Re: [PATCH] hugetlb: select PREEMPT_COUNT if HUGETLB_PAGE for in_atomic use

[Michal Hocko]

On Thu 11-03-21 12:36:51, Peter Zijlstra wrote:
[...]
> Also, Linus hates constructs like this:
> 
>   https://lkml.kernel.org/r/CAHk-=wht7kAeyR5xEW2ORj7m0hibVxZ3t+2ie8vNHLQfdbN2_g@xxxxxxxxxxxxxx

Btw. I would really appreciate if somebody would explain why it is
_fundamentally broken_ to check for an atomic context and chose a
different handling in a code path, like put_page, which is out of hands
of the called context? This can be called from a wide variety of
contexts. There is no way to pass a context information to the called
function. I do recognize that this is not an act of beauty but why
fundamentally broken?

The put_page context can certainly work towards robustness and operate
on the most restrictive context grounds (I really hope nobody will ever
come up with an idea that put_page can be called from nmi context). This
can make the code more complex and less optimal in normal case (e.g.
hugetlb is almost never freed from an atomic context - one has to be
really creative to achieve that). So where do we draw a line?

-- 
Michal Hocko
SUSE Labs