Re: [PATCH] kmap tracking

From: Randy.Dunlap
Date: Mon May 22 2006 - 14:08:21 EST

On Mon, 22 May 2006 10:58:54 -0700 Zach Brown wrote:

> > I was scratching my head over this patch trying to think of any bug in
> > recent years which it would have detected. I failed.
> 2.4 nfs used to require that it be able to kmap entire RPCs for them to
> make forward progress. Its file->write() required RPC forward progress
> before it would return. And some callers were holding kmaps across
> file->write() calls. So with enough concurrent callers doing that the
> system would get stuck.
> We used the patch to see who the callers were when the system got into
> that state.
> One of them was core dumping, believe it or not. 2.6 elf_core_dump()
> still holds a kmap across file->write(), which seems unwise, but I
> haven't gotten to seeing if it's worth worrying about or not.
> So maybe these days the kmap story is less dreadful and it isn't as
> helpful, but that's what we used it for.

I was planning to add kmap_atomic* variants to the patch.
I could see it being useful for those callers, but maybe problems
with them would be more obvious anyway and wouldn't need such
a patch.

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