Kostik Belousov wrote:
On Mon, Mar 05, 2007 at 10:17:14PM +0300, Yar Tikhiy wrote:
On Mon, Mar 05, 2007 at 01:14:29PM -0500, Mikhail Teterin wrote:
On Monday 05 March 2007 08:23, Yar Tikhiy wrote:
= > How will it break them? swap backing only touches swap if there is
= > memory pressure, i.e. precisely the situation in which malloc backing
= > will panic.
=
= I forgot that in BSD swap wouldn't be allocated in advance to its
= consumers. Then removing the -M flag and making swap backing the
= default is a very sound choice. Thank you for correcting me.
Yar, would you change the man-page's advice and the default, then?
Yes, I'll be glad to if no objections arise until I finish updating
my CURRENT machine, i.e., tomorrow. :-)
Someone still needs to look into the panic... Who would that be?
Obviously, Mr(s). Someone. :-)
The md case exposes a quite tangled nature of the problem. Funnily
enough, kernel malloc() cannot just fail in the case because it
must not fail if called with M_WAITOK. This means that the system
has quite a rough choice:
- put the requesting thread to sleep forever;
- grow kmem_map, eventually sacrifice all RAM to the greedy thread
and die sooner or later;
- panic immediately.
If all malloc() callers in the kernel were ready to deal with
allocation failure, the system could just tell the greedy thread
to buzz off. But too many kernel parts depend on malloc(M_WAITOK)
never failing. Perhaps it's the root of the problem.
Mark callers that are ready for M_WAITOK failure with some additional
flag, like M_FAILOK (feel free to propose meaningful name there).
At least malloc()-based md could then use it.
The panic is a chronic problem that really needs to be fixed in general.
However, the md code should probably be modified to reject any
malloc-backed size larger than some trivial (and arbitrary) value, like
say 1MB. It's really inferior to being swap-backed, and it only
encourages foot-shooting and these unclear panics.
Scott
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