Re: another IDE cleanup: kill duplicated code

[Andrew Morton <akpm@zip.com.au>]

Martin Dalecki wrote:
> 
> If you are already at it, I would like to ask to you consider seriously
> the removal of the
> following entries in the ide drivers /proc control files:
> 
>     ide_add_setting(drive,    "breada_readahead",    ...         1,
> 2,    &read_ahead[major],        NULL);
>     ide_add_setting(drive,    "file_readahead",   ...
> &max_readahead[major][minor],    NULL);
>
> Those calls can be found in ide-cd.c, ide-disk,c and ide-floppy.c

I suspect that if we remove these, we'll one day end up putting them back.
It is appropriate that we be able to control readahead characteristics
on a per-device and per-technology basis.

> The second of them is trying to control a file-system level constant
> inside the actual block device driver.
> This is a blatant violation of the layering principle in software
> design, and should go as soon as
> possible.

Well, the whole design of filesystems and the VM are a blatant
layering violation: all the stuff we do at all levels to try to
perform block-contiguous reads and writes, and to keep related
data at related LBAs is making implicit assumptions about the
underlying physical storage technology.  We've been doing that
for 30 years - if a constant access time storage technology
takes over from rotating and seeking disks, we have a lot of stuff
to toss out.

That being said, it is a horrid user interface wart that IDE readhead
is controlled from /proc/ide/hda/settings:file_readhead, and that IDE
ignores /proc/sys/vm/max-readahead, whereas devices which forget to
set up their max_readhead entries are controlled by /proc/sys/vm/max-readhead.

My vote would be to remove both of them.  Move the readhead controls
into the filemap level, but implement per-device controls.  Say,
/proc/sys/vm/readhead/hda, /proc/sys/vm/readhead/sdc, etc.

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