Re: [PATCH 03/11] mm: Protect operations adding pages to page cache with invalidate_lock

Matthew Wilcox
2021-05-12T14:41:57Z

On Wed, May 12, 2021 at 03:46:11PM +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. inode->i_rwsem does not work because page fault happens under
> mmap_sem which ranks below inode->i_rwsem. Also using it for reads makes
> the performance for mixed read-write workloads suffer.
>
> So create a new rw_semaphore in the address_space - invalidate_lock -
> that protects adding of pages to page cache for page faults / reads /
> readahead.

Remind me (or, rather, add to the documentation) why we have to hold the
invalidate_lock during the call to readpage / readahead, and we don't just
hold it around the call to add_to_page_cache / add_to_page_cache_locked
/ add_to_page_cache_lru ? I appreciate that ->readpages is still going
to suck, but we're down to just three implementations of ->readpages now
(9p, cifs & nfs).

Also, could I trouble you to run the comments through 'fmt' (or
equivalent)? It's easier to read if you're not kissing right up on 80
columns.

> +++ b/fs/inode.c
> @@ -190,6 +190,9 @@ int inode_init_always(struct super_block *sb, struct inode *inode)
> mapping_set_gfp_mask(mapping, GFP_HIGHUSER_MOVABLE);
> mapping->private_data = NULL;
> mapping->writeback_index = 0;
> + init_rwsem(&mapping->invalidate_lock);
> + lockdep_set_class(&mapping->invalidate_lock,
> + &sb->s_type->invalidate_lock_key);

Why not:

__init_rwsem(&mapping->invalidate_lock, "mapping.invalidate_lock",
&sb->s_type->invalidate_lock_key);