linux-kernel - Re: [PATCH v2] mm/mm_init: Don't iterate pages below ARCH_PFN

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Message-ID: <1cf8ff5dd37054c8430c0a7578e84b7eec3f2a05.camel@infradead.org>
Date: Tue, 22 Apr 2025 21:25:27 +0100
From: David Woodhouse <dwmw2@...radead.org>
To: Andrew Morton <akpm@...ux-foundation.org>, Ruihan Li <lrh2000@....edu.cn>
Cc: Mike Rapoport <rppt@...nel.org>, linux-mm@...ck.org, 
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] mm/mm_init: Don't iterate pages below ARCH_PFN_OFFSET

On Tue, 2025-04-22 at 13:20 -0700, Andrew Morton wrote:
> On Tue, 22 Apr 2025 17:08:06 +0800 Ruihan Li <lrh2000@....edu.cn> wrote:
> 
> > 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@....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?
> 
> (cc David)

Want to add a patch on top of my tree at
https://git.infradead.org/users/dwmw2/linux.git/shortlog/refs/heads/for_each_valid_pfn
and I'll send it on with the rest?


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