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Date:   Fri, 23 Apr 2021 19:30:40 +0100
From:   Matthew Wilcox <willy@infradead.org>
To:     Jan Kara <jack@suse.cz>
Cc:     linux-fsdevel@vger.kernel.org,
        Christoph Hellwig <hch@infradead.org>,
        Amir Goldstein <amir73il@gmail.com>,
        Dave Chinner <david@fromorbit.com>, Ted Tso <tytso@mit.edu>,
        ceph-devel@vger.kernel.org, Chao Yu <yuchao0@huawei.com>,
        Damien Le Moal <damien.lemoal@wdc.com>,
        "Darrick J. Wong" <darrick.wong@oracle.com>,
        Hugh Dickins <hughd@google.com>,
        Jaegeuk Kim <jaegeuk@kernel.org>,
        Jeff Layton <jlayton@kernel.org>,
        Johannes Thumshirn <jth@kernel.org>,
        linux-cifs@vger.kernel.org, linux-ext4@vger.kernel.org,
        linux-f2fs-devel@lists.sourceforge.net, linux-mm@kvack.org,
        linux-xfs@vger.kernel.org, Miklos Szeredi <miklos@szeredi.hu>,
        Steve French <sfrench@samba.org>
Subject: Re: [PATCH 02/12] mm: Protect operations adding pages to page cache
 with invalidate_lock
Message-ID: <20210423183040.GD235567@casper.infradead.org>
References: <20210423171010.12-1-jack@suse.cz>
 <20210423173018.23133-2-jack@suse.cz>
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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.