From: Peter Zijlstra <peterz@infradead.org>
To: Nadav Amit <namit@vmware.com>
Cc: Will Deacon <will.deacon@arm.com>,
Yang Shi <yang.shi@linux.alibaba.com>,
"jstancek@redhat.com" <jstancek@redhat.com>,
Andrew Morton <akpm@linux-foundation.org>,
"stable@vger.kernel.org" <stable@vger.kernel.org>,
"linux-mm@kvack.org" <linux-mm@kvack.org>,
"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
"aneesh.kumar@linux.vnet.ibm.com"
<aneesh.kumar@linux.vnet.ibm.com>,
"npiggin@gmail.com" <npiggin@gmail.com>,
"minchan@kernel.org" <minchan@kernel.org>,
Mel Gorman <mgorman@suse.de>
Subject: Re: [PATCH] mm: mmu_gather: remove __tlb_reset_range() for force flush
Date: Thu, 9 May 2019 20:24:35 +0200 [thread overview]
Message-ID: <20190509182435.GA2623@hirez.programming.kicks-ass.net> (raw)
In-Reply-To: <F22533A7-016F-4506-809A-7E86BAF24D5A@vmware.com>
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)
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.
This should not be too hard to make happen.
next prev parent reply other threads:[~2019-05-09 18:24 UTC|newest]
Thread overview: 26+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-05-07 21:34 [PATCH] mm: mmu_gather: remove __tlb_reset_range() for force flush Yang Shi
2019-05-09 8:37 ` Will Deacon
2019-05-09 10:38 ` Peter Zijlstra
2019-05-09 10:54 ` Peter Zijlstra
2019-05-09 18:35 ` Yang Shi
2019-05-09 18:40 ` Peter Zijlstra
2019-05-09 12:44 ` Jan Stancek
2019-05-09 17:36 ` Nadav Amit
2019-05-09 18:24 ` Peter Zijlstra [this message]
2019-05-09 19:10 ` Yang Shi
2019-05-09 21:06 ` Jan Stancek
2019-05-09 21:48 ` Yang Shi
2019-05-09 22:12 ` Jan Stancek
[not found] ` <04668E51-FD87-4D53-A066-5A35ABC3A0D6@vmware.com>
[not found] ` <20190509191120.GD2623@hirez.programming.kicks-ass.net>
2019-05-09 21:21 ` Nadav Amit
2019-05-13 8:36 ` Peter Zijlstra
2019-05-13 9:11 ` Nadav Amit
2019-05-13 11:30 ` Peter Zijlstra
2019-05-13 16:37 ` Will Deacon
2019-05-13 17:06 ` Nadav Amit
2019-05-14 8:58 ` Mel Gorman
2019-05-13 9:12 ` Peter Zijlstra
2019-05-13 9:21 ` Nadav Amit
2019-05-13 11:27 ` Peter Zijlstra
2019-05-13 17:41 ` Nadav Amit
2019-05-09 18:22 ` Yang Shi
2019-05-09 19:56 ` Peter Zijlstra
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