[RFC v2 8/8] shmem: add support to customize block size order

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This allows tmpfs mounts to use a custom block size order. We
only allow block sizes greater than PAGE_SIZE, and these must
also be a multiple of the PAGE_SIZE too. To simplify these
requirements and the math we just use power of 2 order, so
block order.

Only simple tests have been run so far:

mkdir -p /data-tmpfs/
time for i in $(seq 1 1000000); do echo $i >> /root/ordered.txt; done

real    0m21.392s
user    0m8.077s
sys     0m13.098s

du -h /root/ordered.txt
6.6M    /root/ordered.txt

sha1sum /root/ordered.txt
2dcc06b7ca3b7dd8b5626af83c1be3cb08ddc76c  /root/ordered.txt

stat /root/ordered.txt
  File: /root/ordered.txt
  Size: 6888896         Blocks: 13456      IO Block: 4096   regular file
Device: 254,1   Inode: 655717      Links: 1
Access: (0644/-rw-r--r--)  Uid: (    0/    root)   Gid: (    0/    root)
Access: 2023-04-21 19:34:20.709869093 +0000
Modify: 2023-04-21 19:34:43.833900042 +0000
Change: 2023-04-21 19:34:43.833900042 +0000
 Birth: 2023-04-21 19:34:20.709869093 +0000

8 KiB block size:

sha1sum /root/ordered.txt
mount -t tmpfs            -o size=10M,border=13 -o noswap tmpfs /data-tmpfs/
cp /root/ordered.txt
sha1sum /data-tmpfs/ordered.txt
stat /data-tmpfs/ordered.txt
2dcc06b7ca3b7dd8b5626af83c1be3cb08ddc76c  /root/ordered.txt
2dcc06b7ca3b7dd8b5626af83c1be3cb08ddc76c  /data-tmpfs/ordered.txt
  File: /data-tmpfs/ordered.txt
  Size: 6888896         Blocks: 13456      IO Block: 8192   regular file
Device: 0,42    Inode: 2           Links: 1
Access: (0644/-rw-r--r--)  Uid: (    0/    root)   Gid: (    0/    root)
Access: 2023-04-21 19:31:16.078390405 +0000
Modify: 2023-04-21 19:31:16.070391363 +0000
Change: 2023-04-21 19:31:16.070391363 +0000
 Birth: 2023-04-21 19:31:16.034395676 +0000

64 KiB block size:

sha1sum /root/ordered.txt
mount -t tmpfs            -o size=10M,border=16 -o noswap tmpfs /data-tmpfs/
cp /root/ordered.txt /data-tmpfs/; sha1sum /data-tmpfs/ordered.txt
stat /data-tmpfs/ordered.txt
2dcc06b7ca3b7dd8b5626af83c1be3cb08ddc76c  /root/ordered.txt
2dcc06b7ca3b7dd8b5626af83c1be3cb08ddc76c  /data-tmpfs/ordered.txt
  File: /data-tmpfs/ordered.txt
  Size: 6888896         Blocks: 13568      IO Block: 65536  regular file
Device: 0,42    Inode: 2           Links: 1
Access: (0644/-rw-r--r--)  Uid: (    0/    root)   Gid: (    0/    root)
Access: 2023-04-21 19:32:14.669796970 +0000
Modify: 2023-04-21 19:32:14.661796959 +0000
Change: 2023-04-21 19:32:14.661796959 +0000
 Birth: 2023-04-21 19:32:14.649796944 +0000

4 MiB works too.

Signed-off-by: Luis Chamberlain <mcgrof@xxxxxxxxxx>
---
 mm/shmem.c | 44 +++++++++++++++++++++++++++++++++++++++-----
 1 file changed, 39 insertions(+), 5 deletions(-)

diff --git a/mm/shmem.c b/mm/shmem.c
index 080864949fe5..777e953df62e 100644
--- a/mm/shmem.c
+++ b/mm/shmem.c
@@ -115,11 +115,13 @@ struct shmem_options {
 	int huge;
 	int seen;
 	bool noswap;
+	unsigned char block_order;
 #define SHMEM_SEEN_BLOCKS 1
 #define SHMEM_SEEN_INODES 2
 #define SHMEM_SEEN_HUGE 4
 #define SHMEM_SEEN_INUMS 8
 #define SHMEM_SEEN_NOSWAP 16
+#define SHMEM_SEEN_BLOCKORDER 32
 };
 
 static u64 shmem_default_block_order(void)
@@ -3661,6 +3663,7 @@ enum shmem_param {
 	Opt_inode32,
 	Opt_inode64,
 	Opt_noswap,
+	Opt_border,
 };
 
 static const struct constant_table shmem_param_enums_huge[] = {
@@ -3683,6 +3686,7 @@ const struct fs_parameter_spec shmem_fs_parameters[] = {
 	fsparam_flag  ("inode32",	Opt_inode32),
 	fsparam_flag  ("inode64",	Opt_inode64),
 	fsparam_flag  ("noswap",	Opt_noswap),
+	fsparam_u32   ("border",	Opt_border),
 	{}
 };
 
@@ -3709,7 +3713,15 @@ static int shmem_parse_one(struct fs_context *fc, struct fs_parameter *param)
 		}
 		if (*rest)
 			goto bad_value;
-		ctx->blocks = DIV_ROUND_UP(size, PAGE_SIZE);
+		if (!(ctx->seen & SHMEM_SEEN_BLOCKORDER) ||
+		    ctx->block_order == shmem_default_block_order())
+			ctx->blocks = DIV_ROUND_UP(size, PAGE_SIZE);
+		else {
+			if (size < (1UL << ctx->block_order) ||
+			    size % (1UL << ctx->block_order) != 0)
+				goto bad_value;
+			ctx->blocks = size >> ctx->block_order;
+		}
 		ctx->seen |= SHMEM_SEEN_BLOCKS;
 		break;
 	case Opt_nr_blocks:
@@ -3774,6 +3786,19 @@ static int shmem_parse_one(struct fs_context *fc, struct fs_parameter *param)
 		ctx->noswap = true;
 		ctx->seen |= SHMEM_SEEN_NOSWAP;
 		break;
+	case Opt_border:
+		ctx->block_order = result.uint_32;
+		ctx->seen |= SHMEM_SEEN_BLOCKORDER;
+		if (ctx->block_order < PAGE_SHIFT)
+			goto bad_value;
+		/*
+		 * We cap this to allow a block to be at least allowed to
+		 * be allocated using the buddy allocator. That's MAX_ORDER
+		 * pages. So 8 MiB on x86_64.
+		 */
+		if (ctx->block_order > (MAX_ORDER + PAGE_SHIFT))
+			goto bad_value;
+		break;
 	}
 	return 0;
 
@@ -3845,6 +3870,12 @@ static int shmem_reconfigure(struct fs_context *fc)
 	raw_spin_lock(&sbinfo->stat_lock);
 	inodes = sbinfo->max_inodes - sbinfo->free_inodes;
 
+	if (ctx->seen & SHMEM_SEEN_BLOCKORDER) {
+		if (ctx->block_order != shmem_block_order(sbinfo)) {
+			err = "Cannot modify block order on remount";
+			goto out;
+		}
+	}
 	if ((ctx->seen & SHMEM_SEEN_BLOCKS) && ctx->blocks) {
 		if (!sbinfo->max_blocks) {
 			err = "Cannot retroactively limit size";
@@ -3960,6 +3991,8 @@ static int shmem_show_options(struct seq_file *seq, struct dentry *root)
 	shmem_show_mpol(seq, sbinfo->mpol);
 	if (sbinfo->noswap)
 		seq_printf(seq, ",noswap");
+	if (shmem_block_order(sbinfo) != shmem_default_block_order())
+		seq_printf(seq, ",border=%llu", shmem_block_order(sbinfo));
 	return 0;
 }
 
@@ -3997,10 +4030,12 @@ static int shmem_fill_super(struct super_block *sb, struct fs_context *fc)
 	 * but the internal instance is left unlimited.
 	 */
 	if (!(sb->s_flags & SB_KERNMOUNT)) {
+		if (!(ctx->seen & SHMEM_SEEN_BLOCKORDER))
+			ctx->block_order = shmem_default_block_order();
 		if (!(ctx->seen & SHMEM_SEEN_BLOCKS))
-			ctx->blocks = shmem_default_max_blocks(shmem_default_block_order());
+			ctx->blocks = shmem_default_max_blocks(ctx->block_order);
 		if (!(ctx->seen & SHMEM_SEEN_INODES))
-			ctx->inodes = shmem_default_max_inodes(shmem_default_block_order());
+			ctx->inodes = shmem_default_max_inodes(ctx->block_order);
 		if (!(ctx->seen & SHMEM_SEEN_INUMS))
 			ctx->full_inums = IS_ENABLED(CONFIG_TMPFS_INODE64);
 		sbinfo->noswap = ctx->noswap;
@@ -4009,7 +4044,7 @@ static int shmem_fill_super(struct super_block *sb, struct fs_context *fc)
 	}
 	sb->s_export_op = &shmem_export_ops;
 	sb->s_flags |= SB_NOSEC | SB_I_VERSION;
-	sbinfo->block_order = shmem_default_block_order();
+	sbinfo->block_order = ctx->block_order;
 #else
 	sb->s_flags |= SB_NOUSER;
 #endif
@@ -4037,7 +4072,6 @@ static int shmem_fill_super(struct super_block *sb, struct fs_context *fc)
 	sb->s_maxbytes = MAX_LFS_FILESIZE;
 	sb->s_blocksize = shmem_sb_blocksize(sbinfo);
 	sb->s_blocksize_bits = shmem_block_order(sbinfo);
-	WARN_ON_ONCE(sb->s_blocksize_bits != PAGE_SHIFT);
 	sb->s_magic = TMPFS_MAGIC;
 	sb->s_op = &shmem_ops;
 	sb->s_time_gran = 1;
-- 
2.39.2





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