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

[Jan Stancek <jstancek@redhat.com>]

----- Original Message -----
> 
> 
> On 5/9/19 2:06 PM, Jan Stancek wrote:
> > ----- 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)
> 
> I'm not quite familiar with the implementation detail of pthread_exit(),
> does pthread_exit() call MADV_DONTNEED all the time? I don't see your
> test call it.

It's called by glibc:
  https://sourceware.org/git/?p=glibc.git;a=blob;f=nptl/allocatestack.c;h=fcbc46f0d796abce8d58970d4a1d3df685981e33;hb=refs/heads/master#l380
  https://sourceware.org/git/?p=glibc.git;a=blob;f=nptl/pthread_create.c;h=18b7bbe7659c027dfd7b0ce3b0c83f54a6f15b18;hb=refs/heads/master#l569

(gdb) bt
#0  madvise () at ../sysdeps/unix/syscall-template.S:78
#1  0x0000ffffbe7679f8 in advise_stack_range (guardsize=<optimized out>, pd=281474976706191, size=<optimized out>, mem=0xffffbddd0000)
    at allocatestack.c:392
#2  start_thread (arg=0xffffffffee8f) at pthread_create.c:576
#3  0x0000ffffbe6b157c in thread_start () at ../sysdeps/unix/sysv/linux/aarch64/clone.S:78

Dump of assembler code for function madvise:
=> 0x0000ffffbe6adaf0 <+0>:     mov     x8, #0xe9                       // #233
   0x0000ffffbe6adaf4 <+4>:     svc     #0x0
   0x0000ffffbe6adaf8 <+8>:     cmn     x0, #0xfff
   0x0000ffffbe6adafc <+12>:    b.cs    0xffffbe6adb04 <madvise+20>  // b.hs, b.nlast
   0x0000ffffbe6adb00 <+16>:    ret
   0x0000ffffbe6adb04 <+20>:    b       0xffffbe600e18 <__GI___syscall_error>


> If so this pattern is definitely possible.
> 
> >                                            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.
> >>
> 
>