Messages in this thread Patch in this message | | | Subject | Re: [RFC PATCH 1/1] s390/cio: make ccw_device_dma_* more robust | From | Pierre Morel <> | Date | Mon, 11 Oct 2021 15:45:55 +0200 |
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On 10/11/21 1:59 PM, Halil Pasic wrote: > Since commit 48720ba56891 ("virtio/s390: use DMA memory for ccw I/O and > classic notifiers") we were supposed to make sure that > virtio_ccw_release_dev() completes before the ccw device and the > attached dma pool are torn down, but unfortunately we did not. Before > that commit it used to be OK to delay cleaning up the memory allocated > by virtio-ccw indefinitely (which isn't really intuitive for guys used > to destruction happens in reverse construction order), but now we > trigger a BUG_ON if the genpool is destroyed before all memory allocated > form it. Which brings down the guest. We can observe this problem, when > unregister_virtio_device() does not give up the last reference to the > virtio_device (e.g. because a virtio-scsi attached scsi disk got removed > without previously unmounting its previously mounted partition). > > To make sure that the genpool is only destroyed after all the necessary > freeing is done let us take a reference on the ccw device on each > ccw_device_dma_zalloc() and give it up on each ccw_device_dma_free(). > > Actually there are multiple approaches to fixing the problem at hand > that can work. The upside of this one is that it is the safest one while > remaining simple. We don't crash the guest even if the driver does not > pair allocations and frees. The downside is the reference counting > overhead, that the reference counting for ccw devices becomes more > complex, in a sense that we need to pair the calls to the aforementioned > functions for it to be correct, and that if we happen to leak, we leak > more than necessary (the whole ccw device instead of just the genpool). > > Some alternatives to this approach are taking a reference in > virtio_ccw_online() and giving it up in virtio_ccw_release_dev() or > making sure virtio_ccw_release_dev() completes its work before > virtio_ccw_remove() returns. The downside of these approaches is that > these are less safe against programming errors. > > Cc: <stable@vger.kernel.org> # v5.3 > Signed-off-by: Halil Pasic <pasic@linux.ibm.com> > Fixes: 48720ba56891 ("virtio/s390: use DMA memory for ccw I/O and > classic notifiers") > Reported-by: bfu@redhat.com > > --- > > FYI I've proposed a different fix to this very same problem: > https://lore.kernel.org/lkml/20210915215742.1793314-1-pasic@linux.ibm.com/ > > This patch is more or less a result of that discussion. > --- > drivers/s390/cio/device_ops.c | 12 +++++++++++- > 1 file changed, 11 insertions(+), 1 deletion(-) > > diff --git a/drivers/s390/cio/device_ops.c b/drivers/s390/cio/device_ops.c > index 0fe7b2f2e7f5..c533d1dadc6b 100644 > --- a/drivers/s390/cio/device_ops.c > +++ b/drivers/s390/cio/device_ops.c > @@ -825,13 +825,23 @@ EXPORT_SYMBOL_GPL(ccw_device_get_chid); > */ > void *ccw_device_dma_zalloc(struct ccw_device *cdev, size_t size) > { > - return cio_gp_dma_zalloc(cdev->private->dma_pool, &cdev->dev, size); > + void *addr; > + > + if (!get_device(&cdev->dev)) > + return NULL; > + addr = cio_gp_dma_zalloc(cdev->private->dma_pool, &cdev->dev, size); > + if (IS_ERR_OR_NULL(addr))
I can be wrong but it seems that only dma_alloc_coherent() used in cio_gp_dma_zalloc() report an error but the error is ignored and used as a valid pointer.
So shouldn't we modify this function and just test for a NULL address here?
here what I mean:---------------------------------
diff --git a/drivers/s390/cio/css.c b/drivers/s390/cio/css.c index 2bc55ccf3f23..b45fbaa7131b 100644 --- a/drivers/s390/cio/css.c +++ b/drivers/s390/cio/css.c @@ -1176,7 +1176,7 @@ void *cio_gp_dma_zalloc(struct gen_pool *gp_dma, struct device *dma_dev, chunk_size = round_up(size, PAGE_SIZE); addr = (unsigned long) dma_alloc_coherent(dma_dev, chunk_size, &dma_addr, CIO_DMA_GFP); - if (!addr) + if (IS_ERR_OR_NULL(addr)) return NULL; gen_pool_add_virt(gp_dma, addr, dma_addr, chunk_size, -1); addr = gen_pool_alloc(gp_dma, size); --------------------------------- > + put_device(&cdev->dev);
addr is not null if addr is ERR.
> + return addr;
may be return IS_ERR_OR_NULL(addr)? NULL : addr;
> } > EXPORT_SYMBOL(ccw_device_dma_zalloc); > > void ccw_device_dma_free(struct ccw_device *cdev, void *cpu_addr, size_t size) > { > + if (!cpu_addr) > + return;
no need, cpu_addr is already tested in cio_gp_dma_free()
> cio_gp_dma_free(cdev->private->dma_pool, cpu_addr, size); > + put_device(&cdev->dev); > } > EXPORT_SYMBOL(ccw_device_dma_free); > > > base-commit: 64570fbc14f8d7cb3fe3995f20e26bc25ce4b2cc >
-- Pierre Morel IBM Lab Boeblingen
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