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From:   David Howells <dhowells@redhat.com>
In-Reply-To: <d4167c15-b3ce-73b2-1d66-97d651723305@linux.alibaba.com>
References: <d4167c15-b3ce-73b2-1d66-97d651723305@linux.alibaba.com> <163819575444.215744.318477214576928110.stgit@warthog.procyon.org.uk> <163819612321.215744.9738308885948264476.stgit@warthog.procyon.org.uk>
To:     JeffleXu <jefflexu@linux.alibaba.com>
Cc:     dhowells@redhat.com, linux-cachefs@redhat.com,
        Trond Myklebust <trondmy@hammerspace.com>,
        Anna Schumaker <anna.schumaker@netapp.com>,
        Steve French <sfrench@samba.org>,
        Dominique Martinet <asmadeus@codewreck.org>,
        Jeff Layton <jlayton@kernel.org>,
        Matthew Wilcox <willy@infradead.org>,
        Alexander Viro <viro@zeniv.linux.org.uk>,
        Omar Sandoval <osandov@osandov.com>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        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,
        linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org
Subject: 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
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JeffleXu <jefflexu@linux.alibaba.com> wrote:

> > @@ -466,7 +466,7 @@ static void netfs_rreq_short_read(struct netfs_rea=
d_request *rreq,
> >  	netfs_get_read_subrequest(subreq);
> >  	atomic_inc(&rreq->nr_rd_ops);
> >  	if (subreq->source =3D=3D 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 zero=
s
from the cache in order to optimise its extent list.  This instructs the c=
ache
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' cas=
e.

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

David