Re: [PATCH 1/2] xen/netback: don't do grant copy across page boundary

[Juergen Gross <jgross@xxxxxxxx>]

On 27.03.23 11:49, Jan Beulich wrote:
> On 27.03.2023 10:36, Juergen Gross wrote:
> > Fix xenvif_get_requests() not to do grant copy operations across local
> > page boundaries. This requires to double the maximum number of copy
> > operations per queue, as each copy could now be split into 2.
> >
> > Make sure that struct xenvif_tx_cb doesn't grow too large.
> >
> > Cc: stable@xxxxxxxxxxxxxxx
> > Fixes: ad7f402ae4f4 ("xen/netback: Ensure protocol headers don't fall in the 
> > non-linear area")
> > Signed-off-by: Juergen Gross <jgross@xxxxxxxx>
> > ---
> >   drivers/net/xen-netback/common.h  |  2 +-
> >   drivers/net/xen-netback/netback.c | 25 +++++++++++++++++++++++--
> >   2 files changed, 24 insertions(+), 3 deletions(-)
> >
> > diff --git a/drivers/net/xen-netback/common.h b/drivers/net/xen-netback/common.h
> > index 3dbfc8a6924e..1fcbd83f7ff2 100644
> > --- a/drivers/net/xen-netback/common.h
> > +++ b/drivers/net/xen-netback/common.h
> > @@ -166,7 +166,7 @@ struct xenvif_queue { /* Per-queue data for xenvif */
> >         struct pending_tx_info pending_tx_info[MAX_PENDING_REQS];
> >         grant_handle_t grant_tx_handle[MAX_PENDING_REQS];
> >   
> > -	struct gnttab_copy tx_copy_ops[MAX_PENDING_REQS];
> > +       struct gnttab_copy tx_copy_ops[2 * MAX_PENDING_REQS];
> >         struct gnttab_map_grant_ref tx_map_ops[MAX_PENDING_REQS];
> >         struct gnttab_unmap_grant_ref tx_unmap_ops[MAX_PENDING_REQS];
> >         /* passed to gnttab_[un]map_refs with pages under (un)mapping */
> > diff --git a/drivers/net/xen-netback/netback.c 
> > b/drivers/net/xen-netback/netback.c
> > index 1b42676ca141..111c179f161b 100644
> > --- a/drivers/net/xen-netback/netback.c
> > +++ b/drivers/net/xen-netback/netback.c
> > @@ -334,6 +334,7 @@ static int xenvif_count_requests(struct xenvif_queue *queue,
> >   struct xenvif_tx_cb {
> >         u16 copy_pending_idx[XEN_NETBK_LEGACY_SLOTS_MAX + 1];
> >         u8 copy_count;
> > +       u32 split_mask;
> >   };
> >   
> >   #define XENVIF_TX_CB(skb) ((struct xenvif_tx_cb *)(skb)->cb)
> > @@ -361,6 +362,8 @@ static inline struct sk_buff *xenvif_alloc_skb(unsigned int 
> > size)
> >         struct sk_buff *skb =
> >                 alloc_skb(size + NET_SKB_PAD + NET_IP_ALIGN,
> >                           GFP_ATOMIC | __GFP_NOWARN);
> > +
> > +       BUILD_BUG_ON(sizeof(*XENVIF_TX_CB(skb)) > sizeof(skb->cb));
> >         if (unlikely(skb == NULL))
> >                 return NULL;
> >   
> > @@ -396,11 +399,13 @@ static void xenvif_get_requests(struct xenvif_queue *queue,
> >         nr_slots = shinfo->nr_frags + 1;
> >   
> >   	copy_count(skb) = 0;
> > +       XENVIF_TX_CB(skb)->split_mask = 0;
> >   
> >   	/* Create copy ops for exactly data_len bytes into the skb head. */
> >         __skb_put(skb, data_len);
> >         while (data_len > 0) {
> >                 int amount = data_len > txp->size ? txp->size : data_len;
> > +               bool split = false;
> >   
> >   		cop->source.u.ref = txp->gref;
> >                 cop->source.domid = queue->vif->domid;
> > @@ -413,6 +418,13 @@ static void xenvif_get_requests(struct xenvif_queue *queue,
> >                 cop->dest.u.gmfn = virt_to_gfn(skb->data + skb_headlen(skb)
> >                                                - data_len);
> >   
> > +		/* Don't cross local page boundary! */
> > +               if (cop->dest.offset + amount > XEN_PAGE_SIZE) {
> > +                       amount = XEN_PAGE_SIZE - cop->dest.offset;
> > +                       XENVIF_TX_CB(skb)->split_mask |= 1U << copy_count(skb);
> Maybe worthwhile to add a BUILD_BUG_ON() somewhere to make sure this
> shift won't grow too large a shift count. The number of slots accepted
> could conceivably be grown past XEN_NETBK_LEGACY_SLOTS_MAX (i.e.
> XEN_NETIF_NR_SLOTS_MIN) at some point.
This is basically impossible due to the size restriction of struct
xenvif_tx_cb.

> > +                       split = true;
> > +               }
> > +
> >                 cop->len = amount;
> >                 cop->flags = GNTCOPY_source_gref;
> >   
> > @@ -420,7 +432,8 @@ static void xenvif_get_requests(struct xenvif_queue *queue,
> >                 pending_idx = queue->pending_ring[index];
> >                 callback_param(queue, pending_idx).ctx = NULL;
> >                 copy_pending_idx(skb, copy_count(skb)) = pending_idx;
> > -               copy_count(skb)++;
> > +               if (!split)
> > +                       copy_count(skb)++;
> >   
> >   		cop++;
> >                 data_len -= amount;
> > @@ -441,7 +454,8 @@ static void xenvif_get_requests(struct xenvif_queue *queue,
> >                         nr_slots--;
> >                 } else {
> >                         /* The copy op partially covered the tx_request.
> > -                        * The remainder will be mapped.
> > +                        * The remainder will be mapped or copied in the next
> > +                        * iteration.
> >                          */
> >                         txp->offset += amount;
> >                         txp->size -= amount;
> > @@ -539,6 +553,13 @@ static int xenvif_tx_check_gop(struct xenvif_queue *queue,
> >                 pending_idx = copy_pending_idx(skb, i);
> >   
> >   		newerr = (*gopp_copy)->status;
> > +
> > +               /* Split copies need to be handled together. */
> > +               if (XENVIF_TX_CB(skb)->split_mask & (1U << i)) {
> > +                       (*gopp_copy)++;
> > +                       if (!newerr)
> > +                               newerr = (*gopp_copy)->status;
> > +               }
> It isn't guaranteed that a slot may be split only once, is it? Assuming a
I think it is guaranteed.

No slot can cover more than XEN_PAGE_SIZE bytes due to the grants being
restricted to that size. There is no way how such a data packet could cross
2 page boundaries.

In the end the problem isn't the copies for the linear area not crossing
multiple page boundaries, but the copies for a single request slot not
doing so. And this can't happen IMO.

> near-64k packet with all tiny non-primary slots, that'll cause those tiny
> slots to all be mapped, but due to
>
>                 if (ret >= XEN_NETBK_LEGACY_SLOTS_MAX - 1 && data_len < 
> txreq.size)
>                         data_len = txreq.size;
>
> will, afaict, cause a lot of copying for the primary slot. Therefore I
> think you need a loop here, not just an if(). Plus tx_copy_ops[]'es
> dimension also looks to need further growing to accommodate this. Or
> maybe not - at least the extreme example given would still be fine; more
> generally packets being limited to below 64k means 2*16 slots would
> suffice at one end of the scale, while 2*MAX_PENDING_REQS would at the
> other end (all tiny, including the primary slot). What I haven't fully
> convinced myself of is whether there might be cases in the middle which
> are yet worse.
See above reasoning. I think it is okay, but maybe I'm missing something.


Juergen

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