[Qemu-devel] [Patch 1/1] net/net: Allocating Large sized arrays to heap

[Qemu-devel] [Patch 1/1] net/net: Allocating Large sized arrays to heap

[Pooja Dhannawat]

Signed-off-by: Pooja Dhannawat <dhannawatpooja1@gmail.com>
---
 net/net.c | 8 ++++++--
 1 file changed, 6 insertions(+), 2 deletions(-)

diff --git a/net/net.c b/net/net.c
index b0c832e..5399758 100644
--- a/net/net.c
+++ b/net/net.c
@@ -709,16 +709,18 @@ ssize_t qemu_send_packet_raw(NetClientState *nc, const uint8_t *buf, int size)
 static ssize_t nc_sendv_compat(NetClientState *nc, const struct iovec *iov,
                                int iovcnt, unsigned flags)
 {
-    uint8_t buf[NET_BUFSIZE];
+    uint8_t *buf;
     uint8_t *buffer;
     size_t offset;
 
+    buf = g_new(uint8_t, 1);
+
     if (iovcnt == 1) {
         buffer = iov[0].iov_base;
         offset = iov[0].iov_len;
     } else {
         buffer = buf;
-        offset = iov_to_buf(iov, iovcnt, 0, buf, sizeof(buf));
+        offset = iov_to_buf(iov, iovcnt, 0, (uint8_t *)buf, sizeof(uint8_t));
     }
 
     if (flags & QEMU_NET_PACKET_FLAG_RAW && nc->info->receive_raw) {
@@ -726,6 +728,8 @@ static ssize_t nc_sendv_compat(NetClientState *nc, const struct iovec *iov,
     } else {
         return nc->info->receive(nc, buffer, offset);
     }
+
+    g_free(buf);
 }
 
 ssize_t qemu_deliver_packet_iov(NetClientState *sender,
-- 
2.5.0

Re: [Qemu-devel] [Patch 1/1] net/net: Allocating Large sized arrays to heap

[Li Zhijian]

could you explain why we need this change ?


On 03/12/2016 04:39 PM, Pooja Dhannawat wrote:
> Signed-off-by: Pooja Dhannawat <dhannawatpooja1@gmail.com>
> ---
>   net/net.c | 8 ++++++--
>   1 file changed, 6 insertions(+), 2 deletions(-)
>
> diff --git a/net/net.c b/net/net.c
> index b0c832e..5399758 100644
> --- a/net/net.c
> +++ b/net/net.c
> @@ -709,16 +709,18 @@ ssize_t qemu_send_packet_raw(NetClientState *nc, const uint8_t *buf, int size)
>   static ssize_t nc_sendv_compat(NetClientState *nc, const struct iovec *iov,
>                                  int iovcnt, unsigned flags)
>   {
> -    uint8_t buf[NET_BUFSIZE];
> +    uint8_t *buf;
>       uint8_t *buffer;
>       size_t offset;
>
> +    buf = g_new(uint8_t, 1);
> +
>       if (iovcnt == 1) {
>           buffer = iov[0].iov_base;
>           offset = iov[0].iov_len;
>       } else {
>           buffer = buf;
> -        offset = iov_to_buf(iov, iovcnt, 0, buf, sizeof(buf));
> +        offset = iov_to_buf(iov, iovcnt, 0, (uint8_t *)buf, sizeof(uint8_t));
>       }
>
>       if (flags & QEMU_NET_PACKET_FLAG_RAW && nc->info->receive_raw) {
> @@ -726,6 +728,8 @@ static ssize_t nc_sendv_compat(NetClientState *nc, const struct iovec *iov,
>       } else {
forget to release 'buf'

>           return nc->info->receive(nc, buffer, offset);
>       }
> +
> +    g_free(buf);
>   }
>
>   ssize_t qemu_deliver_packet_iov(NetClientState *sender,
>

-- 
Best regards.
Li Zhijian

Re: [Qemu-devel] [Patch 1/1] net/net: Allocating Large sized arrays to heap

[Eric Blake]

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On 03/12/2016 01:39 AM, Pooja Dhannawat wrote:
> Signed-off-by: Pooja Dhannawat <dhannawatpooja1@gmail.com>
> ---
>  net/net.c | 8 ++++++--
>  1 file changed, 6 insertions(+), 2 deletions(-)
> 
> diff --git a/net/net.c b/net/net.c
> index b0c832e..5399758 100644
> --- a/net/net.c
> +++ b/net/net.c
> @@ -709,16 +709,18 @@ ssize_t qemu_send_packet_raw(NetClientState *nc, const uint8_t *buf, int size)
>  static ssize_t nc_sendv_compat(NetClientState *nc, const struct iovec *iov,
>                                 int iovcnt, unsigned flags)
>  {
> -    uint8_t buf[NET_BUFSIZE];
> +    uint8_t *buf;
>      uint8_t *buffer;
>      size_t offset;
>  
> +    buf = g_new(uint8_t, 1);

If you're only going to malloc() one byte, it's more efficient to just
stack-allocate it:

uint8_t buf[1];

Did you mean to do:

buf = g_new(uint8_t, NET_BUFSIZE)

instead?

> +
>      if (iovcnt == 1) {
>          buffer = iov[0].iov_base;
>          offset = iov[0].iov_len;
>      } else {
>          buffer = buf;
> -        offset = iov_to_buf(iov, iovcnt, 0, buf, sizeof(buf));
> +        offset = iov_to_buf(iov, iovcnt, 0, (uint8_t *)buf, sizeof(uint8_t));

This is wrong.  The old code used NET_BUFSIZE bytes for iov_to_buf(),
the new code uses only one byte.  By the way, sizeof(char) and
sizeof(uint8_t) are pointless in code, as they are strictly equivalent to 1.

I agree that the idea behind the patch is useful (NET_BUFSIZE is 68k,
which is too large for our goal of at most 4k stack allocation if we
never want to overflow a stack guard page), but you'll need a correct
working version of the patch.

-- 
Eric Blake   eblake redhat com    +1-919-301-3266
Libvirt virtualization library http://libvirt.org


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Re: [Qemu-devel] [Patch 1/1] net/net: Allocating Large sized arrays to heap

[Pooja Dhannawat]

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On Mon, Mar 14, 2016 at 7:41 PM, Eric Blake <eblake@redhat.com> wrote:

> On 03/12/2016 01:39 AM, Pooja Dhannawat wrote:
> > Signed-off-by: Pooja Dhannawat <dhannawatpooja1@gmail.com>
> > ---
> >  net/net.c | 8 ++++++--
> >  1 file changed, 6 insertions(+), 2 deletions(-)
> >
> > diff --git a/net/net.c b/net/net.c
> > index b0c832e..5399758 100644
> > --- a/net/net.c
> > +++ b/net/net.c
> > @@ -709,16 +709,18 @@ ssize_t qemu_send_packet_raw(NetClientState *nc,
> const uint8_t *buf, int size)
> >  static ssize_t nc_sendv_compat(NetClientState *nc, const struct iovec
> *iov,
> >                                 int iovcnt, unsigned flags)
> >  {
> > -    uint8_t buf[NET_BUFSIZE];
> > +    uint8_t *buf;
> >      uint8_t *buffer;
> >      size_t offset;
> >
> > +    buf = g_new(uint8_t, 1);
>
> If you're only going to malloc() one byte, it's more efficient to just
> stack-allocate it:
>
> uint8_t buf[1];
>
> Did you mean to do:
>
> buf = g_new(uint8_t, NET_BUFSIZE)
>
> instead?
>
>     Yes, my bad, I meant NET_BUFSIZE.

> > +
> >      if (iovcnt == 1) {
> >          buffer = iov[0].iov_base;
> >          offset = iov[0].iov_len;
> >      } else {
> >          buffer = buf;
> > -        offset = iov_to_buf(iov, iovcnt, 0, buf, sizeof(buf));
> > +        offset = iov_to_buf(iov, iovcnt, 0, (uint8_t *)buf,
> sizeof(uint8_t));
>
> This is wrong.  The old code used NET_BUFSIZE bytes for iov_to_buf(),
> the new code uses only one byte.  By the way, sizeof(char) and
> sizeof(uint8_t) are pointless in code, as they are strictly equivalent to
> 1.
>
> I agree that the idea behind the patch is useful (NET_BUFSIZE is 68k,
> which is too large for our goal of at most 4k stack allocation if we
> never want to overflow a stack guard page), but you'll need a correct
> working version of the patch.
>
>   Yes, I will correct it and send next version of patch.

> --
> Eric Blake   eblake redhat com    +1-919-301-3266
> Libvirt virtualization library http://libvirt.org
>
>

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[Qemu-devel] [PATCH v2] net: Allocating Large sized arrays to heap

[Pooja Dhannawat]

nc_sendv_compat has a huge stack usage of 69680 bytes approx.
Moving large arrays to heap to reduce stack usage.

Signed-off-by: Pooja Dhannawat <dhannawatpooja1@gmail.com>
---
 net/net.c | 14 ++++++++++----
 1 file changed, 10 insertions(+), 4 deletions(-)

diff --git a/net/net.c b/net/net.c
index b0c832e..89589c0 100644
--- a/net/net.c
+++ b/net/net.c
@@ -709,23 +709,29 @@ ssize_t qemu_send_packet_raw(NetClientState *nc, const uint8_t *buf, int size)
 static ssize_t nc_sendv_compat(NetClientState *nc, const struct iovec *iov,
                                int iovcnt, unsigned flags)
 {
-    uint8_t buf[NET_BUFSIZE];
+    uint8_t *buf;
     uint8_t *buffer;
     size_t offset;
+    ssize_t ret;
+
+    buf = g_new(uint8_t, NET_BUFSIZE);
 
     if (iovcnt == 1) {
         buffer = iov[0].iov_base;
         offset = iov[0].iov_len;
     } else {
         buffer = buf;
-        offset = iov_to_buf(iov, iovcnt, 0, buf, sizeof(buf));
+        offset = iov_to_buf(iov, iovcnt, 0, (uint8_t *)buf, NET_BUFSIZE);
     }
 
     if (flags & QEMU_NET_PACKET_FLAG_RAW && nc->info->receive_raw) {
-        return nc->info->receive_raw(nc, buffer, offset);
+        ret = nc->info->receive_raw(nc, buffer, offset);
     } else {
-        return nc->info->receive(nc, buffer, offset);
+        ret = nc->info->receive(nc, buffer, offset);
     }
+
+    g_free(buf);
+    return ret;
 }
 
 ssize_t qemu_deliver_packet_iov(NetClientState *sender,
-- 
2.5.0