linux-kernel - Re: [PATCH 24/64] netfs: Pass more information on how to deal with a hole in the cache

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Date:   Thu, 09 Dec 2021 16:49:53 +0000
From:   David Howells <dhowells@...hat.com>
To:     JeffleXu <jefflexu@...ux.alibaba.com>
Cc:     dhowells@...hat.com, linux-cachefs@...hat.com,
        Trond Myklebust <trondmy@...merspace.com>,
        Anna Schumaker <anna.schumaker@...app.com>,
        Steve French <sfrench@...ba.org>,
        Dominique Martinet <asmadeus@...ewreck.org>,
        Jeff Layton <jlayton@...nel.org>,
        Matthew Wilcox <willy@...radead.org>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Omar Sandoval <osandov@...ndov.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        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: [PATCH 24/64] netfs: Pass more information on how to deal with a hole in the cache

JeffleXu <jefflexu@...ux.alibaba.com> wrote:

> > @@ -466,7 +466,7 @@ static void netfs_rreq_short_read(struct netfs_read_request *rreq,
> >  	netfs_get_read_subrequest(subreq);
> >  	atomic_inc(&rreq->nr_rd_ops);
> >  	if (subreq->source == NETFS_READ_FROM_CACHE)
> > -		netfs_read_from_cache(rreq, subreq, true);
> > +		netfs_read_from_cache(rreq, subreq, NETFS_READ_HOLE_CLEAR);
> 
> Hi I'm not sure why NETFS_READ_HOLE_CLEAR style should be used in 'short
> read' case.

The cache backing filesystem (eg. ext4) might have excised a chunk of zeros
from the cache in order to optimise its extent list.  This instructs the cache
to zero over the cracks.  Actually, I need to think a bit further on this.
This was written assuming that the cache tracks its content independently -
but those patches are not in with this set.

> I'm not sure why 'subreq->start' is not incremented with
> 'subreq->transferred' when calling cres->ops->read() in 'short read' case.

subreq->start shouldn't get changed.  subreq->transferred is sufficient.

David