Re: [PATCH V15] mm/debug: Add tests validating architecture page table helpers

[Christophe Leroy]

Le 07/03/2020 Ã 01:56, Anshuman Khandual a ÃcritÂ:
> On 03/07/2020 06:04 AM, Qian Cai wrote:
> > > On Mar 6, 2020, at 7:03 PM, Anshuman Khandual <Anshuman.Khandual@xxxxxxx> wrote:
> > >
> > > Hmm, set_pte_at() function is not preferred here for these tests. The idea
> > > is to avoid or atleast minimize TLB/cache flushes triggered from these sort
> > > of 'static' tests. set_pte_at() is platform provided and could/might trigger
> > > these flushes or some other platform specific synchronization stuff. Just
> >
> > Why is that important for this debugging option?
>
> Primarily reason is to avoid TLB/cache flush instructions on the system
> during these tests that only involve transforming different page table
> level entries through helpers. Unless really necessary, why should it
> emit any TLB/cache flush instructions ?

What's the problem with thoses flushes ?

> > > wondering is there specific reason with respect to the soft lock up problem
> > > making it necessary to use set_pte_at() rather than a simple WRITE_ONCE() ?
> >
> > Looks at the s390 version of set_pte_at(), it has this comment,
> > vmaddr);
> >
> > /*
> >   * Certain architectures need to do special things when PTEs
> >   * within a page table are directly modified.  Thus, the following
> >   * hook is made available.
> >   */
> >
> > I can only guess that powerpc  could be the same here.
>
> This comment is present in multiple platforms while defining set_pte_at().
> Is not 'barrier()' here alone good enough ? Else what exactly set_pte_at()
> does as compared to WRITE_ONCE() that avoids the soft lock up, just trying
> to understand.


Argh ! I didn't realise that you were writing directly into the page 
tables. When it works, that's only by chance I guess.

To properly set the page table entries, set_pte_at() has to be used:
- On powerpc 8xx, with 16k pages, the page table entry must be copied 
four times. set_pte_at() does it, WRITE_ONCE() doesn't.
- On powerpc book3s/32 (hash MMU), the flag _PAGE_HASHPTE must be 
preserved among writes. set_pte_at() preserves it, WRITE_ONCE() doesn't.

set_pte_at() also does a few other mandatory things, like calling 
pte_mkpte()

So, the WRITE_ONCE() must definitely become a set_pte_at()

Christophe