Re: Test case for "mm/thp: carry over dirty bit when thp splits on pmd"

[Peter Xu <peterx@redhat.com>]

On Fri, Nov 25, 2022 at 07:38:36PM +0800, hev wrote:
> On Tue, Nov 22, 2022 at 2:55 AM Peter Xu <peterx@redhat.com> wrote:
> >
> > Hi, Anatoly (or/and Hev),
> >
> > On Wed, Nov 16, 2022 at 01:45:15PM +0300, Anatoly Pugachev wrote:
> > > On Wed, Nov 16, 2022 at 11:49 AM hev <r@hev.cc> wrote:
> > > >
> > > > Hello Peter,
> > > >
> > > > I see a random crash issue  on the LoongArch system, that is caused by
> > > > commit 0ccf7f1 ("mm/thp: carry over dirty bit when thp splits on
> > > > pmd").
> > > >
> > > > Now, the thing is already resolved. The root cause is arch's mkdirty
> > > > is set hardware writable bit in unconditional. That breaks
> > > > write-protect and then breaks COW.
> > > >
> > > > Here is a simple and fast testcase (It may be helpful for sparc64):
> > > > https://gist.github.com/heiher/72919fae6b53f04cac606a9631100506
> > > > (assertion: c sum == 0)
> > >
> > > Just tried on my sparc64 VM -  fixed vs old (non-patched) kernels...
> > >
> > > fixed kernel (6.1.0-rc5) running ./a.out:
> > > mator@ttip:~$ ./a.out
> > > c sum: 0
> > > p sum: 35184372088832
> > > c sum: 0
> > > p sum: 35184372088832
> > > c sum: 0
> > > p sum: 35184372088832
> > > c sum: 0
> > > p sum: 35184372088832
> > > c sum: 0
> > > p sum: 35184372088832
> > > ...
> > >
> > > old (non-patched) kernel (6.1.0-rc4) :
> > > mator@ttip:~$ ./a.out
> > > c sum: 35150012350464
> > > p sum: 35184372088832
> > > c sum: 35150012350464
> > > p sum: 35184372088832
> > > ...
> >
> > I've got another patch attached that might be nicer to fix this same
> > problem for both archs but without dropping the dirty bit, could you help
> > check whether it works?
> 
> The tesecase PASSED with this patch and without:
>  * "Partly revert "mm/thp: carry over dirty bit when thp splits on pmd"
>  * "LoongArch: Set _PAGE_DIRTY only if _PAGE_WRITE is set in
> {pmd,pte}_mkdirty()"

My fault to not have noticed that the partly revert patch already landed
6.1-rc5, so it'll need to be another patch upon it.

I'll post a formal patch.  Thanks Hev.

-- 
Peter Xu