Re: [PATCH v2 04/13] fs: split off __alloc_page_buffers function

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On Sun, Feb 20, 2022 at 04:23:50AM +0000, Matthew Wilcox wrote:
> On Fri, Feb 18, 2022 at 11:35:10PM -0800, Christoph Hellwig wrote:
> > Err, hell no.  Please do not add any new functionality to the legacy
> > buffer head code.  If you want new features do that on the
> > non-bufferhead iomap code path only please.
> 
> I think "first convert the block device code from buffer_heads to iomap"
> might be a bit much of a prerequisite.  I think running ext4 on top of a
> block device still requires buffer_heads, for example (I tried to convert
> the block device to use mpage in order to avoid creating buffer_heads
> when possible, and ext4 stopped working.  I didn't try too hard to debug
> it as it was a bit of a distraction at the time).

Oh, I did not spot the users here is the block device.  Which is really
weird, why would anyone do buffered writes to a block devices?  Doing
so is a bit of a data integrity nightmare.

Can we please develop this feature for iomap based file systems first,
and if by then a use case for block devices arises I'll see what we can
do there.  I've been planning to get the block device code to stop using
buffer_heads by default, but taking them into account if used by a
legacy buffer_head user anyway.



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