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

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Message-ID: <f0ccb18b-4297-4741-9dd9-d020b171c28d@redhat.com>
Date: Mon, 30 Jun 2025 11:08:34 +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 11:04, 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? ;)

-- 
Cheers,

David / dhildenb