GFS2: Add a next-resource-group pointer to resource groups

Add a new rg_skip field to struct gfs2_rgrp, replacing __pad. The rg_skip field has the following meaning:

- If rg_skip is zero, it is considered unset and not useful.
- If rg_skip is non-zero, its value will be the number of blocks between this rgrp's address and the next rgrp's address. This can be used as a hint by fsck.gfs2 when rebuilding a bad rindex, for example.

When gfs2_rgrp_bh_get() reads a resource group header and finds rg_skip to be 0 it will attempt to set it to the difference between its rd_addr and the rd_addr of the next resource group.

The only special case is the final rgrp, which always has a rg_skip of
0. It is not set to a special value (like -1) because, when the filesystem is grown, the rgrp will no longer be the final one and it will then need to have its rg_skip field set. The overhead of this special case is a gfs2_rgrpd_get_next() call each time gfs2_rgrp_bh_get() is called for the final resource group.

For the other resource groups, if the rg_skip field is 0, it is set appropriately and then the only overhead becomes the rgd->rg_skip == 0 comparison in gfs2_rgrp_bh_get().

Before this patch, gfs2_rgrp_out() zeroes the __pad field explicitly, so the rg_skip field can get set back to 0 in cases where nodes with and without this patch are mixed in a cluster. In some cases, the field may bounce between being set by one node and then zeroed by another which may harm performance slightly, e.g. when two nodes create many small files. In testing this situation is rare but it becomes more likely as the filesystem fills up and there are fewer resource groups to choose from. The problem goes away when all nodes are running with this patch. Dipping into the space currently occupied by the rg_reserved field would have resulted in the same problem as it is also explicitly zeroed, so unfortunately there is no other way around it.

Signed-off-by Andrew Price <[email protected]>

---
  fs/gfs2/incore.h                 |  1 +
  fs/gfs2/rgrp.c                   | 27 ++++++++++++++++++++++++++-
  include/uapi/linux/gfs2_ondisk.h |  2 +-
  3 files changed, 28 insertions(+), 2 deletions(-)

fs/gfs2/incore.h +1 -0

index 845fb09..84cc1fd 100644
@@ -88,6 +88,7 @@ struct gfs2_rgrpd {
  	u32 rd_reserved;                /* number of blocks reserved */
  	u32 rd_free_clone;
  	u32 rd_dinodes;
+	u32 rd_skip;                    /* Distance to the next rgrp in fs blocks */
  	u64 rd_igeneration;
  	struct gfs2_bitmap *rd_bits;
  	struct gfs2_sbd *rd_sbd;

fs/gfs2/rgrp.c +26 -1

index 07c0265..9779258 100644
@@ -1048,6 +1048,7 @@ static void gfs2_rgrp_in(struct gfs2_rgrpd *rgd, const void *buf)
  	rgd->rd_flags |= rg_flags;
  	rgd->rd_free = be32_to_cpu(str->rg_free);
  	rgd->rd_dinodes = be32_to_cpu(str->rg_dinodes);
+	rgd->rd_skip = be32_to_cpu(str->rg_skip);
  	rgd->rd_igeneration = be64_to_cpu(str->rg_igeneration);
  }
 

@@ -1058,7 +1059,7 @@ static void gfs2_rgrp_out(struct gfs2_rgrpd *rgd, void *buf)
  	str->rg_flags = cpu_to_be32(rgd->rd_flags & ~GFS2_RDF_MASK);
  	str->rg_free = cpu_to_be32(rgd->rd_free);
  	str->rg_dinodes = cpu_to_be32(rgd->rd_dinodes);
-	str->__pad = cpu_to_be32(0);
+	str->rg_skip = cpu_to_be32(rgd->rd_skip);
  	str->rg_igeneration = cpu_to_be64(rgd->rd_igeneration);
  	memset(&str->rg_reserved, 0, sizeof(str->rg_reserved));
  }

@@ -1118,6 +1119,28 @@ static u32 count_unlinked(struct gfs2_rgrpd *rgd)
  	return count;
  }
 
+/**
+ * Set the rg_next field if this isn't the final rgrp.
+ */
+static void gfs2_rgrp_set_skip(struct gfs2_rgrpd *rgd)
+{
+	struct gfs2_sbd *sdp = rgd->rd_sbd;
+	struct buffer_head *bh = rgd->rd_bits[0].bi_bh;
+	struct gfs2_rgrpd *next = gfs2_rgrpd_get_next(rgd);
+
+	if (next == NULL || next->rd_addr <= rgd->rd_addr)
+		return;
+
+	if (gfs2_trans_begin(sdp, RES_RG_HDR, 0) != 0)
+		return;
+
+	rgd->rd_skip = next->rd_addr - rgd->rd_addr;
+	gfs2_trans_add_meta(rgd->rd_gl, bh);
+	gfs2_rgrp_out(rgd, bh->b_data);
+	gfs2_rgrp_ondisk2lvb(rgd->rd_rgl, bh->b_data);
+	gfs2_trans_end(sdp);
+	return;
+}
 
  /**
   * gfs2_rgrp_bh_get - Read in a RG's header and bitmaps

@@ -1183,6 +1206,8 @@ static int gfs2_rgrp_bh_get(struct gfs2_rgrpd *rgd)
  		if (rgd->rd_rgl->rl_unlinked == 0)
  			rgd->rd_flags &= ~GFS2_RDF_CHECK;
  	}
+	if (rgd->rd_skip == 0)
+		gfs2_rgrp_set_skip(rgd);
  	return 0;
 
  fail:

include/uapi/linux/gfs2_ondisk.h +0 -0

index 7c4be77..a35c26c 100644

include/uapi/linux/gfs2_ondisk.h +0 -0

  	__be64 rg_igeneration;
 
  	__u8 rg_reserved[80]; /* Several fields from gfs1 now reserved */

Add a new rg_skip field to struct gfs2_rgrp, replacing __pad. The rg_skip field has the following meaning:

- If rg_skip is zero, it is considered unset and not useful.
- If rg_skip is non-zero, its value will be the number of blocks between this rgrp's address and the next rgrp's address. This can be used as a hint by fsck.gfs2 when rebuilding a bad rindex, for example.

When gfs2_rgrp_bh_get() reads a resource group header and finds rg_skip to be 0 it will attempt to set it to the difference between its rd_addr and the rd_addr of the next resource group.

The only special case is the final rgrp, which always has a rg_skip of
0. It is not set to a special value (like -1) because, when the filesystem is grown, the rgrp will no longer be the final one and it will then need to have its rg_skip field set. The overhead of this special case is a gfs2_rgrpd_get_next() call each time gfs2_rgrp_bh_get() is called for the final resource group.

For the other resource groups, if the rg_skip field is 0, it is set appropriately and then the only overhead becomes the rgd->rg_skip == 0 comparison in gfs2_rgrp_bh_get().

Before this patch, gfs2_rgrp_out() zeroes the __pad field explicitly, so the rg_skip field can get set back to 0 in cases where nodes with and without this patch are mixed in a cluster. In some cases, the field may bounce between being set by one node and then zeroed by another which may harm performance slightly, e.g. when two nodes create many small files. In testing this situation is rare but it becomes more likely as the filesystem fills up and there are fewer resource groups to choose from. The problem goes away when all nodes are running with this patch. Dipping into the space currently occupied by the rg_reserved field would have resulted in the same problem as it is also explicitly zeroed, so unfortunately there is no other way around it.

Signed-off-by Andrew Price <[email protected]>

----- Original Message -----

Quoted 35 lines

Add a new rg_skip field to struct gfs2_rgrp, replacing __pad. The rg_skip field has the following meaning:

- If rg_skip is zero, it is considered unset and not useful.
- If rg_skip is non-zero, its value will be the number of blocks between this rgrp's address and the next rgrp's address. This can be used as a hint by fsck.gfs2 when rebuilding a bad rindex, for example.

When gfs2_rgrp_bh_get() reads a resource group header and finds rg_skip to be 0 it will attempt to set it to the difference between its rd_addr and the rd_addr of the next resource group.

The only special case is the final rgrp, which always has a rg_skip of
0. It is not set to a special value (like -1) because, when the filesystem is grown, the rgrp will no longer be the final one and it will then need to have its rg_skip field set. The overhead of this special case is a gfs2_rgrpd_get_next() call each time gfs2_rgrp_bh_get() is called for the final resource group.

For the other resource groups, if the rg_skip field is 0, it is set appropriately and then the only overhead becomes the rgd->rg_skip == 0 comparison in gfs2_rgrp_bh_get().

Before this patch, gfs2_rgrp_out() zeroes the __pad field explicitly, so the rg_skip field can get set back to 0 in cases where nodes with and without this patch are mixed in a cluster. In some cases, the field may bounce between being set by one node and then zeroed by another which may harm performance slightly, e.g. when two nodes create many small files. In testing this situation is rare but it becomes more likely as the filesystem fills up and there are fewer resource groups to choose from. The problem goes away when all nodes are running with this patch. Dipping into the space currently occupied by the rg_reserved field would have resulted in the same problem as it is also explicitly zeroed, so unfortunately there is no other way around it.

Quoted 1 line

Signed-off-by Andrew Price <[email protected]>

Hi Andy,

I've been talking about doing something like this for years, so it's good to see someone finally acting on it.

Although this is a good first stab at the solution, my main concern about this implementation is that, AFAICT, it doesn't take read-only mounts into account. In fact, a "spectator" mount might even cause it to BUG_ON from gfs2_trans_begin, since there's no journal. But it's close.

Add a new rg_skip field to struct gfs2_rgrp, replacing __pad. The rg_skip field has the following meaning:

- If rg_skip is zero, it is considered unset and not useful.
- If rg_skip is non-zero, its value will be the number of blocks between this rgrp's address and the next rgrp's address. This can be used as a hint by fsck.gfs2 when rebuilding a bad rindex, for example.

When gfs2_rgrp_bh_get() reads a resource group header and finds rg_skip to be 0 it will attempt to set it to the difference between its rd_addr and the rd_addr of the next resource group.

The only special case is the final rgrp, which always has a rg_skip of
0. It is not set to a special value (like -1) because, when the filesystem is grown, the rgrp will no longer be the final one and it will then need to have its rg_skip field set. The overhead of this special case is a gfs2_rgrpd_get_next() call each time gfs2_rgrp_bh_get() is called for the final resource group.

For the other resource groups, if the rg_skip field is 0, it is set appropriately and then the only overhead becomes the rgd->rg_skip == 0 comparison in gfs2_rgrp_bh_get().

Before this patch, gfs2_rgrp_out() zeroes the __pad field explicitly, so the rg_skip field can get set back to 0 in cases where nodes with and without this patch are mixed in a cluster. In some cases, the field may bounce between being set by one node and then zeroed by another which may harm performance slightly, e.g. when two nodes create many small files. In testing this situation is rare but it becomes more likely as the filesystem fills up and there are fewer resource groups to choose from. The problem goes away when all nodes are running with this patch. Dipping into the space currently occupied by the rg_reserved field would have resulted in the same problem as it is also explicitly zeroed, so unfortunately there is no other way around it.

Signed-off-by Andrew Price <[email protected]>

---
 fs/gfs2/incore.h                 |  1 +
 fs/gfs2/rgrp.c                   | 27 ++++++++++++++++++++++++++-
 include/uapi/linux/gfs2_ondisk.h |  5 ++++-
 3 files changed, 31 insertions(+), 2 deletions(-)

fs/gfs2/incore.h +1 -0

index a6a3389..2c03287 100644
@@ -88,6 +88,7 @@ struct gfs2_rgrpd {
 	u32 rd_reserved;                /* number of blocks reserved */
 	u32 rd_free_clone;
 	u32 rd_dinodes;
+	u32 rd_skip;                    /* Distance to the next rgrp in fs blocks */
 	u64 rd_igeneration;
 	struct gfs2_bitmap *rd_bits;
 	struct gfs2_sbd *rd_sbd;

fs/gfs2/rgrp.c +26 -1

index 86ccc015..aaf435d 100644
@@ -1049,6 +1049,7 @@ static void gfs2_rgrp_in(struct gfs2_rgrpd *rgd, const void *buf)
 	rgd->rd_flags |= rg_flags;
 	rgd->rd_free = be32_to_cpu(str->rg_free);
 	rgd->rd_dinodes = be32_to_cpu(str->rg_dinodes);
+	rgd->rd_skip = be32_to_cpu(str->rg_skip);
 	rgd->rd_igeneration = be64_to_cpu(str->rg_igeneration);
 }
 

@@ -1059,7 +1060,7 @@ static void gfs2_rgrp_out(struct gfs2_rgrpd *rgd, void *buf)
 	str->rg_flags = cpu_to_be32(rgd->rd_flags & ~GFS2_RDF_MASK);
 	str->rg_free = cpu_to_be32(rgd->rd_free);
 	str->rg_dinodes = cpu_to_be32(rgd->rd_dinodes);
-	str->__pad = cpu_to_be32(0);
+	str->rg_skip = cpu_to_be32(rgd->rd_skip);
 	str->rg_igeneration = cpu_to_be64(rgd->rd_igeneration);
 	memset(&str->rg_reserved, 0, sizeof(str->rg_reserved));
 }

@@ -1119,6 +1120,28 @@ static u32 count_unlinked(struct gfs2_rgrpd *rgd)
 	return count;
 }
 
+/**
+ * Set the rg_next field if this isn't the final rgrp.
+ */
+static void gfs2_rgrp_set_skip(struct gfs2_rgrpd *rgd)
+{
+	struct gfs2_sbd *sdp = rgd->rd_sbd;
+	struct buffer_head *bh = rgd->rd_bits[0].bi_bh;
+	struct gfs2_rgrpd *next = gfs2_rgrpd_get_next(rgd);
+
+	if (next == NULL || next->rd_addr <= rgd->rd_addr)
+		return;
+
+	if (gfs2_trans_begin(sdp, RES_RG_HDR, 0) != 0)
+		return;
+
+	rgd->rd_skip = next->rd_addr - rgd->rd_addr;
+	gfs2_trans_add_meta(rgd->rd_gl, bh);
+	gfs2_rgrp_out(rgd, bh->b_data);
+	gfs2_rgrp_ondisk2lvb(rgd->rd_rgl, bh->b_data);
+	gfs2_trans_end(sdp);
+	return;
+}
 
 /**
  * gfs2_rgrp_bh_get - Read in a RG's header and bitmaps

@@ -1184,6 +1207,8 @@ static int gfs2_rgrp_bh_get(struct gfs2_rgrpd *rgd)
 		if (rgd->rd_rgl->rl_unlinked == 0)
 			rgd->rd_flags &= ~GFS2_RDF_CHECK;
 	}
+	if (rgd->rd_skip == 0 && !(sdp->sd_vfs->s_flags & MS_RDONLY))
+		gfs2_rgrp_set_skip(rgd);
 	return 0;
 
 fail:

include/uapi/linux/gfs2_ondisk.h +4 -1

index 7c4be77..0064381f 100644
@@ -186,7 +186,10 @@ struct gfs2_rgrp {
 	__be32 rg_flags;
 	__be32 rg_free;
 	__be32 rg_dinodes;
-	__be32 __pad;
+	union {
+		__be32 __pad;
+		__be32 rg_skip; /* Distance to the next rgrp in fs blocks */
+	};
 	__be64 rg_igeneration;
 
 	__u8 rg_reserved[80]; /* Several fields from gfs1 now reserved */

Add a new rg_skip field to struct gfs2_rgrp, replacing __pad. The rg_skip field has the following meaning:

- If rg_skip is zero, it is considered unset and not useful.
- If rg_skip is non-zero, its value will be the number of blocks between this rgrp's address and the next rgrp's address. This can be used as a hint by fsck.gfs2 when rebuilding a bad rindex, for example.

When gfs2_rgrp_bh_get() reads a resource group header and finds rg_skip to be 0 it will attempt to set it to the difference between its rd_addr and the rd_addr of the next resource group.

The only special case is the final rgrp, which always has a rg_skip of
0. It is not set to a special value (like -1) because, when the filesystem is grown, the rgrp will no longer be the final one and it will then need to have its rg_skip field set. The overhead of this special case is a gfs2_rgrpd_get_next() call each time gfs2_rgrp_bh_get() is called for the final resource group.

For the other resource groups, if the rg_skip field is 0, it is set appropriately and then the only overhead becomes the rgd->rg_skip == 0 comparison in gfs2_rgrp_bh_get().

Before this patch, gfs2_rgrp_out() zeroes the __pad field explicitly, so the rg_skip field can get set back to 0 in cases where nodes with and without this patch are mixed in a cluster. In some cases, the field may bounce between being set by one node and then zeroed by another which may harm performance slightly, e.g. when two nodes create many small files. In testing this situation is rare but it becomes more likely as the filesystem fills up and there are fewer resource groups to choose from. The problem goes away when all nodes are running with this patch. Dipping into the space currently occupied by the rg_reserved field would have resulted in the same problem as it is also explicitly zeroed, so unfortunately there is no other way around it.

Signed-off-by Andrew Price <[email protected]>

---
  fs/gfs2/incore.h                 |  1 +
  fs/gfs2/rgrp.c                   | 27 ++++++++++++++++++++++++++-
  include/uapi/linux/gfs2_ondisk.h |  5 ++++-
  3 files changed, 31 insertions(+), 2 deletions(-)

fs/gfs2/incore.h +1 -0

index a6a3389..2c03287 100644
@@ -88,6 +88,7 @@ struct gfs2_rgrpd {
  	u32 rd_reserved;                /* number of blocks reserved */
  	u32 rd_free_clone;
  	u32 rd_dinodes;
+	u32 rd_skip;                    /* Distance to the next rgrp in fs blocks */
  	u64 rd_igeneration;
  	struct gfs2_bitmap *rd_bits;
  	struct gfs2_sbd *rd_sbd;

fs/gfs2/rgrp.c +24 -1

index 86ccc015..aaf435d 100644
@@ -1049,6 +1049,7 @@ static void gfs2_rgrp_in(struct gfs2_rgrpd *rgd, const void *buf)
  	rgd->rd_flags |= rg_flags;
  	rgd->rd_free = be32_to_cpu(str->rg_free);
  	rgd->rd_dinodes = be32_to_cpu(str->rg_dinodes);
+	rgd->rd_skip = be32_to_cpu(str->rg_skip);
  	rgd->rd_igeneration = be64_to_cpu(str->rg_igeneration);
  }
 

@@ -1059,7 +1060,7 @@ static void gfs2_rgrp_out(struct gfs2_rgrpd *rgd, void *buf)
  	str->rg_flags = cpu_to_be32(rgd->rd_flags & ~GFS2_RDF_MASK);
  	str->rg_free = cpu_to_be32(rgd->rd_free);
  	str->rg_dinodes = cpu_to_be32(rgd->rd_dinodes);
-	str->__pad = cpu_to_be32(0);
+	str->rg_skip = cpu_to_be32(rgd->rd_skip);
  	str->rg_igeneration = cpu_to_be64(rgd->rd_igeneration);
  	memset(&str->rg_reserved, 0, sizeof(str->rg_reserved));
  }

@@ -1119,6 +1120,28 @@ static u32 count_unlinked(struct gfs2_rgrpd *rgd)
  	return count;
  }
 
+/**
+ * Set the rg_next field if this isn't the final rgrp.
+ */
+static void gfs2_rgrp_set_skip(struct gfs2_rgrpd *rgd)
+{
+	struct gfs2_sbd *sdp = rgd->rd_sbd;
+	struct buffer_head *bh = rgd->rd_bits[0].bi_bh;
+	struct gfs2_rgrpd *next = gfs2_rgrpd_get_next(rgd);
+
+	if (next == NULL || next->rd_addr <= rgd->rd_addr)
+		return;
+
+	if (gfs2_trans_begin(sdp, RES_RG_HDR, 0) != 0)
+		return;
+
+	rgd->rd_skip = next->rd_addr - rgd->rd_addr;
+	gfs2_trans_add_meta(rgd->rd_gl, bh);
+	gfs2_rgrp_out(rgd, bh->b_data);
+	gfs2_rgrp_ondisk2lvb(rgd->rd_rgl, bh->b_data);
+	gfs2_trans_end(sdp);
+	return;
+}
 
  /**
   * gfs2_rgrp_bh_get - Read in a RG's header and bitmaps

fs/gfs2/rgrp.c +0 -0

  fail:

include/uapi/linux/gfs2_ondisk.h +0 -0

index 7c4be77..0064381f 100644
@@ -186,7 +186,10 @@ struct gfs2_rgrp {
  	__be32 rg_flags;

Hi,

Quoted 36 lines · Andrew Price

Add a new rg_skip field to struct gfs2_rgrp, replacing __pad. The rg_skip field has the following meaning:

- If rg_skip is zero, it is considered unset and not useful.
- If rg_skip is non-zero, its value will be the number of blocks between this rgrp's address and the next rgrp's address. This can be used as a hint by fsck.gfs2 when rebuilding a bad rindex, for example.

When gfs2_rgrp_bh_get() reads a resource group header and finds rg_skip to be 0 it will attempt to set it to the difference between its rd_addr and the rd_addr of the next resource group.

The only special case is the final rgrp, which always has a rg_skip of
0. It is not set to a special value (like -1) because, when the filesystem is grown, the rgrp will no longer be the final one and it will then need to have its rg_skip field set. The overhead of this special case is a gfs2_rgrpd_get_next() call each time gfs2_rgrp_bh_get() is called for the final resource group.

For the other resource groups, if the rg_skip field is 0, it is set appropriately and then the only overhead becomes the rgd->rg_skip == 0 comparison in gfs2_rgrp_bh_get().

Before this patch, gfs2_rgrp_out() zeroes the __pad field explicitly, so the rg_skip field can get set back to 0 in cases where nodes with and without this patch are mixed in a cluster. In some cases, the field may bounce between being set by one node and then zeroed by another which may harm performance slightly, e.g. when two nodes create many small files. In testing this situation is rare but it becomes more likely as the filesystem fills up and there are fewer resource groups to choose from. The problem goes away when all nodes are running with this patch. Dipping into the space currently occupied by the rg_reserved field would have resulted in the same problem as it is also explicitly zeroed, so unfortunately there is no other way around it.

Quoted 1 line

Signed-off-by Andrew Price <[email protected]>

We will also need patches for gfs2_convert, mkfs.gfs2, fsck.gfs2, gfs2_edit and gfs2_grow too most likely. Ideally we want to be in a situation where we can default to using only the info in the rgrp headers, and fall back to looking at the rindex only in case that information is not there/broken in the rgrp headers.

Quoted 72 lines

---
  fs/gfs2/incore.h                 |  1 +
  fs/gfs2/rgrp.c                   | 27 ++++++++++++++++++++++++++-
  include/uapi/linux/gfs2_ondisk.h |  5 ++++-
  3 files changed, 31 insertions(+), 2 deletions(-)

fs/gfs2/incore.h +1 -0

index a6a3389..2c03287 100644
@@ -88,6 +88,7 @@ struct gfs2_rgrpd {
        u32 rd_reserved;                /* number of blocks reserved */
        u32 rd_free_clone;
        u32 rd_dinodes;
+       u32 rd_skip;                    /* Distance to the next rgrp in fs

blocks */

        u64 rd_igeneration;
        struct gfs2_bitmap *rd_bits;
        struct gfs2_sbd *rd_sbd;

fs/gfs2/rgrp.c +1 -0

index 86ccc015..aaf435d 100644
@@ -1049,6 +1049,7 @@ static void gfs2_rgrp_in(struct gfs2_rgrpd *rgd,
 const void *buf)
        rgd->rd_flags |= rg_flags;
        rgd->rd_free = be32_to_cpu(str->rg_free);
        rgd->rd_dinodes = be32_to_cpu(str->rg_dinodes);
+       rgd->rd_skip = be32_to_cpu(str->rg_skip);
        rgd->rd_igeneration = be64_to_cpu(str->rg_igeneration);
  }

@@ -1059,7 +1060,7 @@ static void gfs2_rgrp_out(struct gfs2_rgrpd *rgd,
void *buf)

        str->rg_flags = cpu_to_be32(rgd->rd_flags & ~GFS2_RDF_MASK);
        str->rg_free = cpu_to_be32(rgd->rd_free);
        str->rg_dinodes = cpu_to_be32(rgd->rd_dinodes);

fs/gfs2/rgrp.c +22 -1

-       str->__pad = cpu_to_be32(0);
+       str->rg_skip = cpu_to_be32(rgd->rd_skip);
        str->rg_igeneration = cpu_to_be64(rgd->rd_igeneration);
        memset(&str->rg_reserved, 0, sizeof(str->rg_reserved));
  }

@@ -1119,6 +1120,28 @@ static u32 count_unlinked(struct gfs2_rgrpd *rgd)
        return count;
  }
  +/**
+ * Set the rg_next field if this isn't the final rgrp.
+ */
+static void gfs2_rgrp_set_skip(struct gfs2_rgrpd *rgd)
+{
+       struct gfs2_sbd *sdp = rgd->rd_sbd;
+       struct buffer_head *bh = rgd->rd_bits[0].bi_bh;
+       struct gfs2_rgrpd *next = gfs2_rgrpd_get_next(rgd);
+
+       if (next == NULL || next->rd_addr <= rgd->rd_addr)
+               return;
+
+       if (gfs2_trans_begin(sdp, RES_RG_HDR, 0) != 0)
+               return;
+
+       rgd->rd_skip = next->rd_addr - rgd->rd_addr;
+       gfs2_trans_add_meta(rgd->rd_gl, bh);
+       gfs2_rgrp_out(rgd, bh->b_data);
+       gfs2_rgrp_ondisk2lvb(rgd->rd_rgl, bh->b_data);
+       gfs2_trans_end(sdp);
+       return;
+}
    /**
   * gfs2_rgrp_bh_get - Read in a RG's header and bitmaps

@@ -1184,6 +1207,8 @@ static int gfs2_rgrp_bh_get(struct gfs2_rgrpd *rgd)
                if (rgd->rd_rgl->rl_unlinked == 0)
                        rgd->rd_flags &= ~GFS2_RDF_CHECK;
        }

fs/gfs2/rgrp.c static int gfs2_rgrp_bh_get(struct gfs2_rgrpd *rgd) · line 1212

Quoted 3 lines

fs/gfs2/rgrp.c +2 -0

+       if (rgd->rd_skip == 0 && !(sdp->sd_vfs->s_flags & MS_RDONLY))
+               gfs2_rgrp_set_skip(rgd);
        return 0;

This looks like it can be called under a read lock. It might be better to move the setting of the field so that we only set it when we are going to write the rgrp anyway. That will remove the additional overhead, and also avoid the complication of trying to figure out when it is ok to set it.

You
could also avoid storing it directly by calculating it on write, and using the existing rindex fields in struct gfs2_rgrpd - no need to duplicate the information there.

Quoted 16 lines

fs/gfs2/rgrp.c +0 -0

    fail:

include/uapi/linux/gfs2_ondisk.h +4 -1

index 7c4be77..0064381f 100644
@@ -186,7 +186,10 @@ struct gfs2_rgrp {
        __be32 rg_flags;
        __be32 rg_free;
        __be32 rg_dinodes;
-       __be32 __pad;
+       union {
+               __be32 __pad;
+               __be32 rg_skip; /* Distance to the next rgrp in fs blocks
 */
+       };

include/uapi/linux/gfs2_ondisk.h struct gfs2_rgrp { · line 195

Quoted 2 lines

include/uapi/linux/gfs2_ondisk.h +0 -0

        __be64 rg_igeneration;
        __u8 rg_reserved[80]; /* Several fields from gfs1 now reserved */

That looks like a good place to fit rg_skip in, but gfs2_rindex contains ri_data0, ri_data and ri_bitbytes and to allow the eventual removal of the rindex, those need also to be duplicated into the rgrp. There is plenty of space there to fit those fields in, so we should do that at the same time. We could also take the opportunity to add a checksum too,

since rg_skip
being non-zero would tell us whether we should be checking the checksum - I don't think that should be too tricky to do, but we could always add it later too, if required.

Other thoughts... if we also added a field to give us the size of the following rgrp in disk blocks, then we could do proper readahead on them. So as well as rg_skip we would need rg_next_len too, which would be the same as ri_length of the following rgrp header,

Steve.

Add a new rg_skip field to struct gfs2_rgrp, replacing __pad. The rg_skip field has the following meaning:

- If rg_skip is zero, it is considered unset and not useful.
- If rg_skip is non-zero, its value will be the number of blocks between this rgrp's address and the next rgrp's address. This can be used as a hint by fsck.gfs2 when rebuilding a bad rindex, for example.

Quoted 7 lines · Andrew Price

Add a new rg_skip field to struct gfs2_rgrp, replacing __pad. The rg_skip field has the following meaning:

- If rg_skip is zero, it is considered unset and not useful.
- If rg_skip is non-zero, its value will be the number of blocks between this rgrp's address and the next rgrp's address. This can be used as a hint by fsck.gfs2 when rebuilding a bad rindex, for example.

How is that better than a resource group size field that wouldn't need to special case the last resource group?

Quoted 6 lines · Andreas Gruenbacher

Quoted 4 lines · Andrew Price

Add a new rg_skip field to struct gfs2_rgrp, replacing __pad. The rg_skip field has the following meaning:

Quoted 8 lines

Quoted 4 lines

- If rg_skip is zero, it is considered unset and not useful.
- If rg_skip is non-zero, its value will be the number of blocks between this rgrp's address and the next rgrp's address. This can be used as a hint by fsck.gfs2 when rebuilding a bad rindex, for example.

How is that better than a resource group size field that wouldn't need to special case the last resource group?

The last block of the mapped space in a resource group (ri_data0 + ri_data) is not necessarily at the block address immediately preceding the next resource group, due to device alignment and bitmap rounding. Also, if we assume we know the position of the first new resource group that would be created by gfs2_grow then we might get it wrong, so we can't set the final resource group's rg_skip to a useful value.

Is it guaranteed that the kernel will never set the new rg_skip field if it hasn't verified that the next resource group is where it thinks it is?

Otherwise we could end up with incorrect fsck "hints" for corrupted filesystems, which would make things worse.

Quoted 4 lines · Andreas Gruenbacher

Is it guaranteed that the kernel will never set the new rg_skip field if it hasn't verified that the next resource group is where it thinks it is?

No, but rg_skip is set from the ri_addr of the next resource group in the rindex and gfs2 generally trusts the rindex so if it's corrupt then there'll be other problems down the line.

Quoted 2 lines

Otherwise we could end up with incorrect fsck "hints" for corrupted filesystems, which would make things worse.

I don't think it would make things worse. fsck.gfs2 will be able to check whether the rg_skip field is correct by checking it against the rindex and whether there's an rgrp where it's pointing (and checking that it's within a sensible range). Before, we only had the rindex and the rgrp header to check against each other but now we have a third factor.

That said, the fsck.gfs2 case is only one of the ways this would be useful. The original plan was to generally reduce dependence on the rindex, effectively allowing us to iterate over all rgrps without reading it in. Though I don't recall the specific issue which motivated it, I can think of cases where it would improve things.