Re: [RFC] Generic deferred file writing

[Roman Zippel]

Hi,

On Sun, 31 Dec 2000, Linus Torvalds wrote:

> 		cached_allocation = NULL;
> 
> 	repeat:
> 		spin_lock();
> 		result = try_to_find_existing();
> 		if (!result) {
> 			if (!cached_allocation) {
> 				spin_unlock();
> 				cached_allocation = allocate_block();
> 				goto repeat;
> 			}
> 			result = cached_allocation;
> 			add_to_datastructures(result);
> 		}
> 		spin_unlock();
> 		return result;
> 
> This is quite standard, and Linux does it in many places. It doesn't have
> to be complex or ugly.

No problem with that.

> Also, I don't see why you claim the current get_block() is recursive and
> hard to use: it obviously isn't. If you look at the current ext2
> get_block(), the way it protects most of its data structures is by the
> super-block-global lock. That wouldn't work if your claims of recursive
> invocation were true. 

I just rechecked that, but I don't see no superblock lock here, it uses
the kernel_lock instead. Although Al could give the definitive answer for
this, he wrote it. :)

> The way the Linux MM works, if the lower levels need to do buffer
> allocations, they will use GFP_BUFFER (which "bread()" does internally),
> which will mean that the VM layer will _not_ call down recursively to
> write stuff out while it's already trying to write something else. This is
> exactly so that filesystems don't have to release and re-try if they don't
> want to.
> 
> In short, I don't see any of your arguments.

Then I must have misunderstood Al. Al?
If you were right here, I would see absolutely no reason for the current
complexity. (Me is a bit confused here.)

bye, Roman