Re: [RFC] Generic deferred file writing

[Alexander Viro]

On Tue, 2 Jan 2001, Roman Zippel wrote:

> Block allocation is not my problem right now (and even directory handling
> is not that difficult), but I will post somethings about this on fsdevel
> later.
> But one question is still open, I'd really like an answer for:
> Is it possible to use a per-inode-indirect-block-semaphore?

Depends on a filesystem. Generally you don't want asynchronous operations
to grab semaphores shared with something else. kswapd knows to skip the locked
pages, but that's it - if writepage() is going to block on a semaphore you
will not know what had hit you. And while buffer-cache operations will not
trigger writepage() (grep for GFP_BUFFER and GFP_IO and you'll see) you have
no such warranties for other sources of memory pressure. If one of them
hits while you are holding such semaphore - you are toast.

We probably could pull it off for ext2_truncate() vs. ext2_get_block()
but it would not do us any good. It would give excessive exclusion for
operations that can be done in parallel just fine (example: we have
a hole from 100Kb to 200Kb. Pageouts in that area can be trivially
done i parallel - current code will not even try to do unrolls. With
your locking they will be serialized for no good reason). What for?