On Wed, Jan 20, 2021 at 05:06:08PM +0100, Jan Kara wrote:

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
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().

Doesn't this indicate that we're doing truncates in the wrong order?
ie first we should deallocate the blocks, then we should free the page
cache that was caching the contents of those blocks.  We'd need to
make sure those pages in the page cache don't get written back to disc
(either by taking pages in the page cache off the lru list or having
the filesystem handle writeback of pages to a freed extent as a no-op).