linux-ext4 - Re: [PATCH 0/7 RFC v3] fs: Hole punch vs page cache filling races

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Date:   Tue, 13 Apr 2021 14:09:50 +0100
From:   Christoph Hellwig <hch@...radead.org>
To:     Jan Kara <jack@...e.cz>
Cc:     linux-fsdevel@...r.kernel.org, linux-ext4@...r.kernel.org,
        linux-xfs@...r.kernel.org, Ted Tso <tytso@....edu>,
        Christoph Hellwig <hch@...radead.org>,
        Amir Goldstein <amir73il@...il.com>,
        Dave Chinner <david@...morbit.com>
Subject: Re: [PATCH 0/7 RFC v3] fs: Hole punch vs page cache filling races

> Also when writing the documentation I came across one question: Do we mandate
> i_mapping_sem for truncate + hole punch for all filesystems or just for
> filesystems that support hole punching (or other complex fallocate operations)?
> I wrote the documentation so that we require every filesystem to use
> i_mapping_sem. This makes locking rules simpler, we can also add asserts when
> all filesystems are converted. The downside is that simple filesystems now pay
> the overhead of the locking unnecessary for them. The overhead is small
> (uncontended rwsem acquisition for truncate) so I don't think we care and the
> simplicity is worth it but I wanted to spell this out.

I think all makes for much better to understand and document rules,
so I'd shoot for that eventually.

Btw, what about locking for DAX faults?  XFS seems to take
the mmap sem for those as well currently.