Re: [PATCH bpf-next v2 2/3] xdp: xdp_umem: replace kmap on vmap for umem map

["Jonathan Lemon" <jonathan.lemon@xxxxxxxxx>]

On 15 Aug 2019, at 12:19, Ivan Khoronzhuk wrote:

> On Thu, Aug 15, 2019 at 11:23:16AM -0700, Jonathan Lemon wrote:
> > On 15 Aug 2019, at 5:13, Ivan Khoronzhuk wrote:
> >
> > > For 64-bit there is no reason to use vmap/vunmap, so use 
> > > page_address
> > > as it was initially. For 32 bits, in some apps, like in samples
> > > xdpsock_user.c when number of pgs in use is quite big, the kmap
> > > memory can be not enough, despite on this, kmap looks like is
> > > deprecated in such cases as it can block and should be used rather
> > > for dynamic mm.
> > >
> > > Signed-off-by: Ivan Khoronzhuk <ivan.khoronzhuk@xxxxxxxxxx>
> > > ---
> > > net/xdp/xdp_umem.c | 36 ++++++++++++++++++++++++++++++------
> > > 1 file changed, 30 insertions(+), 6 deletions(-)
> > >
> > > diff --git a/net/xdp/xdp_umem.c b/net/xdp/xdp_umem.c
> > > index a0607969f8c0..d740c4f8810c 100644
> > > --- a/net/xdp/xdp_umem.c
> > > +++ b/net/xdp/xdp_umem.c
> > > @@ -14,7 +14,7 @@
> > > #include <linux/netdevice.h>
> > > #include <linux/rtnetlink.h>
> > > #include <linux/idr.h>
> > > -#include <linux/highmem.h>
> > > +#include <linux/vmalloc.h>
> > >
> > > #include "xdp_umem.h"
> > > #include "xsk_queue.h"
> > > @@ -170,7 +170,30 @@ static void xdp_umem_unmap_pages(struct 
> > > xdp_umem *umem)
> > > 	unsigned int i;
> > >
> > > 	for (i = 0; i < umem->npgs; i++)
> > > -		kunmap(umem->pgs[i]);
> > > +		if (PageHighMem(umem->pgs[i]))
> > > +			vunmap(umem->pages[i].addr);
> > > +}
> > > +
> > > +static int xdp_umem_map_pages(struct xdp_umem *umem)
> > > +{
> > > +	unsigned int i;
> > > +	void *addr;
> > > +
> > > +	for (i = 0; i < umem->npgs; i++) {
> > > +		if (PageHighMem(umem->pgs[i]))
> > > +			addr = vmap(&umem->pgs[i], 1, VM_MAP, PAGE_KERNEL);
> > > +		else
> > > +			addr = page_address(umem->pgs[i]);
> > > +
> > > +		if (!addr) {
> > > +			xdp_umem_unmap_pages(umem);
> > > +			return -ENOMEM;
> > > +		}
> > > +
> > > +		umem->pages[i].addr = addr;
> > > +	}
> > > +
> > > +	return 0;
> > > }
> >
> > You'll want a __xdp_umem_unmap_pages() helper here that takes an
> > count of the number of pages to unmap, so it can be called from
> > xdp_umem_unmap_pages() in the normal case, and xdp_umem_map_pages()
> > in the error case.  Otherwise the error case ends up calling
> > PageHighMem on a null page.
> > --
> > Jonathan
>
> Do you mean null address?
> If so, then vunmap do nothing if it's null, and addr is null if it's 
> not
> assigned... and it's not assigned w/o correct mapping...
>
> If you mean null page, then it is not possible after all they are
> pinned above, here: xdp_umem_pin_pages(), thus assigned.
>
> Or I missed smth?

No - I forgot about umem_pin_pages() - feel free to ignore my comments.
--
Jonathan

> Despite of this, seems like here should be one more patch, adding 
> unpinning page
> in error path, but this not related to this change. Will do this in 
> follow up
> fix patch, if no objection to my explanation, ofc.
>
> --
> Regards,
> Ivan Khoronzhuk