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

[Daniel Jordan]

Hi Alex,

On Thu, Apr 30, 2020 at 02:43:28PM -0700, Alexander Duyck wrote:
> On 4/30/2020 1:11 PM, Daniel Jordan wrote:
> > padata will soon divide up pfn ranges between threads when parallelizing
> > deferred init, and deferred_init_maxorder() complicates that by using an
> > opaque index in addition to start and end pfns.  Move the index outside
> > the function to make splitting the job easier, and simplify the code
> > while at it.
> > 
> > deferred_init_maxorder() now always iterates within a single pfn range
> > instead of potentially multiple ranges, and advances start_pfn to the
> > end of that range instead of the max-order block so partial pfn ranges
> > in the block aren't skipped in a later iteration.  The section alignment
> > check in deferred_grow_zone() is removed as well since this alignment is
> > no longer guaranteed.  It's not clear what value the alignment provided
> > originally.
> > 
> > Signed-off-by: Daniel Jordan <daniel.m.jordan@xxxxxxxxxx>
> 
> So part of the reason for splitting it up along section aligned boundaries
> was because we already had an existing functionality in deferred_grow_zone
> that was going in and pulling out a section aligned chunk and processing it
> to prepare enough memory for other threads to keep running. I suspect that
> the section alignment was done because normally I believe that is also the
> alignment for memory onlining.

I think Pavel added that functionality, maybe he could confirm.

My impression was that the reason deferred_grow_zone aligned the requested
order up to a section was to make enough memory available to avoid being called
on every allocation.

> With this already breaking things up over multiple threads how does this
> work with deferred_grow_zone? Which thread is it trying to allocate from if
> it needs to allocate some memory for itself?

I may not be following your question, but deferred_grow_zone doesn't allocate
memory during the multithreading in deferred_init_memmap because the latter
sets first_deferred_pfn so that deferred_grow_zone bails early.

> Also what is to prevent a worker from stop deferred_grow_zone from bailing
> out in the middle of a max order page block if there is a hole in the middle
> of the block?

deferred_grow_zone remains singlethreaded.  It could stop in the middle of a
max order block, but it can't run concurrently with deferred_init_memmap, as
per above, so if deferred_init_memmap were to init 'n free the remaining part
of the block, the previous portion would have already been initialized.