[PATCH 2/3] btrfs: map uncontinuous extent buffer pages into virtual address space

[Qu Wenruo <wqu@suse.com>]

Currently btrfs implements its extent buffer read-write using various
helpers doing cross-page handling for the pages array.

However other filesystems like XFS is mapping the pages into kernel
virtual address space, greatly simplify the access.

This patch would learn from XFS and map the pages into virtual address
space, if and only if the pages are not physically continuous.
(Note, a single page counts as physically continuous.)

For now we only do the map, but not yet really utilize the mapped
address.

Signed-off-by: Qu Wenruo <wqu@suse.com>
---
 fs/btrfs/extent_io.c | 70 ++++++++++++++++++++++++++++++++++++++++++++
 fs/btrfs/extent_io.h |  7 +++++
 2 files changed, 77 insertions(+)

diff --git a/fs/btrfs/extent_io.c b/fs/btrfs/extent_io.c
index f13211975e0b..9f9a3ab82f04 100644
--- a/fs/btrfs/extent_io.c
+++ b/fs/btrfs/extent_io.c
@@ -14,6 +14,7 @@
 #include <linux/pagevec.h>
 #include <linux/prefetch.h>
 #include <linux/fsverity.h>
+#include <linux/vmalloc.h>
 #include "misc.h"
 #include "extent_io.h"
 #include "extent-io-tree.h"
@@ -3153,6 +3154,8 @@ static void btrfs_release_extent_buffer_pages(struct extent_buffer *eb)
 	ASSERT(!extent_buffer_under_io(eb));
 
 	num_pages = num_extent_pages(eb);
+	if (eb->vaddr)
+		vm_unmap_ram(eb->vaddr, num_pages);
 	for (i = 0; i < num_pages; i++) {
 		struct page *page = eb->pages[i];
 
@@ -3202,6 +3205,7 @@ struct extent_buffer *btrfs_clone_extent_buffer(const struct extent_buffer *src)
 {
 	int i;
 	struct extent_buffer *new;
+	bool pages_contig = true;
 	int num_pages = num_extent_pages(src);
 	int ret;
 
@@ -3226,6 +3230,9 @@ struct extent_buffer *btrfs_clone_extent_buffer(const struct extent_buffer *src)
 		int ret;
 		struct page *p = new->pages[i];
 
+		if (i && p != new->pages[i - 1] + 1)
+			pages_contig = false;
+
 		ret = attach_extent_buffer_page(new, p, NULL);
 		if (ret < 0) {
 			btrfs_release_extent_buffer(new);
@@ -3233,6 +3240,23 @@ struct extent_buffer *btrfs_clone_extent_buffer(const struct extent_buffer *src)
 		}
 		WARN_ON(PageDirty(p));
 	}
+	if (!pages_contig) {
+		unsigned int nofs_flag;
+		int retried = 0;
+
+		nofs_flag = memalloc_nofs_save();
+		do {
+			new->vaddr = vm_map_ram(new->pages, num_pages, -1);
+			if (new->vaddr)
+				break;
+			vm_unmap_aliases();
+		} while ((retried++) <= 1);
+		memalloc_nofs_restore(nofs_flag);
+		if (!new->vaddr) {
+			btrfs_release_extent_buffer(new);
+			return NULL;
+		}
+	}
 	copy_extent_buffer_full(new, src);
 	set_extent_buffer_uptodate(new);
 
@@ -3243,6 +3267,7 @@ struct extent_buffer *__alloc_dummy_extent_buffer(struct btrfs_fs_info *fs_info,
 						  u64 start, unsigned long len)
 {
 	struct extent_buffer *eb;
+	bool pages_contig = true;
 	int num_pages;
 	int i;
 	int ret;
@@ -3259,11 +3284,29 @@ struct extent_buffer *__alloc_dummy_extent_buffer(struct btrfs_fs_info *fs_info,
 	for (i = 0; i < num_pages; i++) {
 		struct page *p = eb->pages[i];
 
+		if (i && p != eb->pages[i - 1] + 1)
+			pages_contig = false;
+
 		ret = attach_extent_buffer_page(eb, p, NULL);
 		if (ret < 0)
 			goto err;
 	}
 
+	if (!pages_contig) {
+		unsigned int nofs_flag;
+		int retried = 0;
+
+		nofs_flag = memalloc_nofs_save();
+		do {
+			eb->vaddr = vm_map_ram(eb->pages, num_pages, -1);
+			if (eb->vaddr)
+				break;
+			vm_unmap_aliases();
+		} while ((retried++) <= 1);
+		memalloc_nofs_restore(nofs_flag);
+		if (!eb->vaddr)
+			goto err;
+	}
 	set_extent_buffer_uptodate(eb);
 	btrfs_set_header_nritems(eb, 0);
 	set_bit(EXTENT_BUFFER_UNMAPPED, &eb->bflags);
@@ -3486,6 +3529,7 @@ struct extent_buffer *alloc_extent_buffer(struct btrfs_fs_info *fs_info,
 	struct address_space *mapping = fs_info->btree_inode->i_mapping;
 	struct btrfs_subpage *prealloc = NULL;
 	u64 lockdep_owner = owner_root;
+	bool pages_contig = true;
 	int uptodate = 1;
 	int ret;
 
@@ -3558,6 +3602,10 @@ struct extent_buffer *alloc_extent_buffer(struct btrfs_fs_info *fs_info,
 		/* Should not fail, as we have preallocated the memory */
 		ret = attach_extent_buffer_page(eb, p, prealloc);
 		ASSERT(!ret);
+
+		if (i && p != eb->pages[i - 1] + 1)
+			pages_contig = false;
+
 		/*
 		 * To inform we have extra eb under allocation, so that
 		 * detach_extent_buffer_page() won't release the page private
@@ -3583,6 +3631,28 @@ struct extent_buffer *alloc_extent_buffer(struct btrfs_fs_info *fs_info,
 		 * we could crash.
 		 */
 	}
+
+	/*
+	 * If pages are not continuous, here we map it into a continuous virtual
+	 * range to make later access easier.
+	 */
+	if (!pages_contig) {
+		unsigned int nofs_flag;
+		int retried = 0;
+
+		nofs_flag = memalloc_nofs_save();
+		do {
+			eb->vaddr = vm_map_ram(eb->pages, num_pages, -1);
+			if (eb->vaddr)
+				break;
+			vm_unmap_aliases();
+		} while ((retried++) <= 1);
+		memalloc_nofs_restore(nofs_flag);
+		if (!eb->vaddr) {
+			exists = ERR_PTR(-ENOMEM);
+			goto free_eb;
+		}
+	}
 	if (uptodate)
 		set_bit(EXTENT_BUFFER_UPTODATE, &eb->bflags);
 again:
diff --git a/fs/btrfs/extent_io.h b/fs/btrfs/extent_io.h
index 68368ba99321..930a2dc38157 100644
--- a/fs/btrfs/extent_io.h
+++ b/fs/btrfs/extent_io.h
@@ -87,6 +87,13 @@ struct extent_buffer {
 
 	struct rw_semaphore lock;
 
+	/*
+	 * For virtually mapped address.
+	 *
+	 * NULL if the pages are physically continuous.
+	 */
+	void *vaddr;
+
 	struct page *pages[INLINE_EXTENT_BUFFER_PAGES];
 #ifdef CONFIG_BTRFS_DEBUG
 	struct list_head leak_list;
-- 
2.41.0