Re: [PATCH v2] mm/mm_init: Don't iterate pages below ARCH_PFN_OFFSET

[Ruihan Li <lrh2000@pku.edu.cn>]

Hi Mike,

Thanks for your review!

On Mon, Apr 21, 2025 at 07:41:43PM +0300, Mike Rapoport wrote:
> On Sat, Apr 19, 2025 at 08:28:01PM +0800, Ruihan Li wrote:
> > Currently, memmap_init initializes pfn_hole with 0 instead of
> > ARCH_PFN_OFFSET. Then init_unavailable_range will start iterating each
> > page from the page at address zero to the first available page, but it
> > won't do anything for pages below ARCH_PFN_OFFSET because pfn_valid
> > won't pass.
> > 
> > If ARCH_PFN_OFFSET is very large (e.g., something like 2^64-2GiB if the
> > kernel is used as a library and loaded at a very high address), the
> > pointless iteration for pages below ARCH_PFN_OFFSET will take a very
> > long time, and the kernel will look stuck at boot time.
> > 
> > This commit sets the initial value of pfn_hole to ARCH_PFN_OFFSET, which
> > avoids the problematic and useless iteration mentioned above.
> > 
> > This problem has existed since commit 907ec5fca3dc ("mm: zero remaining
> > unavailable struct pages").
> > 
> > Signed-off-by: Ruihan Li <lrh2000@pku.edu.cn>
> > ---
> > Link to v1:
> >  - https://lore.kernel.org/linux-mm/20250418162727.1535335-1-lrh2000@pku.edu.cn/
> > Changes since v1:
> >  - Removed the unnecessary Fixes tag.
> >  - Fixed the build issue for CONFIG_SPARSEMEM.
> > 
> >  mm/mm_init.c | 9 +++++++++
> >  1 file changed, 9 insertions(+)
> > 
> > diff --git a/mm/mm_init.c b/mm/mm_init.c
> > index 84f14fa12..a697a83ff 100644
> > --- a/mm/mm_init.c
> > +++ b/mm/mm_init.c
> > @@ -969,6 +969,15 @@ static void __init memmap_init(void)
> >  	unsigned long hole_pfn = 0;
> >  	int i, j, zone_id = 0, nid;
> >  
> > +#ifdef CONFIG_FLATMEM
> > +	/*
> > +	 * Pages below ARCH_PFN_OFFSET are invalid as far as pfn_valid is
> > +	 * concerned, so don't waste time iterating on them when looking
> > +	 * for holes.
> > +	 */
> > +	hole_pfn = ARCH_PFN_OFFSET;
> > +#endif
> > +
> 
> I'd prefer a solution for both FLATMEM and SPARSMEM. 
> 
> David Woodhouse proposed a for_each_valid_pfn() a while ago:
> 
> https://lore.kernel.org/all/20250404155959.3442111-1-dwmw2@infradead.org
> 
> It can be used in init_unavailable_range() and will essentially skip the
> unpopulated memory map.

for_each_valid_pfn sounds much better. Thanks for your input.

However, the problem is that David's patch is not showing up in the
mainline, so what can I do to move forward with my patch?

Perhaps you mean that I should wait until David's patch is merged and
send another patch to fix the problem?

> 
> >  	for_each_mem_pfn_range(i, MAX_NUMNODES, &start_pfn, &end_pfn, &nid) {
> >  		struct pglist_data *node = NODE_DATA(nid);
> >  
> > -- 
> > 2.49.0
> > 
> 
> -- 
> Sincerely yours,
> Mike.

Thanks,
Ruihan Li