Messages in this thread | | | Date | Mon, 04 Dec 2017 11:46:46 +0800 | From | Wei Wang <> | Subject | Re: [PATCH v18 07/10] virtio-balloon: VIRTIO_BALLOON_F_SG |
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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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