linux-ext4 - Re: [PATCH 03/11] mm: Protect operations adding pages to page cache with invalidate

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Date:   Wed, 12 May 2021 15:40:21 +0100
From:   Matthew Wilcox <willy@...radead.org>
To:     Jan Kara <jack@...e.cz>
Cc:     linux-fsdevel@...r.kernel.org,
        Christoph Hellwig <hch@...radead.org>,
        Dave Chinner <david@...morbit.com>, ceph-devel@...r.kernel.org,
        Chao Yu <yuchao0@...wei.com>,
        Damien Le Moal <damien.lemoal@....com>,
        "Darrick J. Wong" <darrick.wong@...cle.com>,
        Jaegeuk Kim <jaegeuk@...nel.org>,
        Jeff Layton <jlayton@...nel.org>,
        Johannes Thumshirn <jth@...nel.org>,
        linux-cifs@...r.kernel.org, linux-ext4@...r.kernel.org,
        linux-f2fs-devel@...ts.sourceforge.net, linux-mm@...ck.org,
        linux-xfs@...r.kernel.org, Miklos Szeredi <miklos@...redi.hu>,
        Steve French <sfrench@...ba.org>, Ted Tso <tytso@....edu>
Subject: Re: [PATCH 03/11] mm: Protect operations adding pages to page cache
 with invalidate_lock

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);