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SubjectRe: [PATCH v18 07/10] virtio-balloon: VIRTIO_BALLOON_F_SG
On 12/01/2017 11:38 PM, Michael S. Tsirkin wrote:
> On Wed, Nov 29, 2017 at 09:55:23PM +0800, Wei Wang wrote:
>> +static void send_one_desc(struct virtio_balloon *vb,
>> + struct virtqueue *vq,
>> + uint64_t addr,
>> + uint32_t len,
>> + bool inbuf,
>> + bool batch)
>> +{
>> + int err;
>> + unsigned int size;
>> +
>> + /* Detach all the used buffers from the vq */
>> + while (virtqueue_get_buf(vq, &size))
>> + ;
>> +
>> + err = virtqueue_add_one_desc(vq, addr, len, inbuf, vq);
>> + /*
>> + * This is expected to never fail: there is always at least 1 entry
>> + * available on the vq, because when the vq is full the worker thread
>> + * that adds the desc will be put into sleep until at least 1 entry is
>> + * available to use.
>> + */
>> + BUG_ON(err);
>> +
>> + /* If batching is requested, we batch till the vq is full */
>> + if (!batch || !vq->num_free)
>> + kick_and_wait(vq, vb->acked);
>> +}
>> +
> This internal kick complicates callers. I suggest that instead,
> you move this to callers, just return a "kick required" boolean.
> This way callers do not need to play with num_free at all.

Then in what situation would the function return true of "kick required"?

I think this wouldn't make a difference fundamentally. For example, we
have 257 sgs (batching size=256) to send to host:

while (i < 257) {
kick_required = send_sgs();
if (kick_required)
kick(); // After the 256 sgs have been added, the caller
performs a kick().
}

Do we still need a kick here for the 257th sg as before? Only the caller
knows if the last added sgs need a kick (when the send_sgs receives one
sg, it doesn't know if there are more to come).

There is another approach to checking if the last added sgs haven't been
sync-ed to the host: expose "vring_virtqueue->num_added" to the caller
via a virtio_ring API:

unsigned int virtqueue_num_added(struct virtqueue *_vq)
{
struct vring_virtqueue *vq = to_vvq(_vq);

return vq->num_added;
}



>> +/*
>> + * Send balloon pages in sgs to host. The balloon pages are recorded in the
>> + * page xbitmap. Each bit in the bitmap corresponds to a page of PAGE_SIZE.
>> + * The page xbitmap is searched for continuous "1" bits, which correspond
>> + * to continuous pages, to chunk into sgs.
>> + *
>> + * @page_xb_start and @page_xb_end form the range of bits in the xbitmap that
>> + * need to be searched.
>> + */
>> +static void tell_host_sgs(struct virtio_balloon *vb,
>> + struct virtqueue *vq,
>> + unsigned long page_xb_start,
>> + unsigned long page_xb_end)
>> +{
>> + unsigned long pfn_start, pfn_end;
>> + uint64_t addr;
>> + uint32_t len, max_len = round_down(UINT_MAX, PAGE_SIZE);
>> +
>> + pfn_start = page_xb_start;
>> + while (pfn_start < page_xb_end) {
>> + pfn_start = xb_find_next_set_bit(&vb->page_xb, pfn_start,
>> + page_xb_end);
>> + if (pfn_start == page_xb_end + 1)
>> + break;
>> + pfn_end = xb_find_next_zero_bit(&vb->page_xb,
>> + pfn_start + 1,
>> + page_xb_end);
>> + addr = pfn_start << PAGE_SHIFT;
>> + len = (pfn_end - pfn_start) << PAGE_SHIFT;
> This assugnment can overflow. Next line compares with UINT_MAX but by
> that time it is too late. I think you should do all math in 64 bit to
> avoid surprises, then truncate to max_len and then it's safe to assign
> to sg.

Sounds reasonable, thanks.


>> +
>> + xb_clear_bit_range(&vb->page_xb, page_xb_start, page_xb_end);
>> +}
>> +
>> +static inline int xb_set_page(struct virtio_balloon *vb,
>> + struct page *page,
>> + unsigned long *pfn_min,
>> + unsigned long *pfn_max)
>> +{
>> + unsigned long pfn = page_to_pfn(page);
>> + int ret;
>> +
>> + *pfn_min = min(pfn, *pfn_min);
>> + *pfn_max = max(pfn, *pfn_max);
>> +
>> + do {
>> + ret = xb_preload_and_set_bit(&vb->page_xb, pfn,
>> + GFP_NOWAIT | __GFP_NOWARN);
>> + } while (unlikely(ret == -EAGAIN));
> what exactly does this loop do? Does this wait
> forever until there is some free memory? why GFP_NOWAIT?

Basically, "-EAGAIN" is returned from xb_set_bit() in the case when the
pre-allocated per-cpu ida_bitmap is NULL. In that case, the caller
re-invokes xb_preload_and_set_bit(), which re-invokes xb_preload to
allocate ida_bitmap. So "-EAGAIN" actually does not indicate a status
about memory allocation. "-ENOMEM" is the one to indicate the failure of
memory allocation, but the loop doesn't re-try on "-ENOMEM".

GFP_NOWAIT is used to avoid memory reclaiming, which could cause the
deadlock issue we discussed before.




> return num_freed_pages;
> }
>
> +/*
> + * The regular leak_balloon() with VIRTIO_BALLOON_F_SG needs memory allocation
> + * for xbitmap, which is not suitable for the oom case. This function does not
> + * use xbitmap to chunk pages, so it can be used by oom notifier to deflate
> + * pages when VIRTIO_BALLOON_F_SG is negotiated.
> + */
> I guess we can live with this for now.

Agree, the patchset has been big. We can get the basic implementation in
first, and leave the following as future work. I can add it in the
commit log.

> Two things to consider
> - adding support for pre-allocating indirect buffers
> - sorting the internal page queue (how?)


Best,
Wei

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