Re: [PATCH v4 2/2] mm: Init page count in reserve_bootmem_region when MEMINIT_EARLY

[Mike Rapoport]

On Mon, Oct 02, 2023 at 10:56:51AM +0200, David Hildenbrand wrote:
> On 02.10.23 10:47, Mike Rapoport wrote:
> > On Mon, Oct 02, 2023 at 03:03:56PM +0800, Yajun Deng wrote:
> > > 
> > > On 2023/10/2 02:59, Mike Rapoport wrote:
> > > > On Fri, Sep 29, 2023 at 06:27:25PM +0800, Yajun Deng wrote:
> > > > > On 2023/9/29 18:02, Mike Rapoport wrote:
> > > > > > > > > diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> > > > > > > > > index 06be8821d833..b868caabe8dc 100644
> > > > > > > > > --- a/mm/page_alloc.c
> > > > > > > > > +++ b/mm/page_alloc.c
> > > > > > > > > @@ -1285,18 +1285,22 @@ void __free_pages_core(struct page *page, unsigned int order)
> > > > > > > > >      	unsigned int loop;
> > > > > > > > >      	/*
> > > > > > > > > -	 * When initializing the memmap, __init_single_page() sets the refcount
> > > > > > > > > -	 * of all pages to 1 ("allocated"/"not free"). We have to set the
> > > > > > > > > -	 * refcount of all involved pages to 0.
> > > > > > > > > +	 * When initializing the memmap, memmap_init_range sets the refcount
> > > > > > > > > +	 * of all pages to 1 ("reserved" and "free") in hotplug context. We
> > > > > > > > > +	 * have to set the refcount of all involved pages to 0. Otherwise,
> > > > > > > > > +	 * we don't do it, as reserve_bootmem_region only set the refcount on
> > > > > > > > > +	 * reserve region ("reserved") in early context.
> > > > > > > > >      	 */
> > > > > > > > Again, why hotplug and early init should be different?
> > > > > > > I will add a comment that describes it will save boot time.
> > > > > > But why do we need initialize struct pages differently at boot time vs
> > > > > > memory hotplug?
> > > > > > Is there a reason memory hotplug cannot have page count set to 0 just like
> > > > > > for pages reserved at boot time?
> > > > > This patch just save boot time in MEMINIT_EARLY. If someone finds out that
> > > > > it can save time in
> > > > > 
> > > > > MEMINIT_HOTPLUG, I think it can be done in another patch later. I just
> > > > > keeping it in the same.
> > > > But it's not the same. It becomes slower after your patch and the code that
> > > > frees the pages for MEMINIT_EARLY and MEMINIT_HOTPLUG becomes non-uniform
> > > > for no apparent reason.
> > > 
> > > __free_pages_core will also be called by others, such as:
> > > deferred_free_range, do_collection and memblock_free_late.
> > > 
> > > We couldn't remove  'if (page_count(page))' even if we set page count to 0
> > > when MEMINIT_HOTPLUG.
> > 
> > That 'if' breaks the invariant that __free_pages_core is always called for
> > pages with initialized page count. Adding it may lead to subtle bugs and
> > random memory corruption so we don't want to add it at the first place.
> 
> As long as we have to special-case memory hotplug, we know that we are
> always coming via generic_online_page() in that case. We could either move
> some logic over there, or let __free_pages_core() know what it should do.

Looks like the patch rather special cases MEMINIT_EARLY, although I didn't
check throughfully other code paths. 
Anyway, relying on page_count() to be correct in different ways for
different callers of __free_pages_core() does not sound right to me.
 
> -- 
> Cheers,
> 
> David / dhildenb
> 

-- 
Sincerely yours,
Mike.