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

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Date:   Mon, 26 Apr 2021 18:54:12 +0000
From:   Al Viro <viro@...iv.linux.org.uk>
To:     David Howells <dhowells@...hat.com>
Cc:     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

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?

> +		if (WARN_ON(xa_is_value(head)))				\
> +			break;						\
> +		if (WARN_ON(PageHuge(head)))				\
> +			break;						\
> +		for (j = (head->index < index) ? index - head->index : 0; \
> +		     j < thp_nr_pages(head); j++) {			\
> +			__v.bv_page = head + j;				\
> +			offset = (i->xarray_start + skip) & ~PAGE_MASK;	\
> +			seg = PAGE_SIZE - offset;			\
> +			__v.bv_offset = offset;				\
> +			__v.bv_len = min(n, seg);			\
> +			(void)(STEP);					\
> +			n -= __v.bv_len;				\
> +			skip += __v.bv_len;				\
> +			if (n == 0)					\
> +				break;					\
> +		}							\
> +		if (n == 0)						\
> +			break;						\
> +	}							\
> +	rcu_read_unlock();					\
> +	n = wanted - n;						\
> +}