On 6/18/12 1:25 PM, Andreas Dilger wrote:

On 2012-06-18, at 6:08 AM, Christoph Hellwig wrote:

quoted

May saw the release of Linux 3.4, including a decent sized XFS update.
Remarkable XFS features in Linux 3.4 include moving over all metadata
updates to use transactions, the addition of a work queue for the
low-level allocator code to avoid stack overflows due to extreme stack
use in the Linux VM/VFS call chain,

This is essentially a workaround for too-small stacks in the kernel,
which we've had to do at times as well, by doing work in a separate
thread (with a new stack) and waiting for the results?  This is a
generic problem that any reasonably-complex filesystem will have when
running under memory pressure on a complex storage stack (e.g. LVM +
iSCSI), but causes unnecessary context switching.

Any thoughts on a better way to handle this, or will there continue
to be a 4kB stack limit and hack around this with repeated kmalloc

well, 8k on x86_64 (not 4k) right?   But still...

Maybe it's still a partial hack but it's more generic - should we have
IRQ stacks like x86 has?  (I think I'm right that that only exists
on x86 / 32-bit) - is there any downside to that?

We could still get into trouble I'm sure but usually we seem to see
these stack overflows when we take an IRQ while already deep-ish
in the stack.

-Eric

on callpaths for any struct over a few tens of bytes, implementing
memory pools all over the place, and "forking" over to other threads
to continue the stack consumption for another 4kB to work around
the small stack limit?

Cheers, Andreas





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