linux-kernel - Re: [PATCH v1 1/4] mm: convert FPB_IGNORE_* into FPB_HONOR

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Message-ID: <8993dbc9-6c9a-4ac7-8c04-813851eba938@redhat.com>
Date: Mon, 30 Jun 2025 11:24:08 +0200
From: David Hildenbrand <david@...hat.com>
To: Ryan Roberts <ryan.roberts@....com>, Dev Jain <dev.jain@....com>,
 linux-kernel@...r.kernel.org
Cc: linux-mm@...ck.org, Andrew Morton <akpm@...ux-foundation.org>,
 "Liam R. Howlett" <Liam.Howlett@...cle.com>,
 Lorenzo Stoakes <lorenzo.stoakes@...cle.com>,
 Vlastimil Babka <vbabka@...e.cz>, Jann Horn <jannh@...gle.com>,
 Mike Rapoport <rppt@...nel.org>, Suren Baghdasaryan <surenb@...gle.com>,
 Michal Hocko <mhocko@...e.com>, Zi Yan <ziy@...dia.com>,
 Matthew Brost <matthew.brost@...el.com>,
 Joshua Hahn <joshua.hahnjy@...il.com>, Rakie Kim <rakie.kim@...com>,
 Byungchul Park <byungchul@...com>, Gregory Price <gourry@...rry.net>,
 Ying Huang <ying.huang@...ux.alibaba.com>,
 Alistair Popple <apopple@...dia.com>, Pedro Falcato <pfalcato@...e.de>,
 Rik van Riel <riel@...riel.com>, Harry Yoo <harry.yoo@...cle.com>
Subject: Re: [PATCH v1 1/4] mm: convert FPB_IGNORE_* into FPB_HONOR_*

On 30.06.25 11:18, Ryan Roberts wrote:
> On 30/06/2025 10:08, David Hildenbrand wrote:
>> On 30.06.25 11:04, Ryan Roberts wrote:
>>> On 30/06/2025 04:34, Dev Jain wrote:
>>>>
>>>> On 29/06/25 2:30 am, David Hildenbrand wrote:
>>>>> On 28.06.25 05:37, Dev Jain wrote:
>>>>>>
>>>>>> On 27/06/25 5:25 pm, David Hildenbrand wrote:
>>>>>>> Honoring these PTE bits is the exception, so let's invert the meaning.
>>>>>>>
>>>>>>> With this change, most callers don't have to pass any flags.
>>>>>>>
>>>>>>> No functional change intended.
>>>>>>
>>>>>> FWIW I had proposed this kind of change earlier to Ryan (CCed) and
>>>>>> he pointed out: "Doesn't that argument apply in reverse if you want
>>>>>> to ignore something new in future?
>>>>>>
>>>>>> By default we are comparing all the bits in the pte when determining the
>>>>>> batch.
>>>>>> The flags request to ignore certain bits.
>>>>>
>>>>> That statement is not true: as default we ignore the write and young bit. And
>>>>> we don't have flags for that ;)
>>>>>
>>>>> Now we also ignore the dirty and soft-dity bit as default, unless told not to
>>>>> do that by one very specific caller.
>>>>>
>>>>>> If we want to ignore extra bits in
>>>>>> future, we add new flags and the existing callers don't need to be updated.
>>>>>
>>>>> What stops you from using FPB_IGNORE_* for something else in the future?
>>>>>
>>>>> As a side note, there are not that many relevant PTE bits to worry about in
>>>>> the near future ;)
>>>>>
>>>>> I mean, uffd-wp, sure, ... and before we add a FPB_HONOR_UFFD_WP to all users
>>>>> to be safe (and changing the default to ignore), you could add a
>>>>> FPB_IGNORE_UFFD_WP first, to then check who really can tolerate just ignoring
>>>>> it (most of them, I assume).
>>>> I agree.
>>>
>>> Meh. Personally I think if you start mixing HONOR and IGNORE flags, it becomes
>>> very confusing to work out what is being checked for and what is not. I stand by
>>> my original view. But yeah, writable and young confuse it a bit... How about
>>> generalizing by explicitly requiring IGNORE flags for write and young, then also
>>> create a flags macro for the common case?
>>>
>>> #define FPB_IGNORE_COMMON (FPB_IGNORE_WRITE | FPB_IGNORE_YOUNG |    \
>>>                 FPB_IGNORE_DIRTY | FPB_IGNORE_SOFT_DIRTY)
>>>
>>> It's not a hill I'm going to die on though...
>>
>> How about we make this function simpler, not more complicated? ;)
> 
> I think here we both have different views of what is simpler... You are trying
> to optimize for the callers writing less code. I'm trying to optimize for the
> reader to be able to easily determine what the function will do for a given caller.

See patch number #3: I want the default function -- folio_pte_batch() -- 
to not have any flags at all.

And I don't want to achieve that by internally using flags when calling 
folio_pte_batch_ext().

If you don't specify flags (folio_pte_batch()), behave just as if 
calling folio_pte_batch_ext() without flags. Anything else would be more 
confusing IMHO.

I agree that mixing HONOR and IGNORE is not a good idea. But then, it's 
really only uffd-wp that still could be batched, and likely we want it 
to be the default, and respect/honor/whatever instead in the cases where 
we really have to.

(If we really want to go down that path and batch it :) )

-- 
Cheers,

David / dhildenb