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

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Message-ID: <5375208d-2c11-4579-a303-e8416ab07159@arm.com>
Date: Mon, 30 Jun 2025 09:04:22 +0530
From: Dev Jain <dev.jain@....com>
To: 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>,
 Ryan Roberts <ryan.roberts@....com>
Subject: Re: [PATCH v1 1/4] mm: convert FPB_IGNORE_* into FPB_HONOR_*

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.