The answer to your question lies in the e2fsck/problems.c file of the e2fsprogs source code. Looking for the PR_PREEN_OK flag should get you started.
As the complete error handling is a bit more involved, due to the multitude of different error conditions that may occur, you are advised to have a closer look at the code if you are concerned about a specific case. However, the lists below were extracted from the comments to the error conditions and should give you a rough overview about the effects of the preen-mode.
The following errors/warnings are currently handled automatically when the -p flag is specified:
- Relocate hint
- Journal inode is invalid
- Journal superblock is corrupt
- Superblock has_journal flag is clear but has a journal
- Superblock needs_recovery flag is set but not journal is present
- Filesystem revision is 0, but feature flags are set
- Superblock hint for external superblock
- group descriptor N marked uninitialized without feature set.
- group N block bitmap uninitialized but inode bitmap in use.
- Group descriptor N has invalid unused inodes count.
- Last group block bitmap uninitialized.
- The test_fs flag is set (and ext4 is available)
- Last mount time is in the future (fudged)
- Last write time is in the future (fudged)
- Block group checksum (latch question) is invalid.
- Root directory has dtime set
- Reserved inode has bad mode
- Deleted inode has zero dtime
- Inode in use, but dtime set
- Zero-length directory
- Inode has incorrect i_size
- Inode has incorrect i_blocks
- Bad superblock in group
- Bad block group descriptors in group
- Block claimed for no reason
- Error allocating blocks for relocating metadata
- Error allocating block buffer during relocation process
- Relocating metadata group information from X to Y
- Relocating metatdata group information to X
- Block read error during relocation process
- Block write error during relocation process
- Immutable flag set on a device or socket inode
- Non-zero size for device, fifo or socket inode
- Filesystem revision is 0, but feature flags are set
- Journal inode is not in use, but contains data
- Journal has bad mode
- INDEX_FL flag set on a non-HTREE filesystem
- INDEX_FL flag set on a non-directory
- Invalid root node in HTREE directory
- Unsupported hash version in HTREE directory
- Incompatible flag in HTREE root node
- HTREE too deep
- invalid inode->i_extra_isize
- invalid ea entry->e_name_len
- invalid ea entry->e_value_offs
- invalid ea entry->e_value_block
- invalid ea entry->e_value_size
- invalid ea entry->e_hash
- inode missing EXTENTS_FL, but is an extent inode
- Inode should not have EOFBLOCKS_FL set
- Directory entry has deleted or unused inode
- Directory filetype not set
- Directory filetype set on filesystem
- Invalid HTREE root node
- Invalid HTREE limit
- Invalid HTREE count
- HTREE interior node has out-of-order hashes in table
- Inode found in group where _INODE_UNINIT is set
- Inode found in group unused inodes area
- i_blocks_hi should be zero
- /lost+found not found
- Unattached zero-length inode
- Inode ref count wrong
- Padding at end of inode bitmap is not set.
- Padding at end of block bitmap is not set.
- Block bitmap differences header
- Block not used, but marked in bitmap
- Block used, but not marked used in bitmap
- Block bitmap differences end
- Inode bitmap differences header
- Inode not used, but marked in bitmap
- Inode used, but not marked used in bitmap
- Inode bitmap differences end
- Free inodes count for group wrong
- Directories count for group wrong
- Free inodes count wrong
- Free blocks count for group wrong
- Free blocks count wrong
- Block range not used, but marked in bitmap
- Block range used, but not marked used in bitmap
- Inode range not used, but marked in bitmap
- Inode range used, but not marked used in bitmap
- Group N block(s) in use but group is marked BLOCK_UNINIT
- Group N inode(s) in use but group is marked INODE_UNINIT
- Recreate journal if E2F_FLAG_JOURNAL_INODE flag is set
The following error conditions cause the non-interactive fsck process to abort, even if the -p flag is set:
- Block bitmap not in group
- Inode bitmap not in group
- Inode table not in group
- Filesystem size is wrong
- Inode count in superblock is incorrect
- The Hurd does not support the filetype feature
- Journal has an unknown superblock type
- Ask if we should clear the journal
- Journal superblock has an unknown read-only feature flag set
- Journal superblock has an unknown incompatible feature flag set
- Journal has unsupported version number
- Ask if we should run the journal anyway
- Reserved blocks w/o resize_inode
- Resize_inode not enabled, but resize inode is non-zero
- Resize inode invalid
- Last mount time is in the future
- Last write time is in the future
- group descriptor N checksum is invalid.
- Root directory is not an inode
- Block bitmap conflicts with some other fs block
- Inode bitmap conflicts with some other fs block
- Inode table conflicts with some other fs block
- Block bitmap is on a bad block
- Inode bitmap is on a bad block
- Illegal blocknumber in inode
- Block number overlaps fs metadata
- Inode has illegal blocks (latch question)
- Too many bad blocks in inode
- Illegal block number in bad block inode
- Bad block inode has illegal blocks (latch question)
- Bad block used as bad block indirect block
- Inconsistency can't be fixed prompt
- Bad primary block prompt
- Suppress messages prompt
- Imagic flag set on an inode when filesystem doesn't support it
- Compression flag set on an inode when filesystem doesn't support it
- Deal with inodes that were part of orphan linked list
- Deal with inodes that were part of corrupted orphan linked list (latch question)
- Error reading extended attribute block
- Invalid extended attribute block
- Extended attribute reference count incorrect
- Multiple EA blocks not supported
- Error EA allocation collision
- Bad extended attribute name
- Bad extended attribute value
- Inode too big (latch question)
- Directory too big
- Regular file too big
- Symlink too big
- Bad block has indirect block that conflicts with filesystem block
- Resize inode failed
- inode appears to be a directory
- Error while reading extent tree
- Failure to iterate extents
- Bad starting block in extent
- Extent ends beyond filesystem
- EXTENTS_FL flag set on a non-extents filesystem
- inode has extents, superblock missing INCOMPAT_EXTENTS feature
- Fast symlink has EXTENTS_FL set
- Extents are out of order
- Inode has an invalid extent node
- Clone duplicate/bad blocks?
- Bad inode number for '.'
- Directory entry has bad inode number
- Directry entry is link to '.'
- Directory entry points to inode now located in a bad block
- Directory entry contains a link to a directory
- Directory entry contains a link to the root directry
- Directory entry has illegal characters in its name
- Missing '.' in directory inode
- Missing '..' in directory inode
- First entry in directory inode doesn't contain '.'
- Second entry in directory inode doesn't contain '..'
- i_faddr should be zero
- i_file_acl should be zero
- i_dir_acl should be zero
- i_frag should be zero
- i_fsize should be zero
- inode has bad mode
- directory corrupted
- filename too long
- Directory inode has a missing block (hole)
- '.' is not NULL terminated
- '..' is not NULL terminated
- Illegal character device inode
- Illegal block device inode
- Duplicate '.' entry
- Duplicate '..' entry
- Final rec_len is wrong
- Error reading directory block
- Error writing directory block
- Directory entry for '.' is big. Split?
- Illegal FIFO inode
- Illegal socket inode
- Directory filetype incorrect
- Directory filename is null
- Invalid symlink
- i_file_acl (extended attribute block) is bad
- Filesystem contains large files, but has no such flag in sb
- Clear invalid HTREE directory
- Bad block in htree interior node
- Duplicate directory entry found
- Non-unique filename found
- i_blocks_hi should be zero
- Unexpected HTREE block
- Root inode not allocated
- No room in lost+found
- Unconnected directory inode
- .. entry is incorrect
- Lost+found not a directory
- Unattached inode
- Superblock corrupt
- Fragments not supported
- Error determing physical device size of filesystem
- The external journal has (unsupported) multiple filesystems
- Can't find external journal
- External journal has bad superblock
- Superblock has a bad journal UUID
- Error allocating inode bitmap
- Error allocating block bitmap
- Error allocating icount link information
- Error allocating directory block array
- Error while scanning inodes
- Error while iterating over blocks
- Error while storing inode count information
- Error while storing directory block information
- Error while reading inode (for clearing)
- Error allocating refcount structure
- Error reading Extended Attribute block while fixing refcount
- Error writing Extended Attribute block while fixing refcount
- Error allocating EA region allocation structure
- Error while scanning inodes
- Error allocating inode bitmap
- Internal error: couldn't find dir_info
- Error allocating icount structure
- Error iterating over directory blocks
- Error deallocating inode
- Error adjusting EA refcount
- Error allocating inode bitmap
- Error creating root directory
- Root inode is not directory; aborting
- Cannot proceed without a root inode.
- Internal error: couldn't find dir_info
- Programming error: bitmap endpoints don't match
- Internal error: fudging end of bitmap
- Error copying in replacement inode bitmap
- Error copying in replacement block bitmap
