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From: David Howells <dhowells@redhat.com>
To: Matthew Wilcox <willy@infradead.org>
Cc: dhowells@redhat.com, Alexander Viro <viro@zeniv.linux.org.uk>,
	Andrew Morton <akpm@linux-foundation.org>,
	linux-fsdevel@vger.kernel.org, linux-mm@kvack.org,
	linux-kernel@vger.kernel.org
Subject: Re: [RFC][PATCH] iov_iter: Add ITER_MAPPING
Date: Thu, 23 Jan 2020 11:04:59 +0000	[thread overview]
Message-ID: <3785795.1579777499@warthog.procyon.org.uk> (raw)
In-Reply-To: <20200122193306.GB4675@bombadil.infradead.org>

Matthew Wilcox <willy@infradead.org> wrote:

> It's perfectly legal to have compound pages in the page cache.  Call
> find_subpage(page, xas.xa_index) unconditionally.

Like this?

#define iterate_mapping(i, n, __v, skip, STEP) {		\
	struct page *page;					\
	size_t wanted = n, seg, offset;				\
	loff_t start = i->mapping_start + skip;			\
	pgoff_t index = start >> PAGE_SHIFT;			\
								\
	XA_STATE(xas, &i->mapping->i_pages, index);		\
								\
	rcu_read_lock();						\
	xas_for_each(&xas, page, ULONG_MAX) {				\
		if (xas_retry(&xas, page) || xa_is_value(page)) {	\
			WARN_ON(1);					\
			break;						\
		}							\
		__v.bv_page = find_subpage(page, xas.xa_index);		\
		offset = (i->mapping_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;					\
	}							\
	rcu_read_unlock();					\
	n = wanted - n;						\
}

Note that the walk is not restartable - and the array is supposed to have been
fully populated by the caller for the range specified - so I've made it print
a warning and end the loop if xas_retry() or xa_is_value() return true (which
takes care of the !page case too).  Possibly I could just leave it to fault in
this case and not check.

If PageHuge(page) is true, I presume I need to support that too.  How do I
find out how big the page is?

David


  parent reply	other threads:[~2020-01-23 11:05 UTC|newest]

Thread overview: 6+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-01-22 14:57 [RFC][PATCH] iov_iter: Add ITER_MAPPING David Howells
2020-01-22 19:33 ` Matthew Wilcox
2020-01-22 21:42 ` David Howells
2020-01-23 11:04 ` David Howells [this message]
2020-01-24 11:21   ` Matthew Wilcox
2020-01-24 14:58   ` David Howells

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