Re: [PATCH 02/12] mm: Protect operations adding pages to page cache with invalidate_lock

Matthew Wilcox
2021-04-23T18:32:07Z

On Fri, Apr 23, 2021 at 07:29:31PM +0200, Jan Kara wrote:
> Currently, serializing operations such as page fault, read, or readahead
> against hole punching is rather difficult. The basic race scheme is
> like:
>
> fallocate(FALLOC_FL_PUNCH_HOLE) read / fault / ..
> truncate_inode_pages_range()
> <create pages in page
> cache here>
> <update fs block mapping and free blocks>
>
> Now the problem is in this way read / page fault / readahead can
> instantiate pages in page cache with potentially stale data (if blocks
> get quickly reused). Avoiding this race is not simple - page locks do
> not work because we want to make sure there are *no* pages in given
> range.

One of the things I've had in mind for a while is moving the DAX locked
entry concept into the page cache proper. It would avoid creating the
new semaphore, at the cost of taking the i_pages lock twice (once to
insert the entries that cover the range, and once to delete the entries).

It'd have pretty much the same effect, though -- read/fault/... would
block until the entry was deleted from the page cache.