linux-kernel - Re: [PATCH v2 4/6] mm/migrate_device: convert __migrate_device

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Message-ID: <Zc_taXK-X-NYPQli@casper.infradead.org>
Date: Fri, 16 Feb 2024 23:19:05 +0000
From: Matthew Wilcox <willy@...radead.org>
To: Sidhartha Kumar <sidhartha.kumar@...cle.com>
Cc: linux-kernel@...r.kernel.org, linux-mm@...ck.org,
	akpm@...ux-foundation.org, apopple@...dia.com
Subject: Re: [PATCH v2 4/6] mm/migrate_device: convert
 __migrate_device_pages() to folios

On Fri, Feb 16, 2024 at 02:00:31PM -0800, Sidhartha Kumar wrote:
> On 2/16/24 1:55 PM, Matthew Wilcox wrote:
> > On Fri, Feb 16, 2024 at 01:13:18PM -0800, Sidhartha Kumar wrote:
> > > Use migrate_pfn_to_folio() so we can work with folios directly in
> > > __migrate_device_pages().
> > 
> > i don't understand why this would be correct if we have multipage
> > folios.
> > 
> 
> Alistair mentioned that he is working on order > 0 device page support so I
> was under the impression that currently device pages are only order 0.

That might well be true, but I'm *very* uncomfortable with doing a folio
conversion in core MM that won't work with large folios.  We need to
consider what will happen with large device folios.

(for filesystems, I am less bothered.  Individual filesystems control
whether they see large folios or not, and for lesser filesystems it may
never be worth converting them to support large folios)