linux-kernel - Re: [PATCH v6 02/30] mm: Add set/end/wait functions for PG_private

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Message-ID: <20210408145057.GN2531743@casper.infradead.org>
Date:   Thu, 8 Apr 2021 15:50:57 +0100
From:   Matthew Wilcox <willy@...radead.org>
To:     David Howells <dhowells@...hat.com>
Cc:     linux-fsdevel@...r.kernel.org,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Christoph Hellwig <hch@....de>, linux-mm@...ck.org,
        linux-cachefs@...hat.com, linux-afs@...ts.infradead.org,
        linux-nfs@...r.kernel.org, linux-cifs@...r.kernel.org,
        ceph-devel@...r.kernel.org, v9fs-developer@...ts.sourceforge.net,
        Trond Myklebust <trond.myklebust@...merspace.com>,
        Anna Schumaker <anna.schumaker@...app.com>,
        Steve French <sfrench@...ba.org>,
        Dominique Martinet <asmadeus@...ewreck.org>,
        Jeff Layton <jlayton@...hat.com>,
        David Wysochanski <dwysocha@...hat.com>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v6 02/30] mm: Add set/end/wait functions for PG_private_2

On Thu, Apr 08, 2021 at 03:04:20PM +0100, David Howells wrote:
> +static inline void set_page_private_2(struct page *page)
> +{
> +	get_page(page);
> +	SetPagePrivate2(page);

PAGEFLAG(OwnerPriv1, owner_priv_1, PF_ANY)

So we can set Private2 on any tail page ...

> +void end_page_private_2(struct page *page)
> +{
> +	page = compound_head(page);
> +	VM_BUG_ON_PAGE(!PagePrivate2(page), page);
> +	clear_bit_unlock(PG_private_2, &page->flags);
> +	wake_up_page_bit(page, PG_private_2);

... but when we try to end on a tail, we actually wake up the head ...

> +void wait_on_page_private_2(struct page *page)
> +{
> +	while (PagePrivate2(page))
> +		wait_on_page_bit(page, PG_private_2);

... although if we were waiting on a tail, the wake up won't find us ...

if only we had a way to ensure this kind of bug can't happen *cough,
lend your support to the page folio patches*.