Re: [PATCH v5 0/3] kselftest: mm: fix some failure of split_huge_page_test

[Yeoreum Yun]

On Thu, Sep 10, 2026 at 01:23:15PM +0200, David Hildenbrand (Arm) wrote:
> On 9/10/26 13:16, Yeoreum Yun wrote:
> > On Thu, Sep 10, 2026 at 12:45:39PM +0200, David Hildenbrand (Arm) wrote:
> >> On 9/10/26 12:31, Yeoreum Yun wrote:
> >>>
> >>> Well, users currently for using /proc/self/smaps are for check vm-flags:
> >>>   - guard-regions where using check_vmflags_guard()
> >>>   - pfnmap test where uses check_vmflag_pfnmap()
> >>
> >> Most vm-flags should not be an issue when it comes to merging. The only
> >> exception are vmflags that do not prevent VMA merging.
> >>
> >> So it's VM_SOFTDIRTY and VM_MAYBE_GUARD. And I agree that for guard-regions.c we
> >> likely have to care such that we don't merge by accident with other VMAs (guard
> >> regions).
> >>
> >> But that's independent of memalign.
> >>
> >> ptr = mmap_(self, variant, NULL, 10 * page_size, PROT_READ | PROT_WRITE, 0, 0);
> >> ASSERT_FALSE(check_vmflag_guard(ptr));
> >>
> >> could be problematic on its own (unlikely but possible).
> > 
> > Yes. That's why I'm think it would be good to use alloc_isolated_mem()
> > in case of ANON mapping for this case.
> 
> It's really only guest-region code that needs this.
> 
> > 
> >>
> >> For other flags, you really only have to find the smaps area that covers the
> >> given address and look at the vm-flags.
> >>
> >> Or am I missing something important?
> >>
> >> (merging vnas with VM_PFNMAP is impossible right now IIRC)
> > 
> > No. what I want to say including the patch #3 is for the above case
> > where you point out -- ASSERT_FALSE(check_vmflag_guard(ptr)).
> > 
> > Since we don't have any interface to check vm_flags execpt smap
> > and for memory mmaped with anon would have a chance to merge,
> > We need something to replace memalign() with preventing unexpected VMA
> > merge. 
> Only for the cases that actually really needs this, which is in my understanding
> guard-regions.
> 
> And I repeat, this is not a memalign() problem.

Yes. I'm not claim memalign() is problem but want to prevent unwanted
VMA merge. That's all.

And as I mentioned in another reply, if we get rid of
ASSERT_FALSE(check_vmflag_guard(ptr)), this patch would be droppable.

-- 
Sincerely,
Yeoreum Yun