On Fri, 20 Apr 2007, David Chinner wrote:
> So looking at this the main thing for converting a filesystem is some extra
> bits in the mount process and replacing PAGE_CACHE_* macros with
> page_cache_*() wrapper functions.
Right.
> We can probably set all this up trivially with XFS by allowing block size >
> page
> size filesystems to be mounted and modifying the way we feed pages to a bio
> to be aware of compound pages.
That would be great! Anyone volunterering for the block layer?
> > What is currently not supported:
> > - Buffer heads for higher order pages (possible with the compound pages in
> > mm
> > that do not use page->private requires upgrade of the buffer cache
> > layers).
>
> Does this mean that the -mm code will currently support bufferheads on
> compound
> pages? We need that before we can get XFS to work with compound pages.
There needs to be some work done on that level. But page->private can be
used for compound pages now which should make this simple to do.
> > - Higher order pages in the block layer etc.
>
> It's more drivers that we have to worry about, I think. We don't need to
> modify bios to explicitly support compound pages. From bio.h:
>
> /*
> * was unsigned short, but we might as well be ready for > 64kB I/O pages
> */
> struct bio_vec {
> struct page *bv_page;
> unsigned int bv_len;
> unsigned int bv_offset;
> };
>
> So compound pages should be transparent to anything that doesn't
> look at the contents of bio_vecs....
Great!
> > - Mmapping higher order pages
>
> *nod*
>
> hmmm - what about the way we do copyin and copyout from the page cache? ie
> we kmap_atomic() them before we access them. Does this need to change?
kmap_atomic does not do anything if we do not use highmem. If we want to
support highmem with higher order pages then kmap_atomic needs to support
arbitrary page orders.
>> > The ramfs driver can be used to test higher order page cache functionality
> > (and may help troubleshoot the VM support until we get some real filesystem
> > and real devices supporting higher order pages).
>
> I don't think it will take much to get XFS to work with a high order
> page cache and we can probably insulate the block layer initially with some
> kind of bio_add_compound_page() wrapper and some similar
> wrapper on the io completion side.
I'd be happy if we could make this work soon.
> So far it's much less intrusive than I expected ;)
I was surprised too. Seems that multiple people have been preparing for
the great day when we finally support higher order pages in the page
cache.
-
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to [EMAIL PROTECTED]
More majordomo info at http://vger.kernel.org/majordomo-info.html
Please read the FAQ at http://www.tux.org/lkml/
