Re: [PATCH] Enhance AF_PACKET implementation to not require high order contiguous memory allocation (v4)

[Eric Dumazet <eric.dumazet@gmail.com>]

Le mercredi 10 novembre 2010 à 14:09 -0500, nhorman@tuxdriver.com a
écrit :
> From: Neil Horman <nhorman@tuxdriver.com>
> 
> Version 4 of this patch.
> 
> Change notes:
> 1) Removed extra memset.  Didn't think kcalloc added a GFP_ZERO the way kzalloc did :)
> 
> Summary:
> It was shown to me recently that systems under high load were driven very deep
> into swap when tcpdump was run.  The reason this happened was because the
> AF_PACKET protocol has a SET_RINGBUFFER socket option that allows the user space
> application to specify how many entries an AF_PACKET socket will have and how
> large each entry will be.  It seems the default setting for tcpdump is to set
> the ring buffer to 32 entries of 64 Kb each, which implies 32 order 5
> allocation.  Thats difficult under good circumstances, and horrid under memory
> pressure.
> 
> I thought it would be good to make that a bit more usable.  I was going to do a
> simple conversion of the ring buffer from contigous pages to iovecs, but
> unfortunately, the metadata which AF_PACKET places in these buffers can easily
> span a page boundary, and given that these buffers get mapped into user space,
> and the data layout doesn't easily allow for a change to padding between frames
> to avoid that, a simple iovec change is just going to break user space ABI
> consistency.
> 
> So I've done this, I've added a three tiered mechanism to the af_packet set_ring
> socket option.  It attempts to allocate memory in the following order:
> 
> 1) Using __get_free_pages with GFP_NORETRY set, so as to fail quickly without
> digging into swap
> 
> 2) Using vmalloc
> 
> 3) Using __get_free_pages with GFP_NORETRY clear, causing us to try as hard as
> needed to get the memory
> 
> The effect is that we don't disturb the system as much when we're under load,
> while still being able to conduct tcpdumps effectively.
> 
> Tested successfully by me.
> 
> Signed-off-by: Neil Horman <nhorman@tuxdriver.com>

Acked-by: Eric Dumazet <eric.dumazet@gmail.com>

Thanks Neil !