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[0/9,v1,RFC] Generic zcopy_* functions

Message ID 20201218201633.2735367-1-jonathan.lemon@gmail.com
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Series Generic zcopy_* functions | expand

Message

Jonathan Lemon Dec. 18, 2020, 8:16 p.m. UTC
From: Jonathan Lemon <bsd@fb.com>

This is set of cleanup patches for zerocopy which are intended
to allow a introduction of a different zerocopy implementation.

The top level api will use the skb_zcopy_*() functions, while
the current TCP specific zerocopy would use the sock_zerocopy_*()
calls.

There should be no functional changes from these patches.

Patch 1:
  Move zerocopy bits from tx_flags into zc_flags for clarity.
  These bits will be used in the RX path in the future.
Patch 2: remove dead function
Patch 3:
  Replace sock_zerocopy_put() with skb_zcopy_put(), moving
  the zerocopy logic into sock_zerocopy_callback().  Push the
  refcounting into the callback, since not all implemenetations
  will have a refcount.
Patch 4: rename sock_zerocopy_get for consistency.
Patch 5:
  Add an optional skb parameter to callback, allowing access to
  the attached skb from the callback.
Patch 6:
  Add skb_zcopy_put_abort, and move zerocopy logic into the 
  callback function.  There unfortunately is still a check 
  against the callback type here.
Patch 7:
  Set the skb zc_flags from the ubuf being attached, instead
  of a fixed value, allowing different initialization types.
Patch 8: Replace open-coded assignments
Patch 9: Relocate skb_zcopy_clear() in skb_release_data()

Jonathan Lemon (9):
  net: group skb_shinfo zerocopy related bits together.
  skbuff: remove unused skb_zcopy_abort function
  skbuff: replace sock_zerocopy_put() with skb_zcopy_put()
  skbuff: replace sock_zerocopy_get with skb_zcopy_get
  skbuff: Add skb parameter to the ubuf zerocopy callback
  skbuff: Call sock_zerocopy_put_abort from skb_zcopy_put_abort
  skbuff: add zc_flags to ubuf_info for ubuf setup
  tap/tun: use skb_zcopy_set() instead of open coded assignment
  skbuff: Call skb_zcopy_clear() before unref'ing fragments

 drivers/net/tap.c                   |  6 +--
 drivers/net/tun.c                   |  6 +--
 drivers/net/xen-netback/common.h    |  3 +-
 drivers/net/xen-netback/interface.c |  4 +-
 drivers/net/xen-netback/netback.c   |  7 +--
 drivers/vhost/net.c                 |  4 +-
 include/linux/skbuff.h              | 77 +++++++++++++++--------------
 net/core/skbuff.c                   | 48 +++++++++---------
 net/ipv4/ip_output.c                |  3 +-
 net/ipv4/tcp.c                      |  6 +--
 net/ipv6/ip6_output.c               |  3 +-
 net/kcm/kcmsock.c                   |  4 +-
 12 files changed, 85 insertions(+), 86 deletions(-)

Comments

Willem de Bruijn Dec. 19, 2020, 7 p.m. UTC | #1
On Fri, Dec 18, 2020 at 4:27 PM Jonathan Lemon <jonathan.lemon@gmail.com> wrote:
>

> On Fri, Dec 18, 2020 at 03:49:44PM -0500, Willem de Bruijn wrote:

> > On Fri, Dec 18, 2020 at 3:23 PM Jonathan Lemon <jonathan.lemon@gmail.com> wrote:

> > >

> > > From: Jonathan Lemon <bsd@fb.com>

> > >

> > > This is set of cleanup patches for zerocopy which are intended

> > > to allow a introduction of a different zerocopy implementation.

> >

> > Can you describe in more detail what exactly is lacking in the current

> > zerocopy interface for this this different implementation? Or point to

> > a github tree with the feature patches attached, perhaps.

>

> I'll get the zctap features up into a github tree.

>

> Essentially, I need different behavior from ubuf_info:

>   - no refcounts on RX packets (static ubuf)


That is already the case for vhost and tpacket zerocopy use cases.

>   - access to the skb on RX skb free (for page handling)


To refers only to patch 9, moving the callback earlier in
skb_release_data, right?

>   - no page pinning on TX/tx completion


That is not part of the skb zerocopy infrastructure?

>   - marking the skb data as inaccessible so skb_condense()

>     and skb_zeroocopy_clone() leave it alone.


Yep. Skipping content access on the Rx path will be interesting. I
wonder if that should be a separate opaque skb feature, independent
from whether the data is owned by userspace, peripheral memory, the
page cache or anything else.

> > I think it's good to split into multiple smaller patchsets, starting

> > with core stack support. But find it hard to understand which of these

> > changes are truly needed to support a new use case.

>

> Agree - kind of hard to see why this is done without a use case.

> These patches are purely restructuring, and don't introduce any

> new features.

>

>

> > If anything, eating up the last 8 bits in skb_shared_info should be last resort.

>

> I would like to add 2 more bits in the future, which is why I

> moved them.  Is there a compelling reason to leave the bits alone?


Opportunity cost.

We cannot grow skb_shared_info due to colocation with MTU sized linear
skbuff's in half a page.

It took me quite some effort to free up a few bytes in commit
4d276eb6a478 ("net: remove deprecated syststamp timestamp").

If we are very frugal, we could shadow some bits to have different
meaning in different paths. SKBTX_IN_PROGRESS is transmit only, I
think. But otherwise we'll have to just dedicate the byte to more
flags. Yours are likely not to be the last anyway.
Jonathan Lemon Dec. 21, 2020, 7:50 p.m. UTC | #2
On Sat, Dec 19, 2020 at 02:00:55PM -0500, Willem de Bruijn wrote:
> On Fri, Dec 18, 2020 at 4:27 PM Jonathan Lemon <jonathan.lemon@gmail.com> wrote:

> >

> > On Fri, Dec 18, 2020 at 03:49:44PM -0500, Willem de Bruijn wrote:

> > > On Fri, Dec 18, 2020 at 3:23 PM Jonathan Lemon <jonathan.lemon@gmail.com> wrote:

> > > >

> > > > From: Jonathan Lemon <bsd@fb.com>

> > > >

> > > > This is set of cleanup patches for zerocopy which are intended

> > > > to allow a introduction of a different zerocopy implementation.

> > >

> > > Can you describe in more detail what exactly is lacking in the current

> > > zerocopy interface for this this different implementation? Or point to

> > > a github tree with the feature patches attached, perhaps.

> >

> > I'll get the zctap features up into a github tree.

> >

> > Essentially, I need different behavior from ubuf_info:

> >   - no refcounts on RX packets (static ubuf)

> 

> That is already the case for vhost and tpacket zerocopy use cases.

> 

> >   - access to the skb on RX skb free (for page handling)

> 

> To refers only to patch 9, moving the callback earlier in

> skb_release_data, right?


Yes.


> >   - no page pinning on TX/tx completion

> 

> That is not part of the skb zerocopy infrastructure?


That's specific to msg_zerocopy.  zctap uses the same network stack
paths, but pins the pages during setup, not during each each system call.


> >   - marking the skb data as inaccessible so skb_condense()

> >     and skb_zeroocopy_clone() leave it alone.

> 

> Yep. Skipping content access on the Rx path will be interesting. I

> wonder if that should be a separate opaque skb feature, independent

> from whether the data is owned by userspace, peripheral memory, the

> page cache or anything else.


Would that be indicated by a bit on the skb (like pfmemalloc), or 
a bit in the skb_shared structure, as I'm leaning towards doing here?


> > > I think it's good to split into multiple smaller patchsets, starting

> > > with core stack support. But find it hard to understand which of these

> > > changes are truly needed to support a new use case.

> >

> > Agree - kind of hard to see why this is done without a use case.

> > These patches are purely restructuring, and don't introduce any

> > new features.

> >

> >

> > > If anything, eating up the last 8 bits in skb_shared_info should be last resort.

> >

> > I would like to add 2 more bits in the future, which is why I

> > moved them.  Is there a compelling reason to leave the bits alone?

> 

> Opportunity cost.

> 

> We cannot grow skb_shared_info due to colocation with MTU sized linear

> skbuff's in half a page.

> 

> It took me quite some effort to free up a few bytes in commit

> 4d276eb6a478 ("net: remove deprecated syststamp timestamp").

> 

> If we are very frugal, we could shadow some bits to have different

> meaning in different paths. SKBTX_IN_PROGRESS is transmit only, I

> think. But otherwise we'll have to just dedicate the byte to more

> flags. Yours are likely not to be the last anyway.


The zerocopy/enable flags could be encoded in one of the lower 3 bits
in the destructor_arg, (similar to nouarg) but that seems messy.
-- 
Jonathan
Willem de Bruijn Dec. 21, 2020, 10:52 p.m. UTC | #3
> > >   - marking the skb data as inaccessible so skb_condense()

> > >     and skb_zeroocopy_clone() leave it alone.

> >

> > Yep. Skipping content access on the Rx path will be interesting. I

> > wonder if that should be a separate opaque skb feature, independent

> > from whether the data is owned by userspace, peripheral memory, the

> > page cache or anything else.

>

> Would that be indicated by a bit on the skb (like pfmemalloc), or

> a bit in the skb_shared structure, as I'm leaning towards doing here?


I would guide it in part by avoiding cold cacheline accesses. That
might be hard if using skb_shinfo. OTOH, you don't have to worry about
copying the bit during clone operations.

> > > > If anything, eating up the last 8 bits in skb_shared_info should be last resort.

> > >

> > > I would like to add 2 more bits in the future, which is why I

> > > moved them.  Is there a compelling reason to leave the bits alone?

> >

> > Opportunity cost.

> >

> > We cannot grow skb_shared_info due to colocation with MTU sized linear

> > skbuff's in half a page.

> >

> > It took me quite some effort to free up a few bytes in commit

> > 4d276eb6a478 ("net: remove deprecated syststamp timestamp").

> >

> > If we are very frugal, we could shadow some bits to have different

> > meaning in different paths. SKBTX_IN_PROGRESS is transmit only, I

> > think. But otherwise we'll have to just dedicate the byte to more

> > flags. Yours are likely not to be the last anyway.

>

> The zerocopy/enable flags could be encoded in one of the lower 3 bits

> in the destructor_arg, (similar to nouarg) but that seems messy.


Agreed :)

Let's just expand the flags for now. It may be better to have one
general purpose 16 bit flags bitmap, rather than reserving 8 bits
specifically to zerocopy features.
Jonathan Lemon Dec. 22, 2020, 12:07 a.m. UTC | #4
On Mon, Dec 21, 2020 at 05:52:08PM -0500, Willem de Bruijn wrote:
> > > >   - marking the skb data as inaccessible so skb_condense()

> > > >     and skb_zeroocopy_clone() leave it alone.

> > >

> > > Yep. Skipping content access on the Rx path will be interesting. I

> > > wonder if that should be a separate opaque skb feature, independent

> > > from whether the data is owned by userspace, peripheral memory, the

> > > page cache or anything else.

> >

> > Would that be indicated by a bit on the skb (like pfmemalloc), or

> > a bit in the skb_shared structure, as I'm leaning towards doing here?

> 

> I would guide it in part by avoiding cold cacheline accesses. That

> might be hard if using skb_shinfo. OTOH, you don't have to worry about

> copying the bit during clone operations.

> 

> > > > > If anything, eating up the last 8 bits in skb_shared_info should be last resort.

> > > >

> > > > I would like to add 2 more bits in the future, which is why I

> > > > moved them.  Is there a compelling reason to leave the bits alone?

> > >

> > > Opportunity cost.

> > >

> > > We cannot grow skb_shared_info due to colocation with MTU sized linear

> > > skbuff's in half a page.

> > >

> > > It took me quite some effort to free up a few bytes in commit

> > > 4d276eb6a478 ("net: remove deprecated syststamp timestamp").

> > >

> > > If we are very frugal, we could shadow some bits to have different

> > > meaning in different paths. SKBTX_IN_PROGRESS is transmit only, I

> > > think. But otherwise we'll have to just dedicate the byte to more

> > > flags. Yours are likely not to be the last anyway.

> >

> > The zerocopy/enable flags could be encoded in one of the lower 3 bits

> > in the destructor_arg, (similar to nouarg) but that seems messy.

> 

> Agreed :)

> 

> Let's just expand the flags for now. It may be better to have one

> general purpose 16 bit flags bitmap, rather than reserving 8 bits

> specifically to zerocopy features.


I was considering doing that also, but that would need to rearrange
the flags in skb_shared_info.  Then I realized that there are currently
only TX flags and ZC flags, so went with that.  I have no objections
to doing it either way.

My motivation here is when MSG_ZCTAP is added to tcp_sendmsg_locked(),
it the returned uarg is self-contained for the rest of the function.
-- 
Jonathan