Re: [PATCH V3 1/2] arm64/mm: Fix pfn_valid() for ZONE_DEVICE based memory

[Catalin Marinas]

On Fri, Mar 05, 2021 at 07:24:21PM +0100, David Hildenbrand wrote:
> On 05.03.21 19:13, Catalin Marinas wrote:
> > On Fri, Mar 05, 2021 at 10:54:57AM +0530, Anshuman Khandual wrote:
> > > pfn_valid() validates a pfn but basically it checks for a valid struct page
> > > backing for that pfn. It should always return positive for memory ranges
> > > backed with struct page mapping. But currently pfn_valid() fails for all
> > > ZONE_DEVICE based memory types even though they have struct page mapping.
> > > 
> > > pfn_valid() asserts that there is a memblock entry for a given pfn without
> > > MEMBLOCK_NOMAP flag being set. The problem with ZONE_DEVICE based memory is
> > > that they do not have memblock entries. Hence memblock_is_map_memory() will
> > > invariably fail via memblock_search() for a ZONE_DEVICE based address. This
> > > eventually fails pfn_valid() which is wrong. memblock_is_map_memory() needs
> > > to be skipped for such memory ranges. As ZONE_DEVICE memory gets hotplugged
> > > into the system via memremap_pages() called from a driver, their respective
> > > memory sections will not have SECTION_IS_EARLY set.
> > > 
> > > Normal hotplug memory will never have MEMBLOCK_NOMAP set in their memblock
> > > regions. Because the flag MEMBLOCK_NOMAP was specifically designed and set
> > > for firmware reserved memory regions. memblock_is_map_memory() can just be
> > > skipped as its always going to be positive and that will be an optimization
> > > for the normal hotplug memory. Like ZONE_DEVICE based memory, all normal
> > > hotplugged memory too will not have SECTION_IS_EARLY set for their sections
> > > 
> > > Skipping memblock_is_map_memory() for all non early memory sections would
> > > fix pfn_valid() problem for ZONE_DEVICE based memory and also improve its
> > > performance for normal hotplug memory as well.
> > > 
> > > Cc: Catalin Marinas <catalin.marinas@xxxxxxx>
> > > Cc: Will Deacon <will@xxxxxxxxxx>
> > > Cc: Ard Biesheuvel <ardb@xxxxxxxxxx>
> > > Cc: Robin Murphy <robin.murphy@xxxxxxx>
> > > Cc: linux-arm-kernel@xxxxxxxxxxxxxxxxxxx
> > > Cc: linux-kernel@xxxxxxxxxxxxxxx
> > > Acked-by: David Hildenbrand <david@xxxxxxxxxx>
> > > Fixes: 73b20c84d42d ("arm64: mm: implement pte_devmap support")
> > > Signed-off-by: Anshuman Khandual <anshuman.khandual@xxxxxxx>
> > > ---
> > >   arch/arm64/mm/init.c | 12 ++++++++++++
> > >   1 file changed, 12 insertions(+)
> > > 
> > > diff --git a/arch/arm64/mm/init.c b/arch/arm64/mm/init.c
> > > index 0ace5e68efba..5920c527845a 100644
> > > --- a/arch/arm64/mm/init.c
> > > +++ b/arch/arm64/mm/init.c
> > > @@ -230,6 +230,18 @@ int pfn_valid(unsigned long pfn)
> > >   	if (!valid_section(__pfn_to_section(pfn)))
> > >   		return 0;
> > > +
> > > +	/*
> > > +	 * ZONE_DEVICE memory does not have the memblock entries.
> > > +	 * memblock_is_map_memory() check for ZONE_DEVICE based
> > > +	 * addresses will always fail. Even the normal hotplugged
> > > +	 * memory will never have MEMBLOCK_NOMAP flag set in their
> > > +	 * memblock entries. Skip memblock search for all non early
> > > +	 * memory sections covering all of hotplug memory including
> > > +	 * both normal and ZONE_DEVICE based.
> > > +	 */
> > > +	if (!early_section(__pfn_to_section(pfn)))
> > > +		return pfn_section_valid(__pfn_to_section(pfn), pfn);
> > 
> > Would something like this work instead:
> > 
> > 	if (online_device_section(ms))
> > 		return 1;
> > 
> > to avoid the assumptions around early_section()?
> 
> Please keep online section logic out of pfn valid logic. Tow different
> things. (and rather not diverge too much from generic pfn_valid() - we want
> to achieve the opposite in the long term, merging both implementations)

I think I misread the code. I was looking for a new flag to check like
SECTION_IS_DEVICE instead of assuming that !SECTION_IS_EARLY means
device or mhp.

Anyway, staring at this code for a bit more, I came to the conclusion
that the logic in Anshuman's patches is fairly robust - we only need to
check for memblock_is_map_memory() if early_section() as that's the only
case where we can have MEMBLOCK_NOMAP. Maybe the comment above should be
re-written a bit and avoid the ZONE_DEVICE and hotplugged memory
details altogether.

-- 
Catalin