On Wed, Nov 16, 2022 at 01:45:15PM +0300, Anatoly Pugachev wrote:
> On Wed, Nov 16, 2022 at 11:49 AM hev <r@xxxxxx> wrote:
> >
> > Hello Peter,
Hi, Hev,
Thanks for letting me know.
> >
> > 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.
Could you help explain how that happened?
I'm taking example of loongarch here:
static inline pte_t pte_mkdirty(pte_t pte)
{
pte_val(pte) |= (_PAGE_DIRTY | _PAGE_MODIFIED);
return pte;
}
#define _PAGE_MODIFIED (_ULCAST_(1) << _PAGE_MODIFIED_SHIFT)
#define _PAGE_MODIFIED_SHIFT 9
#define _PAGE_DIRTY (_ULCAST_(1) << _PAGE_DIRTY_SHIFT)
#define _PAGE_DIRTY_SHIFT 1
I don't see when write bit is set, which is bit 8 instead:
#define _PAGE_WRITE (_ULCAST_(1) << _PAGE_WRITE_SHIFT)
#define _PAGE_WRITE_SHIFT 8
According to loongarch spec:
https://loongson.github.io/LoongArch-Documentation/LoongArch-Vol1-EN.html#section-multi-level-page-table-structure-supported-by-page-walking
Bits 1 & 8 match the spec D & W definitions. Bit 9 seems not defined but I
didn't quickly spot how that's related to the write bit.
> >
> > 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
> ...
Thanks for the quick run, Anatoly. Obviously I went the wrong way before
on the code patching. It seems we have more chance fixing this.
--
Peter Xu
