linux-kernel - Re: [PATCH v2 2/2] mm/mmu_gather: send tlb_remove_table_smp

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Message-ID: <c08d81286b9986c086e9b68e3440f9bf99086d70.camel@redhat.com>
Date:   Thu, 22 Jun 2023 16:57:39 +0300
From:   ypodemsk@...hat.com
To:     Nadav Amit <nadav.amit@...il.com>
Cc:     mtosatti@...hat.com, ppandit@...hat.com,
        David Hildenbrand <david@...hat.com>,
        "Russell King (Oracle)" <linux@...linux.org.uk>,
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        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
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Subject: Re: [PATCH v2 2/2] mm/mmu_gather: send tlb_remove_table_smp_sync
 IPI only to MM CPUs

On Wed, 2023-06-21 at 11:02 -0700, Nadav Amit wrote:
> > On Jun 20, 2023, at 7:46 AM, Yair Podemsky <ypodemsk@...hat.com>
> > wrote:
> > 
> > @@ -1525,7 +1525,7 @@ static void collapse_and_free_pmd(struct
> > mm_struct *mm, struct vm_area_struct *v
> > 				addr + HPAGE_PMD_SIZE);
> > 	mmu_notifier_invalidate_range_start(&range);
> > 	pmd = pmdp_collapse_flush(vma, addr, pmdp);
> > -	tlb_remove_table_sync_one();
> > +	tlb_remove_table_sync_one(mm);
> 
> Can’t pmdp_collapse_flush() have one additional argument
> “freed_tables”
> that it would propagate, for instance on x86 to flush_tlb_mm_range()
> ?
> Then you would not need tlb_remove_table_sync_one() to issue an
> additional
> IPI, no?
> 
> It just seems that you might still have 2 IPIs in many cases instead
> of
> one, and unless I am missing something, I don’t see why.
> 
Hi Nadav,
Thanks for your comment.
I think you are right and in some configurations 2 IPIs might occur.
However I a am not really dealing with the thp code at the moment,
This patch is about the mmu_gatherer and mostly dealing with IPIs sent
via the other code path.
Thanks,
Yair