linux-kernel - Re: [PATCH v5 10/12] x86,tlb: do targeted broadcast flushing from tlbbatch code

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Message-ID: <11d9f41a-eae3-43a1-90bc-eb5c69020002@gmail.com>
Date: Mon, 20 Jan 2025 19:09:06 +0200
From: Nadav Amit <nadav.amit@...il.com>
To: Rik van Riel <riel@...riel.com>, x86@...nel.org
Cc: linux-kernel@...r.kernel.org, bp@...en8.de, peterz@...radead.org,
 dave.hansen@...ux.intel.com, zhengqi.arch@...edance.com,
 thomas.lendacky@....com, kernel-team@...a.com, linux-mm@...ck.org,
 akpm@...ux-foundation.org, jannh@...gle.com, mhklinux@...look.com,
 andrew.cooper3@...rix.com
Subject: Re: [PATCH v5 10/12] x86,tlb: do targeted broadcast flushing from
 tlbbatch code



On 20/01/2025 18:11, Rik van Riel wrote:
> On Mon, 2025-01-20 at 16:14 +0200, Nadav Amit wrote:
>>
>> I am not sure we are on the same page. What I suggested is:
>>
>> 1. arch_tlbbatch_flush() would still do tlbsync()
>> 2. No migrate_enable() in arch_tlbbatch_flush()
>> 3. No migrate_disable() in arch_tlbbatch_add_pending()
>> 4. arch_tlbbatch_add_pending() sets
>> cpu_tlbstate.pending_tlb_broadcast
>> 5. switch_mm_irqs_off() checks cpu_tlbstate.pending_tlb_broadcast and
>> if
>> it is set performs tlbsync and clears it.
>>
> How does that synchronize the page freeing (from page
> reclaim) with the TLBSYNCs?
> 
> What guarantees that the page reclaim path won't free
> the pages until after TLBSYNC has completed on the CPUs
> that kicked off asynchronous flushes with INVPLGB?

[ you make me lose my confidence, although I see nothing wrong ]

Freeing the pages must be done after the TLBSYNC. I did not imply it 
needs to be changed.

The page freeing (and reclaim) path is only initiated after 
arch_tlbbatch_flush() is completed. If no migration is initiated, since 
we did not remove any tlbsync, it should be fine.

If migration was initiated, and some invlpgb's were already initiated, 
then we need for correctness to initiate tlbsync before the task might 
be scheduled on another core. That's exactly why adding tlbsync to 
switch_mm_irqs_off() is needed in such a case.