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

[Daniel Jordan]

On Mon, May 04, 2020 at 03:10:46PM -0700, Alexander Duyck wrote:
> So we cannot stop in the middle of a max order block. That shouldn't
> be possible as part of the issue is that the buddy allocator will
> attempt to access the buddy for the page which could cause issues if
> it tries to merge the page with one that is not initialized. So if
> your code supports that then it is definitely broken. That was one of
> the reasons for all of the variable weirdness in
> deferred_init_maxorder. I was going through and making certain that
> while we were initializing the range we were freeing the pages in
> MAX_ORDER aligned blocks and skipping over whatever reserved blocks
> were there. Basically it was handling the case where a single
> MAX_ORDER block could span multiple ranges.
> 
> On x86 this was all pretty straightforward and I don't believe we
> needed the code, but I seem to recall there were some other
> architectures that had more complex memory layouts at the time and
> that was one of the reasons why I had to be careful to wait until I
> had processed the full MAX_ORDER block before I could start freeing
> the pages, otherwise it would start triggering memory corruptions.

Yes, thanks, I missed the case where deferred_grow_zone could stop
mid-max-order-block.

Maybe it's better to leave deferred_init_maxorder alone and adapt the
multithreading to the existing implementation.  That'd mean dealing with the
pesky opaque index somehow, so deferred_init_mem_pfn_range_in_zone() could be
generalized to find it in the thread function based on the start/end range, or
it could be maintained as part of the range that padata passes to the thread
function.

Or, keep this patch but make sure deferred_grow_zone stops on a
max-order-aligned boundary.