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

Jan Stancek
2019-05-09T21:06:45Z

----- Original Message -----
>
>
> On 5/9/19 11:24 AM, Peter Zijlstra wrote:
> > On Thu, May 09, 2019 at 05:36:29PM +0000, Nadav Amit wrote:
> >>> On May 9, 2019, at 3:38 AM, Peter Zijlstra <peterz@infradead.org> wrote:
> >>> 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);
> >> As a simple optimization, I think it is possible to hold multiple nesting
> >> counters in the mm, similar to tlb_flush_pending, for freed_tables,
> >> cleared_ptes, etc.
> >>
> >> The first time you set tlb->freed_tables, you also atomically increase
> >> mm->tlb_flush_freed_tables. Then, in tlb_flush_mmu(), you just use
> >> mm->tlb_flush_freed_tables instead of tlb->freed_tables.
> > That sounds fraught with races and expensive; I would much prefer to not
> > go there for this arguably rare case.
> >
> > Consider such fun cases as where CPU-0 sees and clears a PTE, CPU-1
> > races and doesn't see that PTE. Therefore CPU-0 sets and counts
> > cleared_ptes. Then if CPU-1 flushes while CPU-0 is still in mmu_gather,
> > it will see cleared_ptes count increased and flush that granularity,
> > OTOH if CPU-1 flushes after CPU-0 completes, it will not and potentiall
> > miss an invalidate it should have had.
> >
> > This whole concurrent mmu_gather stuff is horrible.
> >
> > /me ponders more....
> >
> > So I think the fundamental race here is this:
> >
> > CPU-0 CPU-1
> >
> > tlb_gather_mmu(.start=1, tlb_gather_mmu(.start=2,
> > .end=3); .end=4);
> >
> > ptep_get_and_clear_full(2)
> > tlb_remove_tlb_entry(2);
> > __tlb_remove_page();
> > if (pte_present(2)) // nope
> >
> > tlb_finish_mmu();
> >
> > // continue without TLBI(2)
> > // whoopsie
> >
> > tlb_finish_mmu();
> > tlb_flush() -> TLBI(2)
>
> I'm not quite sure if this is the case Jan really met. But, according to
> his test, once correct tlb->freed_tables and tlb->cleared_* are set, his
> test works well.

My theory was following sequence:

t1: map_write_unmap() t2: dummy()

map_address = mmap()
map_address[i] = 'b'
munmap(map_address)
downgrade_write(&mm->mmap_sem);
unmap_region()
tlb_gather_mmu()
inc_tlb_flush_pending(tlb->mm);
free_pgtables()
tlb->freed_tables = 1
tlb->cleared_pmds = 1

pthread_exit()
madvise(thread_stack, 8M, MADV_DONTNEED)
zap_page_range()
tlb_gather_mmu()
inc_tlb_flush_pending(tlb->mm);

tlb_finish_mmu()
if (mm_tlb_flush_nested(tlb->mm))
__tlb_reset_range()
tlb->freed_tables = 0
tlb->cleared_pmds = 0
__flush_tlb_range(last_level = 0)
...
map_address = mmap()
map_address[i] = 'b'
<page fault loop>
# PTE appeared valid to me,
# so I suspected stale TLB entry at higher level as result of "freed_tables = 0"

I'm happy to apply/run any debug patches to get more data that would help.

>
> >
> >
> > And we can fix that by having tlb_finish_mmu() sync up. Never let a
> > concurrent tlb_finish_mmu() complete until all concurrenct mmu_gathers
> > have completed.
>
> Not sure if this will scale well.
>
> >
> > This should not be too hard to make happen.
>
>