SMP skas requirements

Since you've now split out delete-hostfs, why don't you merge the new hostfs, possibly labeling it as "EXPERIMENTAL"?

Btw, there's a ton of other patches which I don't see reasons for not merging like Al Viro's cleanups (I've given a look to them and they seem safe).

And is punctuaction_fixes likely to cause instability?

Finally, I'd like to get devshm merged if there aren't problems with the code.

Don't know how do you implemented it this time, but since we had special read/write functions for filehandles, the get_fh() and put_fh() could be simply put in their body. Has this changed?

Quoted 6 lines

In this case, get_fh should just increment a count, put_fh should decrement it, and the only list operation should be to move it to the end of the list so it's last to be reclaimed. The reclaimer would not reclaim filehandles with non-zero counts. I see no point in removing it from the list and adding it back, as that seems not to protect against anything.

You need a spinlock on the list. With the non-reference-counted approach, removing that from the list allowed dropping the spinlock over the I/O call on the host. That's not needed with the refcount.

With the refcount, likely you don't need to move it off-list, but that could maybe be useful to avoid looping on unused fd; however, this requires taking the spinlock when reinserting the element on the list.

Instead, with the refcount, if you decrease to 0 the refcount of FH_1 while a reclaim loop is iterating over FH_1, you only risk that FH_1 is missed on that reclaim pass, which isn't a race.

Note: testing that a fd has refcount 0 must be done atomically with freeing it; i.e. even with an atomic_t refcount, it must be incremented only while you have a lock on the list; and while freeing it, you must use atomic_dec_and_lock() so you get a lock on the list if the refcount goes to 0 (atomic_dec_and_lock() is equivalent to taking the lock, doing dec and test, and releasing it, but is faster).

get_fh() {
	spin_lock(&list_lock);
	<iterate on list>
	atomic_inc(fd->count); //while still holding the lock!!
	if (atomic_read(fd->count) <= 0)
		BUG();
	spin_unlock(&list_lock);
}

put_fh(pointer to fd on list "fd") {

	if(atomic_dec_and_lock(fd->count, &list_lock)) {
		//Here we have the lock!
		//Remove the thing from list and free it
	}
}

Quoted 2 lines

You're envisioning the os_* interfaces calling back into filehandle.c to get a descriptor if needed?

To cause fd reclaim, that's what I mean (not sure if you meant the same).

I suggested last time to rename this as "make_reclaimable" (or set_reclaimable), is_reclaimable() sounds as an interrogation method.

Quoted 2 lines

Finally, I'd like to get devshm merged if there aren't problems with the code.

That one needs to check if /dev/shm is present, and fall back to /tmp if not.

Quoting Jeff Dike <[email protected]>:

Quoted 3 lines · Blaisorblade

Quoted 2 lines

Finally, I'd like to get devshm merged if there aren't problems with the code.

Quoted 2 lines

That one needs to check if /dev/shm is present, and fall back to /tmp if not.

Are you proposing that virtual servers automatically using /dev/shm if available and only falling back to the $TMP variable and /tmp directory if not? Or would this keep the use of $TMP if present and fall back to /dev/shm followed by /tmp?

I'm asking because I've optimized multiple host machines to offer a ramfs for virtual servers. This gives me locked memory (which still provides a significant performance improvement over time) for each instance. I would prefer to avoid using tmpfs if possible.

Alternatively, some means for preventing the host from swapping out instances would be appreciated.

Quoted 4 lines · Anthony Brock

Quoting Jeff Dike <[email protected]>:

Quoted 3 lines · Blaisorblade

Quoted 2 lines

Finally, I'd like to get devshm merged if there aren't problems with the code.

Quoted 2 lines

That one needs to check if /dev/shm is present, and fall back to /tmp if not.

Quoted 4 lines

Are you proposing that virtual servers automatically using /dev/shm if available and only falling back to the $TMP variable and /tmp directory if not? Or would this keep the use of $TMP if present and fall back to /dev/shm followed by /tmp?

By looking at:
http://user-mode-linux.sourceforge.net/work/current/2.6/2.6.16/patches/devs
hm

Sorry, forgot to finish. That patch keeps checking TMP,TMPDIR and TEMP (don't
remember the order), but if none is set, it switches the default from /tmp
to /dev/shm.