Although zeros can be compressed transparently on EROFS using fixed-size
output compression so that it is never prioritized in the Android use
cases, indicating entire pclusters as holes is still useful to preserve
holes in the sparse datasets; otherwise overlayfs will allocate more
space when copying up, and SEEK_HOLE won't report any hole.
This patch introduces two ways to mark a pcluster as a hole:
- A new Z_EROFS_LI_HOLE compatible flag (bit 14) in the HEAD lcluster
advise field for non-compact (full) indexes;
- A 0-block CBLKCNT value on the first NONHEAD lcluster.
The hole tag is preferred for maximum compatibility since pre-existing
kernels that do not understand Z_EROFS_LI_HOLE will decompress at the
stored blkaddr (the same blkaddr will be shared among all sparse
pclusters). While the 0-block CBLKCNT approach also works for compact
indexes, it is limited to big pclusters and depends on a recent implied
behavior change (6.17+) out of commit 1a5223c182fd ("erofs: do sanity
check on m->type in z_erofs_load_compact_lcluster()").
Signed-off-by: Gao Xiang <[email protected]>
---
fs/erofs/erofs_fs.h | 2 ++
fs/erofs/zmap.c | 33 ++++++++++++++++++++++-----------
2 files changed, 24 insertions(+), 11 deletions(-)
diff --git a/fs/erofs/erofs_fs.h b/fs/erofs/erofs_fs.h
index 7871b16c1d33..16ec4fd33ac6 100644
--- a/fs/erofs/erofs_fs.h
+++ b/fs/erofs/erofs_fs.h
@@ -396,6 +396,8 @@ enum {
/* (noncompact only, HEAD) This pcluster refers to partial decompressed data */
#define Z_EROFS_LI_PARTIAL_REF (1 << 15)
+/* (noncompact only, HEAD) This pcluster can also be regarded as a HOLE */
+#define Z_EROFS_LI_HOLE (1 << 14)
/* Set on 1st non-head lcluster to store compressed block counti (in blocks) */
#define Z_EROFS_LI_D0_CBLKCNT (1 << 11)
diff --git a/fs/erofs/zmap.c b/fs/erofs/zmap.c
index e1a02a2c8406..bab521613552 100644
--- a/fs/erofs/zmap.c
+++ b/fs/erofs/zmap.c
@@ -15,8 +15,9 @@ struct z_erofs_maprecorder {
u8 type, headtype;
u16 clusterofs;
u16 delta[2];
- erofs_blk_t pblk, compressedblks;
+ erofs_blk_t pblk;
erofs_off_t nextpackoff;
+ int compressedblks;
bool partialref, in_mbox;
};
@@ -54,7 +55,12 @@ static int z_erofs_load_full_lcluster(struct
z_erofs_maprecorder *m, u64 lcn)
} else {
m->partialref = !!(advise & Z_EROFS_LI_PARTIAL_REF);
m->clusterofs = le16_to_cpu(di->di_clusterofs);
- m->pblk = le32_to_cpu(di->di_u.blkaddr);
+ if (advise & Z_EROFS_LI_HOLE) {
+ m->compressedblks = 0;
+ m->pblk = EROFS_NULL_ADDR;
+ } else {
+ m->pblk = le32_to_cpu(di->di_u.blkaddr);
+ }
}
return 0;
}
@@ -309,9 +315,10 @@ static int z_erofs_get_extent_compressedlen(struct
z_erofs_maprecorder *m,
((m->headtype == Z_EROFS_LCLUSTER_TYPE_PLAIN ||
m->headtype == Z_EROFS_LCLUSTER_TYPE_HEAD2) && !bigpcl2) ||
(lcn << vi->z_lclusterbits) >= inode->i_size)
- m->compressedblks = 1;
+ if (m->compressedblks < 0)
+ m->compressedblks = 1;
- if (m->compressedblks)
+ if (m->compressedblks >= 0)
goto out;
err = z_erofs_load_lcluster_from_disk(m, lcn, false);
@@ -329,19 +336,22 @@ static int z_erofs_get_extent_compressedlen(struct
z_erofs_maprecorder *m,
DBG_BUGON(lcn == initial_lcn &&
m->type == Z_EROFS_LCLUSTER_TYPE_NONHEAD);
- if (m->type == Z_EROFS_LCLUSTER_TYPE_NONHEAD && m->delta[0] != 1) {
+ if (m->type != Z_EROFS_LCLUSTER_TYPE_NONHEAD) {
+ /*
+ * if the 1st NONHEAD lcluster is actually PLAIN or HEAD type
+ * rather than CBLKCNT, it's a 1 block-sized pcluster.
+ */
+ if (m->compressedblks < 0)
+ m->compressedblks = 1;
+ } else if (m->delta[0] != 1 || m->compressedblks < 0) {
erofs_err(sb, "bogus CBLKCNT @ lcn %llu of nid %llu", lcn,
vi->nid);
DBG_BUGON(1);
return -EFSCORRUPTED;
}
- /*
- * if the 1st NONHEAD lcluster is actually PLAIN or HEAD type rather
- * than CBLKCNT, it's a 1 block-sized pcluster.
- */
- if (m->type != Z_EROFS_LCLUSTER_TYPE_NONHEAD || !m->compressedblks)
- m->compressedblks = 1;
out:
+ if (!m->compressedblks)
+ m->map->m_flags &= ~EROFS_MAP_MAPPED;
m->map->m_plen = erofs_pos(sb, m->compressedblks);
return 0;
}
@@ -395,6 +405,7 @@ static int z_erofs_map_blocks_fo(struct inode *inode,
.inode = inode,
.map = map,
.in_mbox = erofs_inode_in_metabox(inode),
+ .compressedblks = -1,
};
unsigned int endoff;
unsigned long initial_lcn;
--
2.43.5
