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

[Yajun Deng]

On 2023/10/2 19:25, David Hildenbrand wrote:
> On 02.10.23 13:10, Mike Rapoport wrote:
> > 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.
>
> Absolutely agreed.
I already sent v5  a few days ago. Comments, please...