Re: [PATCH] btrfs: simplify read_extent_buffer_pages a bit

David Sterba
2019-04-17T12:19:06Z

On Tue, Apr 16, 2019 at 11:16:33AM +0200, Johannes Thumshirn wrote:
>
> On Thu, Apr 11, 2019 at 06:59:35PM +0200, David Sterba wrote:
> > On Thu, Apr 11, 2019 at 06:40:59PM +0200, Johannes Thumshirn wrote:
> > > Currently read_extent_buffer_pages() uses a 4 pass algorithm to read an
> > > extent buffer's pages from disk, all 4 stages looping over all pages of
> > > the extent buffer.
> > >
> > > 1) Loop over all pages and lock them.
> > > 2) Loop over all pages and see if one is not marked as PageUptodate, so we
> > > can break out of the function early.
> > > 3) Loop over all pages and if the page is !PageUptodate read the page,
> > > otherwise unlock the page.
> > > 4) Loop over all pages and wait for stable pages.
> > >
> > > Unify the 1st two for loops, we can count the number of uptodate pages
> > > after we have locked them without the need for re-starting the loop.
> >
> > Effectively reverting 2571e739677f1e4c0c63f5ed49adcc0857923625 that
> > fixed a bug. Please have a look if something has really changed so that
> > the split loops are no longer necessary.
>
> After starring at the .s and .lst files GCC produces I cannot say whether this
> holds true or not.

I'm not sure this is about the assembly. Currently all pages are locked
first, then the bits are checked. Merging that locking and setting bit
to the same iteration has different effect to the outside, as mentioned
in the commit.