Re: [PATCH] MIPS: fix pmd_mkinvalid

[Hongchen Zhang]

On 2022/7/12 下午7:19, Thomas Bogendoerfer wrote:
> On Tue, Jul 12, 2022 at 06:01:08PM +0800, Hongchen Zhang wrote:
> > On 2022/7/7 下午7:12, Hongchen Zhang wrote:
> > > On 2022/7/7 下午5:22, Thomas Bogendoerfer wrote:
> > > > On Thu, Jul 07, 2022 at 04:11:35PM +0800, Hongchen Zhang wrote:
> > > > > When a pmd entry is invalidated by pmd_mkinvalid,pmd_present should
> > > > > return true.
> > > > > So introduce a _PMD_PRESENT_INVALID_SHIFT bit to check if a pmd is
> > > > > present but invalidated by pmd_mkinvalid.
> > > >
> > > > What problem are you trying to fix ? What are the symptoms ?
> > > >
> > > > > Reported-by: kernel test robot <lkp@xxxxxxxxx>
> > > >
> > > > the test robot showed problems with your last version of the patch,
> > > > which hasn't been integrated into at least the MIPS tree, so no
> > > > need to that.
> > > >
> > > > Thomas.
> > >
> > > Hi Thomas,
> > >     The idea come from the commit:
> > >     b65399f6111b(arm64/mm: Change THP helpers to comply with generic MM
> > >   semantics).
> > >     There is an problem now:
> > >           CPU 0        CPU 1
> > >       pmdp_invalidate        do_page_fault
> > >       ...              __handle_mm_fault
> > >                       is_swap_pmd == true
> > >                       trigger VM_BUG_ON() ?
> > >       set_pmd_at
> > >     the reason is that pmd_present return true,after this commit
> > >     pmd_present will return false,and the VM_BUG_ON will not be triggered.
> > >     Like arm64 does,we can introduce a new bit to fix this.
> > >
> > > Thanks.
> > Hi Thomas,
> >   Is there problem of this patch? What's your opinion of this patch?
>
> I haven't dig deeper into it, but needing more page bits is a pain
> for 32bit kernel and would make it nearly impossible to get huge
> page support there. And the description you gave me, needs to be
> in the commit description.
>
> Thomas.
Hi Thomas,
 Thanks for your patiently review. For your question,
  1. I think there may be problem when compile 32bit kernel with huge 
page support,because _PAGE_HUGE_SHIFT is only defined for R4K now.
  2. I will modify the commit as you said and make a v2 patch.

Thanks.
Hongchen Zhang