On 12.03.2013 13:19, Gleb Smirnoff wrote:
Author: glebius
Date: Tue Mar 12 12:19:23 2013
New Revision: 248196
URL: http://svnweb.freebsd.org/changeset/base/248196
Log:
Use m_get2() to get mbuf of appropriate size.
The problem with m_get2() is that it will attempt to use jumbo mbufs
larger than PAGE_SIZE if the request size is sufficiently large.
In light of the recent issues with jumbo > PAGE_SIZE allocations on
loaded systems I don't think it is appropriate to extend this practice
further.
Anything that needs more than PAGE_SIZE (where PAGE_SIZE == 4K) mbuf
space should allocate an mbuf chain with m_getm2(). That is what we
have mbuf chains for.
Sufficient availability of mbufs > PAGE_SIZE cannot be guaranteed after
some uptime and/or a loaded system due to memory fragmentation. Allocation
can fail non-deterministically for mbufs > PAGE_SIZE long before smaller
mbufs become unavailable due to overall (kernel) memory exhaustion.
Mbufs > PAGE_SIZE are not only contiguous in kernel address space but
also in real memory address space. They are intended jumbo frames on
poor NIC's without scatter-capable DMA engines.
When you put the m_get2() function proposal for review and comments I
already highlighted these issues and put forward serious concerns about
this.
Please change m_get2() to limit itself to mbuf allocations <= PAGE_SIZE.
--
Andre
Please note that I applaud your general clean up of mbuf related allocation
functions and users thereof. :-)
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