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Date: Thu, 6 Jun 2024 10:33:26 +0200
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Subject: Re: [PATCH v11 09/12] mm: implement LUF(Lazy Unmap Flush) defering
 tlb flush when folios get unmapped
To: Byungchul Park <byungchul@sk.com>, Dave Hansen <dave.hansen@intel.com>
Cc: Byungchul Park <lkml.byungchul.park@gmail.com>,
 linux-kernel@vger.kernel.org, linux-mm@kvack.org, kernel_team@skhynix.com,
 akpm@linux-foundation.org, ying.huang@intel.com, vernhao@tencent.com,
 mgorman@techsingularity.net, hughd@google.com, willy@infradead.org,
 peterz@infradead.org, luto@kernel.org, tglx@linutronix.de, mingo@redhat.com,
 bp@alien8.de, dave.hansen@linux.intel.com, rjgolo@gmail.com
References: <20240531092001.30428-1-byungchul@sk.com>
 <20240531092001.30428-10-byungchul@sk.com>
 <fab1dd64-c652-4160-93b4-7b483a8874da@intel.com>
 <CAHyrMpxETdVewTH3MCS4qPyD6Xf1zRUfWZf-8SCdpCFj2Pj_Wg@mail.gmail.com>
 <f17f33e8-1c1f-460f-8c5a-713476f524a3@intel.com>
 <26dc4594-430b-483c-a26c-7e68bade74b0@redhat.com>
 <20240603093505.GA12549@system.software.com>
 <d650c29b-129f-4fac-9a9d-ea1fbdae2c3a@intel.com>
 <20240604015348.GB26609@system.software.com>
 <20240604044348.GA40889@system.software.com>
From: David Hildenbrand <david@redhat.com>
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On 04.06.24 06:43, Byungchul Park wrote:
> On Tue, Jun 04, 2024 at 10:53:48AM +0900, Byungchul Park wrote:
>> On Mon, Jun 03, 2024 at 06:23:46AM -0700, Dave Hansen wrote:
>>> On 6/3/24 02:35, Byungchul Park wrote:
>>> ...> In luf's point of view, the points where the deferred flush should be
>>>> performed are simply:
>>>>
>>>> 	1. when changing the vma maps, that might be luf'ed.
>>>> 	2. when updating data of the pages, that might be luf'ed.
>>>
>>> It's simple, but the devil is in the details as always.
>>
>> Agree with that.
>>
>>>> All we need to do is to indentify the points:
>>>>
>>>> 	1. when changing the vma maps, that might be luf'ed.
>>>>
>>>> 	   a) mmap and munmap e.i. fault handler or unmap_region().
>>>> 	   b) permission to writable e.i. mprotect or fault handler.
>>>> 	   c) what I'm missing.
>>>
>>> I'd say it even more generally: anything that installs a PTE which is
>>> inconsistent with the original PTE.  That, of course, includes writes.
>>> But it also includes crazy things that we do like uprobes.  Take a look
>>> at __replace_page().
>>>
>>> I think the page_vma_mapped_walk() checks plus the ptl keep LUF at bay
>>> there.  But it needs some really thorough review.
>>>
>>> But the bigger concern is that, if there was a problem, I can't think of
>>> a systematic way to find it.
>>>
>>>> 	2. when updating data of the pages, that might be luf'ed.
>>>>
>>>> 	   a) updating files through vfs e.g. file_end_write().
>>>> 	   b) updating files through writable maps e.i. 1-a) or 1-b).
>>>> 	   c) what I'm missing.
>>>
>>> Filesystems or block devices that change content without a "write" from
>>> the local system.  Network filesystems and block devices come to mind.
>>
>> AFAIK, every network filesystem eventully "updates" its connected local
>> filesystem.  It could be still handled at the point where updating the
>> local file system.
>>
>>> I honestly don't know what all the rules are around these, but they
>>> could certainly be troublesome.
>>>
>>> There appear to be some interactions for NFS between file locking and
>>> page cache flushing.
>>>
>>> But, stepping back ...
>>>
>>> I'd honestly be a lot more comfortable if there was even a debugging LUF
>>
>> I'd better provide a method for better debugging.  Lemme know whatever
>> it is we need.
>>
>>> mode that enforced a rule that said:
> 
> Do you means a debugging mode that can WARN or inform the situation that
> we don't want?  If yes, sure.  Now that I get this, I will re-read all
> you guys' talk.
> 

In my opinion either for debugging, or for actually enforcing it at 
runtime. Whatever the cost of that would be needs to be determined.

-- 
Cheers,

David / dhildenb