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Date:   Tue, 19 Sep 2023 12:34:17 -0400
From:   "Theodore Ts'o" <tytso@mit.edu>
To:     Matthew Wilcox <willy@infradead.org>
Cc:     Dave Chinner <david@fromorbit.com>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        NeilBrown <neilb@suse.de>,
        James Bottomley <James.Bottomley@hansenpartnership.com>,
        Eric Sandeen <sandeen@sandeen.net>,
        Steven Rostedt <rostedt@goodmis.org>,
        Guenter Roeck <linux@roeck-us.net>,
        Christoph Hellwig <hch@infradead.org>, ksummit@lists.linux.dev,
        linux-fsdevel@vger.kernel.org
Subject: Re: [MAINTAINERS/KERNEL SUMMIT] Trust and maintenance of file systems
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On Tue, Sep 19, 2023 at 06:17:21AM +0100, Matthew Wilcox wrote:
> Frustratingly, it looks like buffer_heads were intended to be used as
> extents; each one has a b_size of its own.  But there's a ridiculous
> amount of code that assumes that all BHs attached to a folio have the
> same b_size as each other.

The primary reason why we need a per-bh b_size is for the benefit of
non-iomap O_DIRECT code paths.  If that goes away, then we can
simplify this significantly, since we flush the buffer cache whenever
we change the blocksize used in the buffer cache; the O_DIRECT bh's
aren't part of the buffer cache, which is when you might have bh's with
a b_size of 8200k (when doing a 8200k O_DIRECT read or write).

Cheers,

						- Ted