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

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Message-ID: <cc846c55-0505-4ad6-9664-ac799d9c0226@redhat.com>
Date: Sat, 28 Jun 2025 23:00:37 +0200
From: David Hildenbrand <david@...hat.com>
To: 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>,
 Ryan Roberts <ryan.roberts@....com>
Subject: Re: [PATCH v1 1/4] mm: convert FPB_IGNORE_* into FPB_HONOR_*

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).

-- 
Cheers,

David / dhildenb