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SubjectRe: Poor floppy performance in kernel 2.4.10
Date
From
>
>
>On Sat, 27 Oct 2001, Alain Knaff wrote:
>
>> Cursory examination of floppy.c (as an example of a block device
>> driver) showed that bdops are also registered using devfs_register and
>> register_disk (what's THAT for?!? Floppies don't have partitions...)
>
>Actually, _that_ is Right Thing(tm) - it should allocate a structure that
>would contain pointer to methods table and would be controlled by
>driver. devfs_register() would get that + prefered name, etc. so that
>we had a common object. Then driver would have a point where it could
>tell the rest of kernel that disk is gone.

Register_disk seems to be related to partitions... and is yet another
place where floppy_fops is handed out. And it doesn't seem to have a
corresponding unregister_disk function, so this worries me
somewhat. Who did that, and why didn't he contact me?

>> Apparently, devfs_register allows a direct mapping from the device's
>> name to its driver, without going through its major/minor number.
>>
>> Thus, a possible solution would be to equip all possible paths leading
>> to the driver's block_device_operations with correct "teardown"
>> function. Thus, not only unregister_blkdev would dump the cache, but
>> also devfs_unregister (maybe near the place in unregister() where
>> de->u.fcb.ops = NULL is done?). Best make this call generic, such as
>
>No go. We can have situations where some of uses come from devfs and
>some - from normal device nodes. struct block_device will be the same.

Ok. So maybe some kind of counter? When it drops to zero, dump the
cache?

>> All this begs of course the following question: what kind of
>> idnetifier does the buffer cache code actually use to refer to the
>> block device, if there is no longer a major?
>
>Right now - major:minor, in 2.5 - struct block_device *.

Good. But then, what's the point of devfs=only ? I assumed this was
intended for situations where we had a direct mapping from filename to
device.

Ok, so in 2.5 will be possible with struct block_device, and the
option will make sense.

So, in the interest of stability, shouldn't we (temporarily) disable
this devfs=only stuff in 2.4 ?

>> We could either use bdev->bd_sem (awkward, as many drivers implement
>> multiple bdev's), or a new per-major device lock to protect that
>> section.
>
>I'd rather have refcount raised by get_blkfops(). Again, that code path
>is not a problem. devfs_get_ops() is.
>
>> unregister_blkdev would need to acquire the same lock while zero-ing
>> blkdevs[major].bdops.
>
>We could put bdev on per-major cyclic list and have it killed on
>unregister_blkdev(). _That_ is easy. The trouble being, with devfs
>we don't have a single removal point. Sometimes it's still
>unregister_blkdev(), sometimes - crapload of devfs_unregister() for
>each minor, sometimes - both. Worse yet, we have one more place that
>holds pointer to block_device_operations - gendisk. Also used by
>devfs (and nothing else) and logics is, to put it mildly, fuzzy.
>
>Frankly, at that point I would prefer to remove the code in devfs that
>tries to provide bdev methods by devfs entry. Rationale:
> a) it's fucked up beyond any repair
> b) it will be useless until we switch buffer cache to block_device *
> c) we will need to change that logics anyway - as it is the thing is
>inherently racy
> d) right now it stands in the way of long-living cache stuff _and_
>introduces an oopsable race between mount and rmmod.
>

I agree. We could maybe just #ifdef those methods out, so that we
could easily add them back in 2.5 once struct block_device is in
place.

Regards,

Alain
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