linux-kernel - Re: [PATCH v7 01/31] iov_iter: Add ITER

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Date:   Mon, 26 Apr 2021 20:23:31 +0100
From:   David Howells <dhowells@...hat.com>
To:     Al Viro <viro@...iv.linux.org.uk>
Cc:     dhowells@...hat.com, linux-fsdevel@...r.kernel.org,
        Dave Wysochanski <dwysocha@...hat.com>,
        Marc Dionne <marc.dionne@...istor.com>,
        "Matthew Wilcox (Oracle)" <willy@...radead.org>,
        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>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v7 01/31] iov_iter: Add ITER_XARRAY

Al Viro <viro@...iv.linux.org.uk> wrote:

> On Fri, Apr 23, 2021 at 02:28:01PM +0100, David Howells wrote:
> > -#define iterate_all_kinds(i, n, v, I, B, K) {			\
> > +#define iterate_xarray(i, n, __v, skip, STEP) {		\
> > +	struct page *head = NULL;				\
> > +	size_t wanted = n, seg, offset;				\
> > +	loff_t start = i->xarray_start + skip;			\
> > +	pgoff_t index = start >> PAGE_SHIFT;			\
> > +	int j;							\
> > +								\
> > +	XA_STATE(xas, i->xarray, index);			\
> > +								\
> > +	rcu_read_lock();						\
> > +	xas_for_each(&xas, head, ULONG_MAX) {				\
> > +		if (xas_retry(&xas, head))				\
> > +			continue;					\
> 
> OK, now I'm really confused; what's to guarantee that restart will not have
> you hit the same entry more than once?  STEP might be e.g.
> 
> 		memcpy_to_page(v.bv_page, v.bv_offset,
> 			       (from += v.bv_len) - v.bv_len, v.bv_len)
> 
> which is clearly not idempotent - from gets incremented, after all.
> What am I missing here?

I really need to defer this question to Willy, but as I understand it,
xas_retry() only restarts the current iteration.  Referring to the comment on
xas_reset():

 * Resets the error or walk state of the @xas so future walks of the
 * array will start from the root.  Use this if you have dropped the
 * xarray lock and want to reuse the xa_state.

I think that the walk returns to the bottom of the tree and whilst xarray
presents an interface that appears to be a contiguous array, it's actually a
tree internally - and 'root' is the root of the tree, not the head of the
array.

Basically, I think it throws away its cached iteration state - which might
have been modified - and rewalks the tree to get back to the same index.

David