On Wed, Jan 12, 2011 at 08:16:05AM -0500, Christoph Hellwig wrote:
> > - if (bp->b_pages != bp->b_page_array) {
> > + if (bp->b_pages && bp->b_pages != bp->b_page_array) {
>
> If bp->p_ages is NULL it's per defintion different from b_page_array.
True. I'll drop that mod.
>
> > STATIC int
> > -_xfs_buf_lookup_pages(
> > +xfs_buf_allocate_buffer(
>
> This is a bit misnamed and rather confusing vs xfs_buf_allocate.
>
> Maybe call it xfs_buf_allocate_memory?
Yes, seems lik a better name.
>
> > + /*
> > + * for buffers that are contained within a single page, just allocate
> > + * the memory from the heap - there's no need for the complexity of
> > + * page arrays to keep allocation down to order 0.
> > + */
> > + if (bp->b_buffer_length <= PAGE_SIZE) {
>
> I think this should be a <, not <= - for page sized buffers avoiding
> the slab overhead should be much more efficient.
*nod*
The patch has been sitting around for a couple of weeks, and when I
did a quick look at it before posting it I wondered if I should make
that exact change before posting it. I decided not to because I
didn't want to wait for retesting before posting...
>
> > + page_count = 1;
> > + } else {
> > + end = bp->b_file_offset + bp->b_buffer_length;
> > + page_count = xfs_buf_btoc(end) -
> > + xfs_buf_btoct(bp->b_file_offset);
> > + }
> >
> > error = _xfs_buf_get_pages(bp, page_count, flags);
> > if (unlikely(error))
> > return error;
> > +
> > + if (bp->b_page_count == 1) {
>
> I'd just duplicate the _xfs_buf_get_pages inside the branches to make
> this a lot cleaner. The page_count calculation in the else clause can
> never end up as 1 anyway. Maybe even make it two different functions
> and let the only caller decide which one to use.
OK, I'll rework the logic here to clean up the flow.
> > + unsigned long pageaddr;
> > +
> > + bp->b_addr = kmem_alloc(bp->b_buffer_length, xb_to_km(flags));
> > + if (!bp->b_addr)
> > + return ENOMEM;
> > +
> > + pageaddr = (unsigned long)bp->b_addr & PAGE_MASK;
> > + ASSERT(((unsigned long)(bp->b_addr + bp->b_buffer_length - 1) & PAGE_MASK) == pageaddr);
> > +
> > + bp->b_offset = (unsigned long)bp->b_addr - pageaddr;
>
> This can just be
>
> bp->b_offset = offset_in_page(bp->b_addr);
>
> > + bp->b_pages[0] = virt_to_page((void *)pageaddr);
>
> and this
>
> bp->b_pages[0] = virt_to_page(bp->b_addr);
>
> with that the above assert can be changed to not need a local variable
> and imho be slightly cleaner:
>
> ASSERT(((unsigned long)bp->b_addr + bp->b_buffer_length - 1) &
> PAGE_MASK) ==
> (unsigned long)bp->b_addr & PAGE_MASK);
Good idea, it can definitely be made cleaner.
> although I'm not sure it's actually correct. slub is a pretty
> aggressive in using high order pages and might give us back memory that
> goes across multiple pages. Adding a loop here to assign multiple pages
> if needed seems safer.
Hmmm. I've been using SLUB and not seen any assert triggers. I put
the assert there because I was concerned about whether this could
occur, but the heap is backed by power-of-two sized slabs so I think
that slub doesn't trigger it.
I'll have a think about how to handle buffers that span multiple
pages cleanly - like you said a loop is probably the easiest way to
handle it.
> > + bp->b_flags |= XBF_MAPPED|_XBF_KMEM;
>
> Space around the | operator, please.
>
> > + bp->b_flags &= XBF_MAPPED|_XBF_KMEM|_XBF_PAGES;
>
> here, too.
Will do. As you've noticed, the code is pretty rough still. :/
> Additional comments:
>
> - the _XBF_PAGE_CACHE and _XBF_PAGE_LOCKED flags are now unused and can
> be removed.
> - the comment describing xfs_buf_t around line 141 in xfs_buf.h is now
> incorrect. It's not overly useful so I'd suggest removing it
> completely.
> - the comments above the buffer locking functions in xfs_buf.c
> (including the meta-comment) are now incorrect and should be
> fixed/removed.
> - the call to mark_page_accessed in xfs_buf_get is now superflous,
> as it only has a meaning for pagecache pages.
I'll clean them up for the next version. Thanks for the initial
sanity check, Christoph.
Cheers,
Dave.
--
Dave Chinner
david@xxxxxxxxxxxxx
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