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

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Message-ID: <aa9c4bd5-f36e-4820-9ca2-1154b44b8908@arm.com>
Date: Mon, 30 Jun 2025 10:04:12 +0100
From: Ryan Roberts <ryan.roberts@....com>
To: Dev Jain <dev.jain@....com>, David Hildenbrand <david@...hat.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 30/06/2025 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...

Thanks,
Ryan