linux-kernel - Re: [PATCH v3 4/6] mm: Introduce a pageflag for partially mapped folios

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Message-ID: <Zr9zx74W6-oRwKXB@casper.infradead.org>
Date: Fri, 16 Aug 2024 16:44:07 +0100
From: Matthew Wilcox <willy@...radead.org>
To: Usama Arif <usamaarif642@...il.com>
Cc: akpm@...ux-foundation.org, linux-mm@...ck.org, hannes@...xchg.org,
	riel@...riel.com, shakeel.butt@...ux.dev, roman.gushchin@...ux.dev,
	yuzhao@...gle.com, david@...hat.com, baohua@...nel.org,
	ryan.roberts@....com, rppt@...nel.org, cerasuolodomenico@...il.com,
	corbet@....net, linux-kernel@...r.kernel.org,
	linux-doc@...r.kernel.org, kernel-team@...a.com
Subject: Re: [PATCH v3 4/6] mm: Introduce a pageflag for partially mapped
 folios

On Tue, Aug 13, 2024 at 01:02:47PM +0100, Usama Arif wrote:
> diff --git a/include/linux/page-flags.h b/include/linux/page-flags.h
> index a0a29bd092f8..cecc1bad7910 100644
> --- a/include/linux/page-flags.h
> +++ b/include/linux/page-flags.h
> @@ -182,6 +182,7 @@ enum pageflags {
>  	/* At least one page in this folio has the hwpoison flag set */
>  	PG_has_hwpoisoned = PG_active,
>  	PG_large_rmappable = PG_workingset, /* anon or file-backed */
> +	PG_partially_mapped, /* was identified to be partially mapped */

No, you can't do this.  You have to be really careful when reusing page
flags, you can't just take the next one.  What made you think it would
be this easy?

I'd suggest using PG_reclaim.  You also need to add PG_partially_mapped
to PAGE_FLAGS_SECOND.  You might get away without that if you're
guaranteeing it'll always be clear when you free the folio; I don't
understand this series so I don't know if that's true or not.