Re: [PATCH] mm: mmu_gather: remove __tlb_reset_range() for force flush

[Peter Zijlstra]

On Thu, May 09, 2019 at 09:37:26AM +0100, Will Deacon wrote:
> Hi all, [+Peter]

Right, mm/mmu_gather.c has a MAINTAINERS entry; use it.

Also added Nadav and Minchan who've poked at this issue before. And Mel,
because he loves these things :-)

> Apologies for the delay; I'm attending a conference this week so it's tricky
> to keep up with email.
> 
> On Wed, May 08, 2019 at 05:34:49AM +0800, Yang Shi wrote:
> > A few new fields were added to mmu_gather to make TLB flush smarter for
> > huge page by telling what level of page table is changed.
> > 
> > __tlb_reset_range() is used to reset all these page table state to
> > unchanged, which is called by TLB flush for parallel mapping changes for
> > the same range under non-exclusive lock (i.e. read mmap_sem).  Before
> > commit dd2283f2605e ("mm: mmap: zap pages with read mmap_sem in
> > munmap"), MADV_DONTNEED is the only one who may do page zapping in
> > parallel and it doesn't remove page tables.  But, the forementioned commit
> > may do munmap() under read mmap_sem and free page tables.  This causes a
> > bug [1] reported by Jan Stancek since __tlb_reset_range() may pass the

Please don't _EVER_ refer to external sources to describe the actual bug
a patch is fixing. That is the primary purpose of the Changelog.

Worse, the email you reference does _NOT_ describe the actual problem.
Nor do you.

> > wrong page table state to architecture specific TLB flush operations.
> 
> Yikes. Is it actually safe to run free_pgtables() concurrently for a given
> mm?

Yeah.. sorta.. it's been a source of 'interesting' things. This really
isn't the first issue here.

Also, change_protection_range() is 'fun' too.

> > So, removing __tlb_reset_range() sounds sane.  This may cause more TLB
> > flush for MADV_DONTNEED, but it should be not called very often, hence
> > the impact should be negligible.
> > 
> > The original proposed fix came from Jan Stancek who mainly debugged this
> > issue, I just wrapped up everything together.
> 
> I'm still paging the nested flush logic back in, but I have some comments on
> the patch below.
> 
> > [1] https://lore.kernel.org/linux-mm/342bf1fd-f1bf-ed62-1127-e911b5032274@xxxxxxxxxxxxxxxxx/T/#m7a2ab6c878d5a256560650e56189cfae4e73217f
> > 
> > Reported-by: Jan Stancek <jstancek@xxxxxxxxxx>
> > Tested-by: Jan Stancek <jstancek@xxxxxxxxxx>
> > Cc: Will Deacon <will.deacon@xxxxxxx>
> > Cc: stable@xxxxxxxxxxxxxxx
> > Signed-off-by: Yang Shi <yang.shi@xxxxxxxxxxxxxxxxx>
> > Signed-off-by: Jan Stancek <jstancek@xxxxxxxxxx>
> > ---
> >  mm/mmu_gather.c | 7 ++++---
> >  1 file changed, 4 insertions(+), 3 deletions(-)
> > 
> > diff --git a/mm/mmu_gather.c b/mm/mmu_gather.c
> > index 99740e1..9fd5272 100644
> > --- a/mm/mmu_gather.c
> > +++ b/mm/mmu_gather.c
> > @@ -249,11 +249,12 @@ void tlb_finish_mmu(struct mmu_gather *tlb,
> >  	 * flush by batching, a thread has stable TLB entry can fail to flush
> 
> Urgh, we should rewrite this comment while we're here so that it makes sense...

Yeah, that's atrocious. We should put the actual race in there.

> >  	 * the TLB by observing pte_none|!pte_dirty, for example so flush TLB
> >  	 * forcefully if we detect parallel PTE batching threads.
> > +	 *
> > +	 * munmap() may change mapping under non-excluse lock and also free
> > +	 * page tables.  Do not call __tlb_reset_range() for it.
> >  	 */
> > -	if (mm_tlb_flush_nested(tlb->mm)) {
> > -		__tlb_reset_range(tlb);
> > +	if (mm_tlb_flush_nested(tlb->mm))
> >  		__tlb_adjust_range(tlb, start, end - start);
> > -	}
> 
> I don't think we can elide the call __tlb_reset_range() entirely, since I
> think we do want to clear the freed_pXX bits to ensure that we walk the
> range with the smallest mapping granule that we have. Otherwise couldn't we
> have a problem if we hit a PMD that had been cleared, but the TLB
> invalidation for the PTEs that used to be linked below it was still pending?

That's tlb->cleared_p*, and yes agreed. That is, right until some
architecture has level dependent TLBI instructions, at which point we'll
need to have them all set instead of cleared.

> Perhaps we should just set fullmm if we see that here's a concurrent
> unmapper rather than do a worst-case range invalidation. Do you have a feeling
> for often the mm_tlb_flush_nested() triggers in practice?

Quite a bit for certain workloads I imagine, that was the whole point of
doing it.

Anyway; am I correct in understanding that the actual problem is that
we've cleared freed_tables and the ARM64 tlb_flush() will then not
invalidate the cache and badness happens?

Because so far nobody has actually provided a coherent description of
the actual problem we're trying to solve. But I'm thinking something
like the below ought to do.


diff --git a/mm/mmu_gather.c b/mm/mmu_gather.c
index 99740e1dd273..fe768f8d612e 100644
--- a/mm/mmu_gather.c
+++ b/mm/mmu_gather.c
@@ -244,15 +244,20 @@ void tlb_finish_mmu(struct mmu_gather *tlb,
 		unsigned long start, unsigned long end)
 {
 	/*
-	 * If there are parallel threads are doing PTE changes on same range
-	 * under non-exclusive lock(e.g., mmap_sem read-side) but defer TLB
-	 * flush by batching, a thread has stable TLB entry can fail to flush
-	 * the TLB by observing pte_none|!pte_dirty, for example so flush TLB
-	 * forcefully if we detect parallel PTE batching threads.
+	 * Sensible comment goes here..
 	 */
-	if (mm_tlb_flush_nested(tlb->mm)) {
-		__tlb_reset_range(tlb);
-		__tlb_adjust_range(tlb, start, end - start);
+	if (mm_tlb_flush_nested(tlb->mm) && !tlb->full_mm) {
+		/*
+		 * Since we're can't tell what we actually should have
+		 * flushed flush everything in the given range.
+		 */
+		tlb->start = start;
+		tlb->end = end;
+		tlb->freed_tables = 1;
+		tlb->cleared_ptes = 1;
+		tlb->cleared_pmds = 1;
+		tlb->cleared_puds = 1;
+		tlb->cleared_p4ds = 1;
 	}
 
 	tlb_flush_mmu(tlb);