From: Andrew Morton <akpm@linux-foundation.org>
To: Dan Williams <dan.j.williams@intel.com>
Cc: dmaengine@vger.kernel.org, Vinod Koul <vinod.koul@intel.com>,
netdev@vger.kernel.org, Joerg Roedel <joro@8bytes.org>,
linux-kernel@vger.kernel.org,
James Bottomley <JBottomley@Parallels.com>,
Russell King <rmk+kernel@arm.linux.org.uk>
Subject: Re: [PATCH v3 4/4] dma debug: introduce debug_dma_assert_idle()
Date: Mon, 13 Jan 2014 17:14:12 -0800 [thread overview]
Message-ID: <20140113171412.dd90c020b103f4a686f8dc34@linux-foundation.org> (raw)
In-Reply-To: <20140114004804.27138.18469.stgit@viggo.jf.intel.com>
On Mon, 13 Jan 2014 16:48:47 -0800 Dan Williams <dan.j.williams@intel.com> wrote:
> Record actively mapped pages and provide an api for asserting a given
> page is dma inactive before execution proceeds. Placing
> debug_dma_assert_idle() in cow_user_page() flagged the violation of the
> dma-api in the NET_DMA implementation (see commit 77873803363c "net_dma:
> mark broken").
Some discussion of the overlap counter thing would be useful.
> --- a/include/linux/dma-debug.h
> +++ b/include/linux/dma-debug.h
>
> ...
>
> +static void __active_pfn_inc_overlap(struct dma_debug_entry *entry)
> +{
> + unsigned long pfn = entry->pfn;
> + int i;
> +
> + for (i = 0; i < RADIX_TREE_MAX_TAGS; i++)
> + if (radix_tree_tag_get(&dma_active_pfn, pfn, i) == 0) {
> + radix_tree_tag_set(&dma_active_pfn, pfn, i);
> + return;
> + }
> + pr_debug("DMA-API: max overlap count (%d) reached for pfn 0x%lx\n",
> + RADIX_TREE_MAX_TAGS, pfn);
> +}
> +
> +static void __active_pfn_dec_overlap(struct dma_debug_entry *entry)
> +{
> + unsigned long pfn = entry->pfn;
> + int i;
> +
> + for (i = RADIX_TREE_MAX_TAGS - 1; i >= 0; i--)
> + if (radix_tree_tag_get(&dma_active_pfn, pfn, i)) {
> + radix_tree_tag_clear(&dma_active_pfn, pfn, i);
> + return;
> + }
> + radix_tree_delete(&dma_active_pfn, pfn);
> +}
> +
> +static int active_pfn_insert(struct dma_debug_entry *entry)
> +{
> + unsigned long flags;
> + int rc;
> +
> + spin_lock_irqsave(&radix_lock, flags);
> + rc = radix_tree_insert(&dma_active_pfn, entry->pfn, entry);
> + if (rc == -EEXIST)
> + __active_pfn_inc_overlap(entry);
> + spin_unlock_irqrestore(&radix_lock, flags);
> +
> + return rc;
> +}
> +
> +static void active_pfn_remove(struct dma_debug_entry *entry)
> +{
> + unsigned long flags;
> +
> + spin_lock_irqsave(&radix_lock, flags);
> + __active_pfn_dec_overlap(entry);
> + spin_unlock_irqrestore(&radix_lock, flags);
> +}
OK, I think I see what's happening. The tags thing acts as a crude
counter and if the map/unmap count ends up imbalanced, we deliberately
leak an entry in the radix-tree so it can later be reported via undescribed
means. Thoughts:
- RADIX_TREE_MAX_TAGS=3 so the code could count to 7, with a bit of
futzing around.
- from a style/readability point of view it is unexpected that
__active_pfn_dec_overlap() actually removes radix-tree items. It
would be better to do:
spin_lock_irqsave(&radix_lock, flags);
if (__active_pfn_dec_overlap(entry) == something) {
/*
* Nice comment goes here
*/
radix_tree_delete(...);
}
spin_unlock_irqrestore(&radix_lock, flags);
next prev parent reply other threads:[~2014-01-14 1:14 UTC|newest]
Thread overview: 16+ messages / expand[flat|nested] mbox.gz Atom feed top
2014-01-14 0:46 [PATCH v3 0/4] net_dma removal, and dma debug extension Dan Williams
2014-01-14 0:46 ` [PATCH v3 1/4] net_dma: simple removal Dan Williams
2014-01-15 21:20 ` saeed bishara
2014-01-15 21:31 ` Dan Williams
2014-01-15 21:33 ` Dan Williams
2014-01-17 20:16 ` saeed bishara
2014-01-21 9:44 ` Dan Williams
2014-01-22 10:38 ` saeed bishara
2014-01-14 0:47 ` [PATCH v3 2/4] net_dma: revert 'copied_early' Dan Williams
2014-01-14 5:16 ` David Miller
2014-01-14 6:04 ` Dan Williams
2014-01-14 0:47 ` [PATCH v3 3/4] net: make tcp_cleanup_rbuf private Dan Williams
2014-01-14 0:48 ` [PATCH v3 4/4] dma debug: introduce debug_dma_assert_idle() Dan Williams
2014-01-14 1:14 ` Andrew Morton [this message]
2014-01-14 2:40 ` Dan Williams
2014-01-14 22:04 ` Dan Williams
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
Avoid top-posting and favor interleaved quoting:
https://en.wikipedia.org/wiki/Posting_style#Interleaved_style
* Reply using the --to, --cc, and --in-reply-to
switches of git-send-email(1):
git send-email \
--in-reply-to=20140113171412.dd90c020b103f4a686f8dc34@linux-foundation.org \
--to=akpm@linux-foundation.org \
--cc=JBottomley@Parallels.com \
--cc=dan.j.williams@intel.com \
--cc=dmaengine@vger.kernel.org \
--cc=joro@8bytes.org \
--cc=linux-kernel@vger.kernel.org \
--cc=netdev@vger.kernel.org \
--cc=rmk+kernel@arm.linux.org.uk \
--cc=vinod.koul@intel.com \
/path/to/YOUR_REPLY
https://kernel.org/pub/software/scm/git/docs/git-send-email.html
* If your mail client supports setting the In-Reply-To header
via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line
before the message body.
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).