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

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Date:   Thu, 15 Aug 2019 12:32:57 -0700
From:   "Jonathan Lemon" <jonathan.lemon@...il.com>
To:     "Ivan Khoronzhuk" <ivan.khoronzhuk@...aro.org>
Cc:     magnus.karlsson@...el.com, bjorn.topel@...el.com,
        davem@...emloft.net, hawk@...nel.org, john.fastabend@...il.com,
        jakub.kicinski@...ronome.com, daniel@...earbox.net,
        netdev@...r.kernel.org, bpf@...r.kernel.org,
        xdp-newbies@...r.kernel.org, linux-kernel@...r.kernel.org,
        yhs@...com, andrii.nakryiko@...il.com
Subject: Re: [PATCH bpf-next v2 2/3] xdp: xdp_umem: replace kmap on vmap for
 umem map



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@...aro.org>
>>> ---
>>> 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