On Tue, Nov 19, 2019 at 4:02 PM Alexander Duyck <alexander.duyck@xxxxxxxxx> wrote: > > On Tue, Nov 19, 2019 at 9:49 AM Dan Williams <dan.j.williams@xxxxxxxxx> wrote: > > > > The debug_dma_assert_idle() infrastructure was put in place to catch a > > data corruption scenario first identified by the now defunct NET_DMA > > receive offload feature. It caught cases where dma was in flight to a > > stale page because the dma raced the cpu writing the page, and the cpu > > write triggered cow_user_page(). > > > > However, the dma-debug tracking is overeager and also triggers in cases > > where the dma device is reading from a page that is also undergoing > > cow_user_page(). > > > > The fix proposed was originally posted in 2016, and Russell reported > > "Yes, that seems to avoid the warning for me from an initial test", and > > now Don is also reporting that this fix is addressing a similar false > > positive report that he is seeing. > > > > Link: https://lore.kernel.org/r/CAPcyv4j8fWqwAaX5oCdg5atc+vmp57HoAGT6AfBFwaCiv0RbAQ@xxxxxxxxxxxxxx > > Reported-by: Russell King <linux@xxxxxxxxxxxxxxx> > > Reported-by: Don Dutile <ddutile@xxxxxxxxxx> > > Fixes: 0abdd7a81b7e ("dma-debug: introduce debug_dma_assert_idle()") > > Cc: <stable@xxxxxxxxxxxxxxx> > > Cc: Christoph Hellwig <hch@xxxxxx> > > Cc: Marek Szyprowski <m.szyprowski@xxxxxxxxxxx> > > Cc: Robin Murphy <robin.murphy@xxxxxxx> > > Signed-off-by: Dan Williams <dan.j.williams@xxxxxxxxx> > > --- > > kernel/dma/debug.c | 2 +- > > 1 file changed, 1 insertion(+), 1 deletion(-) > > > > diff --git a/kernel/dma/debug.c b/kernel/dma/debug.c > > index 099002d84f46..11a6db53d193 100644 > > --- a/kernel/dma/debug.c > > +++ b/kernel/dma/debug.c > > @@ -587,7 +587,7 @@ void debug_dma_assert_idle(struct page *page) > > } > > spin_unlock_irqrestore(&radix_lock, flags); > > > > - if (!entry) > > + if (!entry || entry->direction != DMA_FROM_DEVICE) > > return; > > > > cln = to_cacheline_number(entry); > > If I am understanding right DMA_TO_DEVICE is fine, but won't you also > need to cover the DMA_BIDIRECTIONAL case since it is possible for a > device to also write the memory in that case? True, DMA_BIDIRECTIONAL and DMA_TO_DEVICE are being treated equally in this case. Given this is the second time this facility needed to be taught to be less eager [1], I'd be inclined to let the tie-break / BIDIR case be treated like TO. This facility was always meant as a "there might be a problem here", but not a definitive checker, and it certainly loses value if the reports are ambiguous. [1]: 3b7a6418c749 dma debug: account for cachelines and read-only mappings in overlap tracking
