Messages in this thread |  | | Subject | Re: Poor floppy performance in kernel 2.4.10 | Date | Sat, 27 Oct 2001 20:00:22 +0200 | From | Alain Knaff <> |
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> > >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,
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