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

[Nadav Amit]

> On May 9, 2019, at 3:38 AM, Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
> 
> 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://nam04.safelinks.protection.outlook.com/?url=https%3A%2F%2Flore.kernel.org%2Flinux-mm%2F342bf1fd-f1bf-ed62-1127-e911b5032274%40linux.alibaba.com%2FT%2F%23m7a2ab6c878d5a256560650e56189cfae4e73217f&amp;data=02%7C01%7Cnamit%40vmware.com%7C7be2f2b29b654aba7de308d6d46a7b93%7Cb39138ca3cee4b4aa4d6cd83d9dd62f0%7C0%7C0%7C636929951176903247&amp;sdata=gGptCMeb9vW4jXUnG53amgvrv8TB9F52JYBHmPeHFvs%3D&amp;reserved=0
>>> 
>>> 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);

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.