linux-kernel - Re: [PATCH RFC 08/29] mm/migrate: rename putback_movable_folio() to putback_movable_ops

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Message-ID: <aFMbsdPYhcL8fyOo@casper.infradead.org>
Date: Wed, 18 Jun 2025 21:04:01 +0100
From: Matthew Wilcox <willy@...radead.org>
To: Zi Yan <ziy@...dia.com>
Cc: David Hildenbrand <david@...hat.com>, linux-kernel@...r.kernel.org,
	linux-mm@...ck.org, linux-doc@...r.kernel.org,
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	Shakeel Butt <shakeel.butt@...ux.dev>
Subject: Re: [PATCH RFC 08/29] mm/migrate: rename putback_movable_folio() to
 putback_movable_ops_page()

On Wed, Jun 18, 2025 at 03:25:46PM -0400, Zi Yan wrote:
> On 18 Jun 2025, at 15:18, Matthew Wilcox wrote:
> >> Why not use page version of lock, unlock, and put? Especially you are
> >> thinking about not using folio for these pages. Just a question,
> >> I am OK with current patch.
> >
> > That would reintroduce unnecessary calls to compound_head().
> 
> Got it. But here page is not folio, so it cannot be a compound page.
> Then, we will need page versions without compound_head() for
> non compound pages. Could that happen in the future when only folio
> can be compound and page is only order-0?

I think the assumption that we'll only see compound pages as part of
folios is untrue.  For example, slabs will still allocate multiple
pages (though slabs aren't migratable at this point).  The sketch at
https://kernelnewbies.org/MatthewWilcox/Memdescs supports "misc pages"
with an order stored in bits 12-17 of the memdesc.  I don't know
how useful that will turn out to be; maybe we'll never implement that.