Re: [PATCH 06/43] FS-Cache: Recruit a couple of page flags for cache management [ver #46]

[Nick Piggin <nickpiggin@yahoo.com.au>]

[sorry for that html crap :(]

On Friday 03 April 2009 01:36:12 David Howells wrote:
> Nick Piggin <nickpiggin@yahoo.com.au> wrote:
> 
> > 1) PG_mappedtodisk is basically PG_owner_priv_2. Please alias that and
> > use it? Then at least we're down to 1 extra flag.
> > 2) Why do you need another PG_private?  PG_private for pagecache means
> > that it should call into the filesystem when it needs to handle fs data
> > attached to the page, right? So PG_private_2 doesn't really make sense
> > in that respect.
> 
> Won't that either break fs/buffer.c and fs/mpage.c or preclude the use of
> FS-Cache with block-based filesystems that use the standard buffer wangling
> routines?

Haven't looked closely at how fscache works. Possibly you can't reuse
mappedtodisk....

But PG_private. PG_private from the vm/vfs side means to call into the
filesystem. So from that point, the filesystem should handle it. It just
doesn't seem to make sense to have 2 flags for this.

I mean, there is only the single aop that can be called, so having 2 VM
visible flags doesn't help the VM do anything, and presumably your aop
knows how to handle this, so it should be an fs private bit.

Just give me a situation of why it won't work.


> As I've previously stated, I want to be able to make ISO9660 use FS-Cache.
> That rules out use of PG_mappedtodisk and PG_private for anything FS-Cache
> related.
> 
> We can actually reclaim PG_private, I think.  There are patches to do that.
> At the very least, we can probably reclaim the std buffering code's use of it.

But that's peripheral issue.

 
> If anything, avoiding the need for PG_fscache_write is probably easier - just
> more memory intensive and slower.  I could build a second radix tree for each
> inode that kept track of which pages from that inode FS-Cache knows about, and
> use the status bits in that node to keep track of what pages are being written
> out to the cache.

If it's not much slower, that would be nice.

 
> We still need a way of triggering the page invalidation callbacks for in-use
> pages, however.  PG_private, as I've said, is not currently a viable option.

Can you say exactly why not?