+				     const bool is_nowait)
+{
+	struct diff_buffer *diff_buffer = NULL;
+	sector_t chunk_sectors;
+	size_t page_count;
+	size_t buffer_size;
+
+	spin_lock(&diff_area->free_diff_buffers_lock);
+	diff_buffer = list_first_entry_or_null(&diff_area->free_diff_buffers,
+					       struct diff_buffer, link);
+	if (diff_buffer) {
+		list_del(&diff_buffer->link);
+		atomic_dec(&diff_area->free_diff_buffers_count);
+	}
+	spin_unlock(&diff_area->free_diff_buffers_lock);
+
+	/* Return free buffer if it was found in a pool */
+	if (diff_buffer)
+		return diff_buffer;
+
+	/* Allocate new buffer */
+	chunk_sectors = diff_area_chunk_sectors(diff_area);
+	page_count = round_up(chunk_sectors, PAGE_SECTORS) / PAGE_SECTORS;
+	buffer_size = chunk_sectors << SECTOR_SHIFT;
+
+	diff_buffer =
+		diff_buffer_new(page_count, buffer_size,
+				is_nowait ? (GFP_NOIO | GFP_NOWAIT) : GFP_NOIO);
+	if (unlikely(!diff_buffer)) {
+		if (is_nowait)
+			return ERR_PTR(-EAGAIN);
+		else
+			return ERR_PTR(-ENOMEM);
+	}
+
+	return diff_buffer;
+}
+
+void diff_buffer_release(struct diff_area *diff_area,
+			 struct diff_buffer *diff_buffer)
+{
+	if (atomic_read(&diff_area->free_diff_buffers_count) >
+	    free_diff_buffer_pool_size) {
+		diff_buffer_free(diff_buffer);
+		return;
+	}
+	spin_lock(&diff_area->free_diff_buffers_lock);
+	list_add_tail(&diff_buffer->link, &diff_area->free_diff_buffers);
+	atomic_inc(&diff_area->free_diff_buffers_count);
+	spin_unlock(&diff_area->free_diff_buffers_lock);
+}
+
+void diff_buffer_cleanup(struct diff_area *diff_area)
+{
+	struct diff_buffer *diff_buffer = NULL;
+
+	do {
+		spin_lock(&diff_area->free_diff_buffers_lock);
+		diff_buffer =
+			list_first_entry_or_null(&diff_area->free_diff_buffers,
+						 struct diff_buffer, link);
+		if (diff_buffer) {
+			list_del(&diff_buffer->link);
+			atomic_dec(&diff_area->free_diff_buffers_count);
+		}
+		spin_unlock(&diff_area->free_diff_buffers_lock);
+
+		if (diff_buffer)
+			diff_buffer_free(diff_buffer);
+	} while (diff_buffer);
+}
diff --git a/drivers/block/blksnap/diff_buffer.h b/drivers/block/blksnap/diff_buffer.h
new file mode 100644
index 000000000000..d1ff80452552
--- /dev/null
+++ b/drivers/block/blksnap/diff_buffer.h
@@ -0,0 +1,75 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+#ifndef __BLK_SNAP_DIFF_BUFFER_H
+#define __BLK_SNAP_DIFF_BUFFER_H
+
+#include <linux/types.h>
+#include <linux/slab.h>
+#include <linux/list.h>
+#include <linux/blkdev.h>
+
+struct diff_area;
+
+/**
+ * struct diff_buffer - Difference buffer.
+ * @link:
+ *	The list header allows to create a pool of the diff_buffer structures.
+ * @size:
+ *	Count of bytes in the buffer.
+ * @page_count:
+ *	The number of pages reserved for the buffer.
+ * @pages:
+ *	An array of pointers to pages.
+ *
+ * Describes the memory buffer for a chunk in the memory.
+ */
+struct diff_buffer {
+	struct list_head link;
+	size_t size;
+	size_t page_count;
+	struct page *pages[0];
+};
+
+/**
+ * struct diff_buffer_iter - Iterator for &struct diff_buffer.
+ * @page:
+ *      A pointer to the current page.
+ * @offset:
+ *      The offset in bytes in the current page.
+ * @bytes:
+ *      The number of bytes that can be read or written from the current page.
+ *
+ * It is convenient to use when copying data from or to &struct bio_vec.
+ */
+struct diff_buffer_iter {
+	struct page *page;
+	size_t offset;
+	size_t bytes;
+};
+
+static inline bool diff_buffer_iter_get(struct diff_buffer *diff_buffer,
+					size_t buff_offset,
+					struct diff_buffer_iter *iter)
+{
+	if (diff_buffer->size <= buff_offset)
+		return false;
+
+	iter->page = diff_buffer->pages[buff_offset >> PAGE_SHIFT];
+	iter->offset = (size_t)(buff_offset & (PAGE_SIZE - 1));
+	/*
+	 * The size cannot exceed the size of the page, taking into account
+	 * the offset in this page.
+	 * But at the same time it is unacceptable to go beyond the allocated
+	 * buffer.
+	 */
+	iter->bytes = min_t(size_t, (PAGE_SIZE - iter->offset),
+			    (diff_buffer->size - buff_offset));
+
+	return true;
+};
+
+struct diff_buffer *diff_buffer_take(struct diff_area *diff_area,
+				     const bool is_nowait);
+void diff_buffer_release(struct diff_area *diff_area,
+			 struct diff_buffer *diff_buffer);
+void diff_buffer_cleanup(struct diff_area *diff_area);
+#endif /* __BLK_SNAP_DIFF_BUFFER_H */
-- 
2.20.1

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