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erofs: add sparse support to pcluster layout

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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).  Only the 0-block CBLKCNT approach also works for compact
indexes, but it is limited to big pclusters and new kernels.

Signed-off-by: Gao Xiang <hsiangkao@linux.alibaba.com>
This commit is contained in:
Gao Xiang
2026-06-22 03:44:14 +08:00
parent 28c5d23098
commit e87827da8c
2 changed files with 24 additions and 11 deletions
Download Patch File Download Diff File Expand all files Collapse all files

2
fs/erofs/erofs_fs.h
View File

@@ -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)

33
fs/erofs/zmap.c
View File

@@ -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;