Commit fb32975d authored Aug 13, 2013 by Ed Cashin Committed by Linus Torvalds Aug 13, 2013

aoe: adjust ref of head for compound page tails



Fix a BUG which can trigger when direct-IO is used with AOE.

As discussed previously, the fact that some users of the block layer
provide bios that point to pages with a zero _count means that it is not
OK for the network layer to do a put_page on the skb frags during an
skb_linearize, so the aoe driver gets a reference to pages in bios and
puts the reference before ending the bio.  And because it cannot use
get_page on a page with a zero _count, it manipulates the value
directly.

It is not OK to increment the _count of a compound page tail, though,
since the VM layer will VM_BUG_ON a non-zero _count.  Block users that
do direct I/O can result in the aoe driver seeing compound page tails in
bios.  In that case, the same logic works as long as the head of the
compound page is used instead of the tails.  This patch handles compound
pages and does not BUG.

It relies on the block layer user leaving the relationship between the
page tail and its head alone for the duration between the submission of
the bio and its completion, whether successful or not.

Signed-off-by: Ed Cashin <ecashin@coraid.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

parent dfa9771a

drivers/block/aoe/aoecmd.c

+7 −10

Original line number	Diff line number	Diff line
		@@ -906,16 +906,10 @@ bio_pageinc(struct bio *bio)
		int i;

		bio_for_each_segment(bv, bio, i) {
		page = bv->bv_page;
		/* Non-zero page count for non-head members of
		* compound pages is no longer allowed by the kernel,
		* but this has never been seen here.
		* compound pages is no longer allowed by the kernel.
		*/
		if (unlikely(PageCompound(page)))
		if (compound_trans_head(page) != page) {
		pr_crit("page tail used for block I/O\n");
		BUG();
		}
		page = compound_trans_head(bv->bv_page);
		atomic_inc(&page->_count);
		}
		}
		@@ -924,10 +918,13 @@ static void
		bio_pagedec(struct bio *bio)
		{
		struct bio_vec *bv;
		struct page *page;
		int i;

		bio_for_each_segment(bv, bio, i)
		atomic_dec(&bv->bv_page->_count);
		bio_for_each_segment(bv, bio, i) {
		page = compound_trans_head(bv->bv_page);
		atomic_dec(&page->_count);
		}
		}

		static void

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