Re: [PATCH net] net/mlx5e: Do not recycle pages from emergency reserve

On Sun, Jan 22, 2017 at 7:50 PM, Tom Herbert [off-list ref] wrote:

On Sat, Jan 21, 2017 at 12:31 PM, Saeed Mahameed
[off-list ref] wrote:

quoted

On Sat, Jan 21, 2017 at 9:12 PM, kernel netdev [off-list ref] wrote:

quoted

Den 21. jan. 2017 7.10 PM skrev "Tom Herbert" [off-list ref]:

On Thu, Jan 19, 2017 at 11:14 AM, Saeed Mahameed
[off-list ref] wrote:

quoted

On Thu, Jan 19, 2017 at 9:03 AM, Eric Dumazet [off-list ref]
wrote:

quoted

From: Eric Dumazet <edumazet@google.com>

A driver using dev_alloc_page() must not reuse a page allocated from
emergency memory reserve.

Otherwise all packets using this page will be immediately dropped,
unless for very specific sockets having SOCK_MEMALLOC bit set.

This issue might be hard to debug, because only a fraction of received
packets would be dropped.

Hi Eric,

When you say reuse, you mean point to the same page from several SKBs ?

Because in our page cache implementation we don't reuse pages that
already passed to the stack,
we just keep them in the page cache until the ref count drop back to
one, so we recycle them (i,e they will be re-used only when no one
else is using them).

Saeed,

Speaking of the mlx page cache can we remove this or a least make it
optional to use. It is another example of complex functionality being
put into drivers that makes things like backports more complicated and

Re complexity, I am not sure the mlx page cache is that complex,
we just wrap alloc_page/put_page with our own page cache calls.
Roughly the page cache implementation is 200-300 LOC tops all concentrated
in one place in the code.

Taken as part of the RX buffer management code the whole thing in very
complicated and seems to be completely bereft of any comments in the
code as to how things are supposed to work.

The Idea is fairly simple, maybe we could have made it cleaner if we
better decoupled the page
cache from the rx logic.

The page cache works in parallel to the rx logic, it takes its owns ref counts,
this way the rx logic will continue working as if there was no cache
(get/put_pages).
but instead of of the kernel (get/put_pages) it will call the internal
cache get/put_cache_page.

Yeah I agree, some comments could help. We will work on this when we
fix the HOL issues.