ext4: add nowait mode for ext4_getblk()

Current ext4_getblk() might sleep if some resources are not valid or
could be race with a concurrent extents modifing procedure. So we
cannot call ext4_getblk() and ext4_map_blocks() to get map blocks in
the atomic context in some fast path (e.g. the upcoming procedure of
getting symlink external block in the RCU context), even if the map
extents have already been check and cached.

Signed-off-by: Zhang Yi <yi.zhang@huawei.com>
Link: https://lore.kernel.org/r/20220424140936.1898920-2-yi.zhang@huawei.com

diff --git a/fs/ext4/ext4.h b/fs/ext4/ext4.h
index a743b1e3b89ec2d58e8f427ba345e55801a62a41..797bc572d6fb3d9f5ba9bb4b0e8115773d5cc44c 100644 (file)
--- a/fs/ext4/ext4.h
+++ b/fs/ext4/ext4.h
@@ -673,6 +673,8 @@ enum {
        /* Caller will submit data before dropping transaction handle. This
         * allows jbd2 to avoid submitting data before commit. */
 #define EXT4_GET_BLOCKS_IO_SUBMIT              0x0400
+       /* Caller is in the atomic contex, find extent if it has been cached */
+#define EXT4_GET_BLOCKS_CACHED_NOWAIT          0x0800
 
 /*
  * The bit position of these flags must not overlap with any of the

diff --git a/fs/ext4/inode.c b/fs/ext4/inode.c
index 15165c87c915b33b52f7eea7c6509aa93fb1af59..f7b4787d6679a1eccc13c9fd62bd75fb0c5630b7 100644 (file)
--- a/fs/ext4/inode.c
+++ b/fs/ext4/inode.c
@@ -545,12 +545,21 @@ int ext4_map_blocks(handle_t *handle, struct inode *inode,
                } else {
                        BUG();
                }
+
+               if (flags & EXT4_GET_BLOCKS_CACHED_NOWAIT)
+                       return retval;
 #ifdef ES_AGGRESSIVE_TEST
                ext4_map_blocks_es_recheck(handle, inode, map,
                                           &orig_map, flags);
 #endif
                goto found;
        }
+       /*
+        * In the query cache no-wait mode, nothing we can do more if we
+        * cannot find extent in the cache.
+        */
+       if (flags & EXT4_GET_BLOCKS_CACHED_NOWAIT)
+               return 0;
 
        /*
         * Try to see if we can get the block without requesting a new
@@ -837,10 +846,12 @@ struct buffer_head *ext4_getblk(handle_t *handle, struct inode *inode,
        struct ext4_map_blocks map;
        struct buffer_head *bh;
        int create = map_flags & EXT4_GET_BLOCKS_CREATE;
+       bool nowait = map_flags & EXT4_GET_BLOCKS_CACHED_NOWAIT;
        int err;
 
        ASSERT((EXT4_SB(inode->i_sb)->s_mount_state & EXT4_FC_REPLAY)
                    || handle != NULL || create == 0);
+       ASSERT(create == 0 || !nowait);
 
        map.m_lblk = block;
        map.m_len = 1;
@@ -851,6 +862,9 @@ struct buffer_head *ext4_getblk(handle_t *handle, struct inode *inode,
        if (err < 0)
                return ERR_PTR(err);
 
+       if (nowait)
+               return sb_find_get_block(inode->i_sb, map.m_pblk);
+
        bh = sb_getblk(inode->i_sb, map.m_pblk);
        if (unlikely(!bh))
                return ERR_PTR(-ENOMEM);