linux-kernel - Re: [PATCH v2 4/4] mm: remove boolean output parameters from folio_pte_batch

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Message-ID: <3f8d6f3f-f98b-4f90-95d5-f4a09d61c88a@redhat.com>
Date: Wed, 2 Jul 2025 16:40:30 +0200
From: David Hildenbrand <david@...hat.com>
To: Oscar Salvador <osalvador@...e.de>
Cc: linux-kernel@...r.kernel.org, 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>,
 Lance Yang <ioworker0@...il.com>
Subject: Re: [PATCH v2 4/4] mm: remove boolean output parameters from
 folio_pte_batch_ext()

On 02.07.25 16:00, Oscar Salvador wrote:
> On Wed, Jul 02, 2025 at 12:49:26PM +0200, David Hildenbrand wrote:
>> Instead, let's just allow for specifying through flags whether we want
>> to have bits merged into the original PTE.
>>
>> For the madvise() case, simplify by having only a single parameter for
>> merging young+dirty. For madvise_cold_or_pageout_pte_range() merging the
>> dirty bit is not required, but also not harmful. This code is not that
>> performance critical after all to really force all micro-optimizations.
> 
> Seems a bit odd to me to have the dirty-young bit being treat as "one".
> You mention that this is done because the only user of it doesn't really
> care about dirty vs non-dirty and it's not harmful eitherway, so micro-optimizing
> this isn't worth at this moment.
> 
> But what if we grop another user which wants to make this distinction and
> where it matters dirty vs non-dirty/young vs not-young.
> Wouldn't be better to have it separated from the start?

I mean, it's easy to add later if ever required. :)

But more importantly, usually young+dirty is a moving target as HW can 
usually update it asynchronously.

So in the common case, you really have to rely on the young+dirty bits 
from get_and_clear_full_ptes(), and not on the current snapshot while 
the page remains mapped.

The madvise() use case is rather special in that sense.

-- 
Cheers,

David / dhildenb