Re: [PATCHv2 1/3] x86/mm: Provide pmdp_establish() helper

[Catalin Marinas]

On Tue, Jun 20, 2017 at 12:52:10AM +0300, Kirill A. Shutemov wrote:
> On Mon, Jun 19, 2017 at 06:09:12PM +0100, Catalin Marinas wrote:
> > On Mon, Jun 19, 2017 at 07:00:05PM +0300, Kirill A. Shutemov wrote:
> > > On Mon, Jun 19, 2017 at 04:22:29PM +0100, Catalin Marinas wrote:
> > > > On Thu, Jun 15, 2017 at 05:52:22PM +0300, Kirill A. Shutemov wrote:
> > > > > We need an atomic way to setup pmd page table entry, avoiding races with
> > > > > CPU setting dirty/accessed bits. This is required to implement
> > > > > pmdp_invalidate() that doesn't loose these bits.
> > > > > 
> > > > > On PAE we have to use cmpxchg8b as we cannot assume what is value of new pmd and
> > > > > setting it up half-by-half can expose broken corrupted entry to CPU.
> > > > > 
> > > > > Signed-off-by: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx>
> > > > > Cc: Ingo Molnar <mingo@xxxxxxxxxx>
> > > > > Cc: H. Peter Anvin <hpa@xxxxxxxxx>
> > > > > Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
> > > > 
> > > > I'll look at this from the arm64 perspective. It would be good if we can
> > > > have a generic atomic implementation based on cmpxchg64 but I need to
> > > > look at the details first.
> > > 
> > > Unfortunately, I'm not sure it's possbile.
> > > 
> > > The format of a page table is defined per-arch. We cannot assume much about
> > > it in generic code.
> > > 
> > > I guess we could make it compile by casting to 'unsigned long', but is it
> > > useful?
> > > Every architecture manintainer still has to validate that this assumption
> > > is valid for the architecture.
> > 
> > You are right, not much gained in doing this.
> > 
> > Maybe a stupid question but can we not implement pmdp_invalidate() with
> > something like pmdp_get_and_clear() (usually reusing the ptep_*
> > equivalent). Or pmdp_clear_flush() (again, reusing ptep_clear_flush())?
> > 
> > In my quick grep on pmdp_invalidate, it seems to be followed by
> > set_pmd_at() or pmd_populate() already and the *pmd value after
> > mknotpresent isn't any different from 0 to the hardware (at least on
> > ARM). That's unless Linux expects to see some non-zero value here if
> > walking the page tables on another CPU.
> 
> The whole reason to have pmdp_invalidate() in first place is to never make
> pmd clear in the middle. Otherwise we will get race with MADV_DONTNEED.
> See ced108037c2a for an example of such race.

Thanks for the explanation. So you basically just want to set a !present
and !none pmd. I noticed that with your proposed pmdp_invalidate(),
pmdp_establish(pmd_mknotpresent(*pmdp)) could set a stale *pmdp (with
the present bit cleared) temporarily until updated with what
pmdp_establish() returned. Is there a risk of racing with other parts of
the kernel? I guess not since the pmd is !present.

For arm64, I don't see the point of a cmpxchg, so something like below
would do (it needs proper testing though):

-------------8<---------------------------
diff --git a/arch/arm64/include/asm/pgtable.h b/arch/arm64/include/asm/pgtable.h
index c213fdbd056c..8fe1dad9100a 100644
--- a/arch/arm64/include/asm/pgtable.h
+++ b/arch/arm64/include/asm/pgtable.h
@@ -39,6 +39,7 @@
 
 #ifndef __ASSEMBLY__
 
+#include <asm/cmpxchg.h>
 #include <asm/fixmap.h>
 #include <linux/mmdebug.h>
 
@@ -683,6 +684,11 @@ static inline void pmdp_set_wrprotect(struct mm_struct *mm,
 {
 	ptep_set_wrprotect(mm, address, (pte_t *)pmdp);
 }
+
+static inline pmd_t pmdp_establish(pmd_t *pmdp, pmd_t pmd)
+{
+	return __pmd(xchg_relaxed(&pmd_val(*pmdp), pmd_val(pmd)));
+}
 #endif
 #endif	/* CONFIG_ARM64_HW_AFDBM */
 
-------------8<---------------------------

-- 
Catalin