linux-kernel - Re: [RFC PATCH 54/61] afs: Wait on PG_fscache before modifying/releasing a page

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Message-ID: <713141.1588775072@warthog.procyon.org.uk>
Date:   Wed, 06 May 2020 15:24:32 +0100
From:   David Howells <dhowells@...hat.com>
To:     Matthew Wilcox <willy@...radead.org>
Cc:     dhowells@...hat.com, Trond Myklebust <trondmy@...merspace.com>,
        Anna Schumaker <anna.schumaker@...app.com>,
        Steve French <sfrench@...ba.org>,
        Jeff Layton <jlayton@...hat.com>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        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,
        linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH 54/61] afs: Wait on PG_fscache before modifying/releasing a page

Matthew Wilcox <willy@...radead.org> wrote:

> > Won't that screw up ITER_MAPPING?  Does that mean that ITER_MAPPING isn't
> > viable?
> 
> Can you remind me why ITER_MAPPING needs:
> 
> "The caller must guarantee that the pages are all present and they must be
> locked using PG_locked, PG_writeback or PG_fscache to prevent them from
> going away or being migrated whilst they're being accessed."
> 
> An elevated refcount prevents migration, and it also prevents the pages
> from being freed.  It doesn't prevent them from being truncated out of
> the file, but it does ensure the pages aren't reallocated.

ITER_MAPPING relies on the mapping to maintain the pointers to the pages so
that it can find them rather than being like ITER_BVEC where there's a
separate list.

Truncate removes the pages from the mapping - at which point ITER_MAPPING can
no longer find them.

David