Re: [PATCH 5/7] mm: move zone iterator outside of deferred_init_maxorder()

[Daniel Jordan]

On Thu, May 07, 2020 at 02:18:42PM -0700, Alexander Duyck wrote:
> The idea behind merging ranges it to address possible cases where a
> range is broken up such that there is a hole in a max order block as a
> result.

Gah, yes, you're right, there could be multiple ranges in a max order block, so
the threads have to use the zone iterators to skip the holes.

> By combining the ranges if they both span the same section we
> can guarantee that the entire section will be initialized as a block
> and not potentially have partially initialized sections floating
> around. Without that mo_pfn logic I had in there I was getting panics
> every so often when booting up one of my systems as I recall.
> 
> Also the iterator itself is cheap. It is basically just walking a
> read-only list so it scales efficiently as well. One of the reasons

Agreed, it's not expensive, it's just gnarliness I was hoping to avoid, but
obviously it's not gonna work.

> why I arranged the code the way I did is that it also allowed me to
> get rid of an extra check in the code as the previous code was having
> to verify if the pfn belonged to the node. That is all handled
> directly through the for_each_free_mem_pfn_range_in_zone[_from] call
> now.
> 
> > With the series as it stands plus leaving in the section alignment check in
> > deferred_grow_zone (which I think could be relaxed to a maxorder alignment
> > check) so it doesn't stop mid-max-order-block, threads simply deal with a
> > start/end range and deferred_init_maxorder becomes shorter and simpler too.
> 
> I still think we are better off initializing complete sections since
> the pageblock_flags are fully initialized that way as well.

Fair enough.

> What
> guarantee do you have that all of the memory ranges will be max order
> aligned?

Sure, it's a problem with multiple ranges in a maxorder block, the rest
could've been handled.

> The problem is we have to guarantee all pages are initialized
> before we start freeing the pages in a max order page. If we just
> process each block as-is I believe we can end up with some
> architectures trying to access uninitialized memory in the buddy
> allocator as a result. That is why the deferred_init_maxorder function
> will walk through the iterator, using the _from version to avoid
> unnecessary iteration, the first time initializing the pages it needs
> to cross that max order boundary, and then again to free the max order
> block of pages that have been initialized. The iterator itself is
> farily cheap and only has to get you through the smaller ranges before
> you end up at the one big range that it just kind of sits at while it
> is working on getting it processed.

Right.


Ok, I think we're on the same page for the next version.  Thanks for the
thorough review!