Re: [PATCH] kho: initialize tail pages for higher order folios properly

[Mike Rapoport <rppt@kernel.org>]

Hi Pratyush,

On Fri, Jun 06, 2025 at 06:23:06PM +0200, Pratyush Yadav wrote:
> Hi Mike,
> 
> On Fri, Jun 06 2025, Mike Rapoport wrote:
> 
> > On Thu, Jun 05, 2025 at 07:11:41PM +0200, Pratyush Yadav wrote:
> >> From: Pratyush Yadav <ptyadav@amazon.de>
> >> 
> >> --- a/kernel/kexec_handover.c
> >> +++ b/kernel/kexec_handover.c
> >> @@ -157,11 +157,21 @@ static int __kho_preserve_order(struct kho_mem_track *track, unsigned long pfn,
> >>  }
> >> 
> >>  /* almost as free_reserved_page(), just don't free the page */
> >> -static void kho_restore_page(struct page *page)
> >> +static void kho_restore_page(struct page *page, unsigned int order)
> >>  {
> >> -	ClearPageReserved(page);
> >
> > So now we don't clear PG_Reserved even on order-0 pages? ;-)
> 
> We don't need to. As I mentioned in the commit message as well,
> PG_Reserved is never set for KHO pages since they are reserved with
> MEMBLOCK_RSRV_NOINIT, so memmap_init_reserved_pages() skips over them.

You are right, I missed it.
 
> That said, while reading through some of the code, I noticed another
> bug: because KHO reserves the preserved pages as NOINIT, with
> CONFIG_DEFERRED_STRUCT_PAGE_INIT == n, all the pages get initialized
> when memmap_init_range() is called from setup_arch (paging_init() on
> x86). This happens before kho_memory_init(), so the KHO-preserved pages
> are not marked as reserved to memblock yet.
> 
> With deferred page init, some pages might not get initialized early, and
> get initialized after kho_memory_init(), by which time the KHO-preserved
> pages are marked as reserved. So, deferred_init_maxorder() will skip
> over those pages and leave them uninitialized.
> 
> So we need to either also call init_deferred_page(), or remove the
> memblock_reserved_mark_noinit() call in deserialize_bitmap(). And TBH, I
> am not sure why KHO pages even need to be marked noinit in the first
> place. Probably the only benefit would be if a large chunk of memory is
> KHO-preserved, the pages can be initialized later on-demand, reducing
> bootup time a bit.

One benefit is performance indeed, because in not deferred case the
initialization of reserved pages in memmap_init_reserved_pages() is really
excessive.

But more importantly, if we remove memblock_reserved_mark_noinit(), with
CONFIG_DEFERRED_STRUCT_PAGE_INIT we'd loose page->private because the
struct page will be cleared after kho_mem_deserialize().
 
> What do you think? Should we drop noinit or call init_deferred_page()?
> FWIW, my preference is to drop noinit, since init_deferred_page() is
> __meminit and we would have to make sure it doesn't go away after boot.

We can't drop noinit and calling init_deferred_page() after boot just won't
work because it uses memblock to find the page's node and memblock is gone
after init.

The simplest short-term solution is to disable KHO when
CONFIG_DEFERRED_STRUCT_PAGE_INIT is set and then find an efficient way to
make it all work together.
 
-- 
Sincerely yours,
Mike.