Re: Some questions about DEBUG_PAGEALLOC on ARMv8

[Steve Capper <steve.capper@arm.com>]

On Thu, Sep 04, 2014 at 10:41:52AM +0100, Catalin Marinas wrote:
> Hi Zhichang,
> 
> (cc'ing Steve Capper for the huge page stuff)
> 
> On Fri, Aug 22, 2014 at 01:38:26PM +0100, zhichang.yuan wrote:
> > I am working to implement the DEBUG_PAGEALLOC on ARMv8.
> 
> I assume that's the arm64 kernel.
> 
> > After i investigated the DEBUG_PAGEALLOC implementation on x86 arch,
> > some questions are standing in the way to start coding.
> > 
> > 1. How to handle the large page when DEBUG_PAGEALLOC is enabled In
> > ARMv8, the kernel direct memory page table entries will set the block
> > flag for better performance. When DEBUG_PAGEALLOC is configured, if
> > the size of freed page is not multiply of page block size, there is no
> > corresponding page table entry. In the old x86 kernel version, the
> > large page to be freed will be split into normal page size and build
> > the corresponding PTEs. And afterwards, someone done a patch to remove
> > the splitting process. It will make the code simpler and easily
> > stable.
> 
> Initially, you could either map everything as pages or implement
> splitting of huge pages (if for example the huge page is at the pmd
> level, you allocate and populate a pte).
> 
> > I prefer the current design in x86, what are your thoughts here?
> 
> I haven't looked at it yet.
> 
> > 2. Does ARMv8 support HIBERNATION?
> 
> Not yet.
> 
> > The HIBERNATION has some dependency on DEBUG_PAGEALLOC.
> 
> Like in DEBUG_PAGEALLOC "depends on !HIBERNATION"?
> 
> > 3. Is the hypothesis of DEBUG_PAGEALLOC always true?
> 
> Which hypothesis?
> 
> > From the x86 code, DEBUG_PAGEALLOC use the invalid page table entries
> > to catch the accesses to free pages. This mechanism is based on the
> > hypothesis that all the corresponding page table entries that are
> > corresponding to the free pages are cleared correctly. Supposed this
> > condition is always true, what we need to do is just to clear the
> > kernel linear mapping page entries, since those page tables are
> > fixable after initialization. DEBUG_PAGEALLOC on x86 seems to do like
> > that.
> 
> I guess that's the ARCH_SUPPORTS_DEBUG_PAGEALLOC rather than just the
> simple DEBUG_PAGEALLOC which can be enabled on arm64 as well, you just
> get page poisoning rather than invalid mappings.
> 
> It could be done on arm64 as well but you need to sort out huge page
> splitting or just map everything as pages when the option is enabled.
> 

(cc'ing Laura Abbott for info...)

Hi,
There is support for splitting pmd's and pud's in the direct kernel
mapping in the following series from Laura Abbott:

"[PATCHv3 7/7] arm64: add better page protections to arm64"
http://lists.infradead.org/pipermail/linux-arm-kernel/2014-August/280782.html

Perhaps some of the splitting logic there could be used by the
kernel_map_pages arm64 implementation for ARCH_SUPPORTS_DEBUG_PAGEALLOC?

Cheers,
-- 
Steve