[RFC] should block layer export bd_set_size() or equivalent?

From: Lou Langholtz (ldl@aros.net)
Date: Tue Jul 01 2003 - 14:04:58 EST


Several drivers (like drivers/block/nbd.c or drivers/block/rd.c) need to
be able to update their devices' size after the first open. The way
fs/block_dev.c is currently implemented (as recently as 2.5.73-mm2)
however, updating the size as seen by user processes is not possible
without directly updating the size stored in various locations of the
struct block_device and the bd_inode as well. As an example, we have the
code in drivers/block/rd.c rd_open() which essentially calls what
fs/block_dev.c calls in its bd_set_size() function. But things are
changing within the block layer still as evidenced by changes to
bd_set_size() between 2.5.73 and 2.5.73-mm2 and consequently
drivers/block/rd.c rd_open() may be incorrect now.


To have fs/block_dev.c share bd_set_size(struct block_device *bdev,
loff_t size) or something like this to better abstract the block layer
from drivers and save us from essentially re-writing the code from here.

Next step:

Discuss this. Why isn't it already exported? What are the cons of just
exporting bd_set_size()? Should we even let drivers change a device's
size after the first open? An alternative for nbd at least, is to have
its user space tool (nbd-client) simply close the device after it has
changed the size and then re-open it before continuing on. This way
fs/block_dev.c do_open() re-checks the disk capacity and calls
bd_set_size() again with the new correct size. Also, bd_set_size() is
very similar to set_blocksize() (the former though changes the byte size
of the block device which is what's most critical really to nbd at
least), perhaps these two functions can be rolled into one; at least
set_blocksize() is already exported.

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