Received-SPF: pass (google.com: domain of linux-nfs-owner@vger.kernel.org designates 23.128.96.18 as permitted sender) client-ip=23.128.96.18;
Date:   Thu, 8 Apr 2021 15:50:57 +0100
From:   Matthew Wilcox <willy@infradead.org>
To:     David Howells <dhowells@redhat.com>
Cc:     linux-fsdevel@vger.kernel.org,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Alexander Viro <viro@zeniv.linux.org.uk>,
        Christoph Hellwig <hch@lst.de>, linux-mm@kvack.org,
        linux-cachefs@redhat.com, linux-afs@lists.infradead.org,
        linux-nfs@vger.kernel.org, linux-cifs@vger.kernel.org,
        ceph-devel@vger.kernel.org, v9fs-developer@lists.sourceforge.net,
        Trond Myklebust <trond.myklebust@hammerspace.com>,
        Anna Schumaker <anna.schumaker@netapp.com>,
        Steve French <sfrench@samba.org>,
        Dominique Martinet <asmadeus@codewreck.org>,
        Jeff Layton <jlayton@redhat.com>,
        David Wysochanski <dwysocha@redhat.com>,
        linux-kernel@vger.kernel.org
Subject: Re: [PATCH v6 02/30] mm: Add set/end/wait functions for PG_private_2
Message-ID: <20210408145057.GN2531743@casper.infradead.org>
References: <161789062190.6155.12711584466338493050.stgit@warthog.procyon.org.uk>
 <161789066013.6155.9816857201817288382.stgit@warthog.procyon.org.uk>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <161789066013.6155.9816857201817288382.stgit@warthog.procyon.org.uk>
Precedence: bulk

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*.