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

From: Jan Stancek
Date: Thu May 09 2019 - 18:13:50 EST


----- 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@xxxxxxxxxxxxx>
> >>>>> 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.
> >>
>
>