erofs: add sparse support to pcluster layout

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>

diff --git a/fs/erofs/erofs_fs.h b/fs/erofs/erofs_fs.h
index 7871b16c1d3336f18fa9d75e97f3c6b830158948..16ec4fd33ac6758ec1487d17de79a758f0217638 100644 (file)
--- 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 e1a02a2c8406bf2d09ad0241498c4ab25a3a8cdc..bab52161355247c122d673216f2a869fd69d3cd6 100644 (file)
--- 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;