[PATCH] erofs: use dedicated meta inodes for file-backed mounts

[Gao Xiang <xiang@kernel.org>]

Currently, metadata access for file-backed mounts reuses the page cache
of backing inodes directly.

Switch to per-sb dedicated pseudo inodes ("managed cache") for metadata
access on file-backed mounts (although I still don't think it is
necessary due to the EROFS immutable model). As the result, metadata
cache won't use the page cache of backing inodes anymore.

The "managed cache" was originally used to cache physical compressed
data according to the current cache strategy and I/O patterns; since
file-backed mounts also need to access physical data for metadata
access, it's natural to reuse the managed cache for this too, providing
a unique inode for all physical data access.

Signed-off-by: Gao Xiang <xiang@kernel.org>
---
Anyway, this is a late follow-up to address the previous Christoph's
comments: https://lore.kernel.org/r/af2kDfHe0B3LnvVm@infradead.org

 fs/erofs/data.c     | 28 ++++++++--------------------
 fs/erofs/fileio.c   | 21 +++++++++++++++++++++
 fs/erofs/internal.h | 12 ++++++++----
 fs/erofs/super.c    | 33 ++++++++++++++++++++++++---------
 fs/erofs/zdata.c    | 13 ++++---------
 5 files changed, 65 insertions(+), 42 deletions(-)

diff --git a/fs/erofs/data.c b/fs/erofs/data.c
index 9aa48c8d67d1..e3f3073779ac 100644
--- a/fs/erofs/data.c
+++ b/fs/erofs/data.c
@@ -30,20 +30,6 @@ void *erofs_bread(struct erofs_buf *buf, erofs_off_t offset, bool need_kmap)
 {
 	pgoff_t index = (buf->off + offset) >> PAGE_SHIFT;
 	struct folio *folio = NULL;
-	loff_t fpos;
-	int err;
-
-	/*
-	 * Metadata access for file-backed mounts reuses page cache of backing
-	 * fs inodes (only folio data will be needed) to prevent double caching.
-	 * However, the data access range must be verified here in advance.
-	 */
-	if (buf->file) {
-		fpos = (loff_t)index << PAGE_SHIFT;
-		err = rw_verify_area(READ, buf->file, &fpos, PAGE_SIZE);
-		if (err < 0)
-			return ERR_PTR(err);
-	}
 
 	if (buf->page) {
 		folio = page_folio(buf->page);
@@ -52,7 +38,8 @@ void *erofs_bread(struct erofs_buf *buf, erofs_off_t offset, bool need_kmap)
 	}
 	if (!folio || !folio_contains(folio, index)) {
 		erofs_put_metabuf(buf);
-		folio = read_mapping_folio(buf->mapping, index, buf->file);
+		folio = read_cache_folio(buf->mapping, index,
+				buf->mc ? erofs_read_meta_folio : NULL, NULL);
 		if (IS_ERR(folio))
 			return folio;
 	}
@@ -69,19 +56,20 @@ int erofs_init_metabuf(struct erofs_buf *buf, struct super_block *sb,
 {
 	struct erofs_sb_info *sbi = EROFS_SB(sb);
 
-	buf->file = NULL;
+	buf->mc = false;
 	if (in_metabox) {
 		if (unlikely(!sbi->metabox_inode))
 			return -EFSCORRUPTED;
 		buf->mapping = sbi->metabox_inode->i_mapping;
 		return 0;
 	}
-	buf->off = sbi->dif0.fsoff;
 	if (erofs_is_fileio_mode(sbi)) {
-		buf->file = sbi->dif0.file;	/* some fs like FUSE needs it */
-		buf->mapping = buf->file->f_mapping;
-	} else
+		buf->mapping = sbi->managed_cache->i_mapping;
+		buf->mc = true;
+	} else {
+		buf->off = sbi->dif0.fsoff;
 		buf->mapping = sb->s_bdev->bd_mapping;
+	}
 	return 0;
 }
 
diff --git a/fs/erofs/fileio.c b/fs/erofs/fileio.c
index 98cdaa1cd1a7..ebb81a7ffd4c 100644
--- a/fs/erofs/fileio.c
+++ b/fs/erofs/fileio.c
@@ -194,3 +194,24 @@ const struct address_space_operations erofs_fileio_aops = {
 	.read_folio = erofs_fileio_read_folio,
 	.readahead = erofs_fileio_readahead,
 };
+
+int erofs_read_meta_folio(struct file *file, struct folio *folio)
+{
+	struct erofs_fileio io = {
+		.dev = { .m_pa = folio_pos(folio), },
+	};
+	struct inode *inode = folio_inode(folio);
+	int err;
+
+	err = erofs_map_dev(inode->i_sb, &io.dev);
+	if (err)
+		return err;
+
+	io.rq = erofs_fileio_rq_alloc(&io.dev);
+	io.rq->bio.bi_iter.bi_sector =
+		(io.dev.m_dif->fsoff + io.dev.m_pa) >> 9;
+	erofs_onlinefolio_init(folio);
+	bio_add_folio_nofail(&io.rq->bio, folio, folio_size(folio), 0);
+	erofs_fileio_rq_submit(io.rq);
+	return 0;
+}
diff --git a/fs/erofs/internal.h b/fs/erofs/internal.h
index 57bd21859c65..6de6e5a58e6b 100644
--- a/fs/erofs/internal.h
+++ b/fs/erofs/internal.h
@@ -98,11 +98,9 @@ struct erofs_sb_info {
 	unsigned int sync_decompress;	/* strategy for sync decompression */
 	unsigned int shrinker_run_no;
 
-	/* pseudo inode to manage cached pages */
-	struct inode *managed_cache;
-
 	struct erofs_sb_lz4_info lz4;
 #endif	/* CONFIG_EROFS_FS_ZIP */
+	struct inode *managed_cache; /* pseudo inode to cache physical data */
 	struct inode *packed_inode;
 	struct inode *metabox_inode;
 	struct erofs_dev_context *devs;
@@ -176,10 +174,10 @@ enum {
 
 struct erofs_buf {
 	struct address_space *mapping;
-	struct file *file;
 	u64 off;
 	struct page *page;
 	void *base;
+	bool mc;
 };
 #define __EROFS_BUF_INITIALIZER	((struct erofs_buf){ .page = NULL })
 
@@ -399,6 +397,12 @@ extern const struct file_operations erofs_ishare_fops;
 
 extern const struct iomap_ops z_erofs_iomap_report_ops;
 
+int erofs_setup_managed_cache(struct super_block *sb);
+#ifdef CONFIG_EROFS_FS_BACKED_BY_FILE
+int erofs_read_meta_folio(struct file *file, struct folio *folio);
+#else
+#define erofs_read_meta_folio NULL
+#endif
 void *erofs_read_metadata(struct super_block *sb, struct erofs_buf *buf,
 			  erofs_off_t *offset, int *lengthp);
 void erofs_unmap_metabuf(struct erofs_buf *buf);
diff --git a/fs/erofs/super.c b/fs/erofs/super.c
index 3d92caec8d3a..ff10b346ef4e 100644
--- a/fs/erofs/super.c
+++ b/fs/erofs/super.c
@@ -633,6 +633,21 @@ static const struct export_operations erofs_export_ops = {
 	.get_parent = erofs_get_parent,
 };
 
+int erofs_setup_managed_cache(struct super_block *sb)
+{
+	if (!EROFS_SB(sb)->managed_cache) {
+		struct inode *inode = new_inode(sb);
+
+		if (!inode)
+			return -ENOMEM;
+		set_nlink(inode, 1);
+		inode->i_size = OFFSET_MAX;
+		mapping_set_gfp_mask(inode->i_mapping, GFP_KERNEL);
+		EROFS_SB(sb)->managed_cache = inode;
+	}
+	return 0;
+}
+
 static int erofs_fc_fill_super(struct super_block *sb, struct fs_context *fc)
 {
 	struct inode *inode;
@@ -654,7 +669,7 @@ static int erofs_fc_fill_super(struct super_block *sb, struct fs_context *fc)
 	}
 
 	sbi->blkszbits = PAGE_SHIFT;
-	if (!sb->s_bdev) {
+	if (erofs_is_fileio_mode(sbi)) {
 		/*
 		 * (File-backed mounts) EROFS claims it's safe to nest other
 		 * fs contexts (including its own) due to self-controlled RO
@@ -669,19 +684,19 @@ static int erofs_fc_fill_super(struct super_block *sb, struct fs_context *fc)
 		 * It MUST change if another fs plans to support them, which
 		 * may also require adjusting FILESYSTEM_MAX_STACK_DEPTH.
 		 */
-		if (erofs_is_fileio_mode(sbi)) {
-			inode = file_inode(sbi->dif0.file);
-			if ((inode->i_sb->s_op == &erofs_sops &&
-			     !inode->i_sb->s_bdev) ||
-			    inode->i_sb->s_stack_depth) {
-				erofs_err(sb, "file-backed mounts cannot be applied to stacked fses");
-				return -ENOTBLK;
-			}
+		inode = file_inode(sbi->dif0.file);
+		if ((inode->i_sb->s_op == &erofs_sops &&
+		     !inode->i_sb->s_bdev) || inode->i_sb->s_stack_depth) {
+			erofs_err(sb, "file-backed mounts cannot be applied to stacked fses");
+			return -ENOTBLK;
 		}
 		sb->s_blocksize = PAGE_SIZE;
 		sb->s_blocksize_bits = PAGE_SHIFT;
 
 		err = super_setup_bdi(sb);
+		if (err)
+			return err;
+		err = erofs_setup_managed_cache(sb);
 		if (err)
 			return err;
 
diff --git a/fs/erofs/zdata.c b/fs/erofs/zdata.c
index 74520e910259..602ba8b7cc79 100644
--- a/fs/erofs/zdata.c
+++ b/fs/erofs/zdata.c
@@ -666,21 +666,16 @@ static const struct address_space_operations z_erofs_cache_aops = {
 
 int z_erofs_init_super(struct super_block *sb)
 {
-	struct inode *inode;
 	int err;
 
 	err = z_erofs_init_pcpu_workers(sb);
 	if (err)
 		return err;
 
-	inode = new_inode(sb);
-	if (!inode)
-		return -ENOMEM;
-	set_nlink(inode, 1);
-	inode->i_size = OFFSET_MAX;
-	inode->i_mapping->a_ops = &z_erofs_cache_aops;
-	mapping_set_gfp_mask(inode->i_mapping, GFP_KERNEL);
-	EROFS_SB(sb)->managed_cache = inode;
+	err = erofs_setup_managed_cache(sb);
+	if (err)
+		return err;
+	EROFS_SB(sb)->managed_cache->i_mapping->a_ops = &z_erofs_cache_aops;
 	xa_init(&EROFS_SB(sb)->managed_pslots);
 	return 0;
 }
-- 
2.47.3