[PATCH 0/2 RFC v2] fs: Hole punch vs page cache filling races

Jan Kara
2021-02-08T16:40:49Z

Hello,

Amir has reported [1] a that ext4 has a potential issues when reads can race
with hole punching possibly exposing stale data from freed blocks or even
corrupting filesystem when stale mapping data gets used for writeout. The
problem is that during hole punching, new page cache pages can get instantiated
and block mapping from the looked up in a punched range after
truncate_inode_pages() has run but before the filesystem removes blocks from
the file. In principle any filesystem implementing hole punching thus needs to
implement a mechanism to block instantiating page cache pages during hole
punching to avoid this race. This is further complicated by the fact that there
are multiple places that can instantiate pages in page cache. We can have
regular read(2) or page fault doing this but fadvise(2) or madvise(2) can also
result in reading in page cache pages through force_page_cache_readahead().

There are couple of ways how to fix this. First way (currently implemented by
XFS) is to protect read(2) and *advise(2) calls with i_rwsem so that they are
serialized with hole punching. This is easy to do but as a result all reads
would then be serialized with writes and thus mixed read-write workloads suffer
heavily on ext4. Thus this series introduces inode->i_mapping_sem and uses it
when creating new pages in the page cache and looking up their corresponding
block mapping. We also replace EXT4_I(inode)->i_mmap_sem with this new rwsem
which provides necessary serialization with hole punching for ext4. If this
approach looks viable, I'll convert also other equivalent fs locks to use this
new VFS semaphore instead - in particular XFS' XFS_MMAPLOCK, f2fs's i_mmap_sem,
fuse's i_mmap_sem and maybe others as well.

Honza

[1] https://lore.kernel.org/linux-fsdevel/CAOQ4uxjQNmxqmtA_VbYW0Su9rKRk2zobJmahcyeaEVOFKVQ5dw@mail.gmail.com/