Re: [RFC PATCH 3/3] mm: Implement for_each_valid_pfn() for CONFIG_SPARSEMEM

[Mike Rapoport <rppt@kernel.org>]

On Thu, Apr 03, 2025 at 08:07:22AM +0100, David Woodhouse wrote:
> On Thu, 2025-04-03 at 09:24 +0300, Mike Rapoport wrote:
> > with a small nit below
> > 
> > > +static inline bool first_valid_pfn(unsigned long *p_pfn)
> > > +{
> > > +	unsigned long pfn = *p_pfn;
> > > +	unsigned long nr = pfn_to_section_nr(pfn);
> > > +	struct mem_section *ms;
> > > +	bool ret = false;
> > > +
> > > +	ms = __pfn_to_section(pfn);
> > > +
> > > +	rcu_read_lock_sched();
> > > +
> > > +	while (!ret && nr <= __highest_present_section_nr) {
> > 
> > This could be just for(;;), we anyway break when ret becomes true or we get
> > past last present section.
> 
> True for the 'ret' part but not *nicely* for the last present section.
> If the original pfn is higher than the last present section it could
> trigger that check before entering the loop.
> 
> Yes, in that case 'ms' will be NULL, valid_section(NULL) is false and
> you're right that it'll make it through to the check in the loop
> without crashing. So it would currently be harmless, but I didn't like
> it. It's relying on the loop not to do the wrong thing with an input
> which is arguably invalid.
> 
> I'll see if I can make it neater. I may drop the 'ret' variable
> completely and just turn the match clause into unlock-and-return-true.
> I *like* having a single unlock site. But I think I like simpler loop
> code more than that.
> 
> FWIW I think the check for (PHYS_PFN(PFN_PHYS(pfn)) != pfn) at the
> start of pfn_valid() a few lines above is similarly redundant. Because
> if the high bits are set in the PFN then pfn_to_section_nr(pfn) is
> surely going to be higher than NR_MEM_SECTIONS and it'll get thrown out
> at the very next check, won't it?

I believe the check for (PHYS_PFN(PFN_PHYS(pfn)) != pfn) got to the generic
version from arm64::pfn_valid() that historically supported both FLATMEM
and SPARSEMEM.

I can't think of a configuration in which (PHYS_PFN(PFN_PHYS(pfn)) != pfn)
and pfn_to_section_nr(pfn) won't be higher than NR_MEM_SECTIONS, but with
all variants that arm64 has for PAGE_SHIFT and ARM64_PA_BITS I could miss
something.
 
> I care because I didn't bother to duplicate that 'redundant' check in
> my first_valid_pfn(), so if there's a reason for it that I'm missing, I
> should take a closer look.
> 
> I'm also missing the reason why the FLATMEM code in memory_model.h does
> 'unsigned long pfn_offset = ARCH_PFN_OFFSET' and then uses its local
> pfn_offset variable, instead of just using ARCH_PFN_OFFSET directly as
> I do in the FLATMEM for_each_valid_pfn() macro.

Don't remember now, but I surely had some $REASON for that :) 

-- 
Sincerely yours,
Mike.