Re: [PATCH RESEND v3 1/2] mm/tlb: skip redundant IPI when TLB flush already synchronized

[Lance Yang <lance.yang@linux.dev>]

On 2026/1/7 00:24, Dave Hansen wrote:
> On 1/6/26 04:03, Lance Yang wrote:
>> From: Lance Yang <lance.yang@linux.dev>
>>
>> When unsharing hugetlb PMD page tables, we currently send two IPIs: one
>> for TLB invalidation, and another to synchronize with concurrent GUP-fast
>> walkers via tlb_remove_table_sync_one().
>>
>> However, if the TLB flush already sent IPIs to all CPUs (when freed_tables
>> or unshared_tables is true), the second IPI is redundant. GUP-fast runs
>> with IRQs disabled, so when the TLB flush IPI completes, any concurrent
>> GUP-fast must have finished.
>>
>> To avoid the redundant IPI, we add a flag to mmu_gather to track whether
>> the TLB flush sent IPIs. We pass the mmu_gather pointer through the TLB
>> flush path via flush_tlb_info, so native_flush_tlb_multi() can set the
>> flag when it sends IPIs for freed_tables. We also set the flag for
>> local-only flushes, since disabling IRQs provides the same guarantee.
> 
> The lack of imperative voice is killing me. :)

Oops.

> 
>> diff --git a/arch/x86/include/asm/tlb.h b/arch/x86/include/asm/tlb.h
>> index 866ea78ba156..c5950a92058c 100644
>> --- a/arch/x86/include/asm/tlb.h
>> +++ b/arch/x86/include/asm/tlb.h
>> @@ -20,7 +20,8 @@ static inline void tlb_flush(struct mmu_gather *tlb)
>>   		end = tlb->end;
>>   	}
>>   
>> -	flush_tlb_mm_range(tlb->mm, start, end, stride_shift, tlb->freed_tables);
>> +	flush_tlb_mm_range(tlb->mm, start, end, stride_shift,
>> +			   tlb->freed_tables || tlb->unshared_tables, tlb);
>>   }
> 
> I think this hunk sums up v3 pretty well. Where there was a single boolean, now there are two. To add to that, the structure that contains the booleans is itself being passed in. The boolean is still named 'freed_tables', and is going from:
> 
> 	tlb->freed_tables
> 
> which is pretty obviously correct to:
> 
> 	tlb->freed_tables || tlb->unshared_tables
> 
> which is _far_ from obviously correct.
> 
> I'm at a loss for why the patch wouldn't just do this:
> 
> -	flush_tlb_mm_range(tlb->mm, start, end, stride_shift, tlb->freed_tables);
> +	flush_tlb_mm_range(tlb->mm, start, end, stride_shift, tlb);
> 
> I suspect these were sent out in a bit of haste, which isn't the first time I've gotten that feeling with this series.
> 
> Could we slow down, please?

Sorry, I went too fast ...

> 
>>   static inline void invlpg(unsigned long addr)
>> diff --git a/arch/x86/include/asm/tlbflush.h b/arch/x86/include/asm/tlbflush.h
>> index 00daedfefc1b..83c260c88b80 100644
>> --- a/arch/x86/include/asm/tlbflush.h
>> +++ b/arch/x86/include/asm/tlbflush.h
>> @@ -220,6 +220,7 @@ struct flush_tlb_info {
>>   	 *   will be zero.
>>   	 */
>>   	struct mm_struct	*mm;
>> +	struct mmu_gather	*tlb;
>>   	unsigned long		start;
>>   	unsigned long		end;
>>   	u64			new_tlb_gen;
> 
> This also gives me pause.
> 
> There is a *lot* of redundant information between 'struct mmu_gather' and 'struct tlb_flush_info'. There needs to at least be a description of what the relationship is and how these relate to each other. I would have naively thought that the right move here would be to pull the mmu_gather data out at one discrete time rather than store a pointer to it.
> 
> What I see here is, I suspect, the most expedient way to do it. I'd _certainly_ have done this myself if I was just hacking something together to play with as quickly as possible.
> 
> So, in the end, I don't hate the approach here (yet). But it is almost impossible to evaluate it because the series is taking some rather egregious shortcuts and is lacking any real semblance of a refactoring effort.

The flag lifetime issue David pointed out is real, and you're right
about the messy parameters :)

And, yeah, I need to think more those. Maybe v3 can be fixed, or maybe
v2 is actually sufficient - it's conservative but safe (no false positives).

Will take more time, thanks!