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SubjectRe: [PATCH v4 05/10] mm: Return faster for non-fatal signals in user mode faults
On Sun, Sep 22, 2019 at 9:26 PM Peter Xu <peterx@redhat.com> wrote:
>
> This patch is a preparation of removing that special path by allowing
> the page fault to return even faster if we were interrupted by a
> non-fatal signal during a user-mode page fault handling routine.

So I really wish saome other vm person would also review these things,
but looking over this series once more, this is the patch I probably
like the least.

And the reason I like it the least is that I have a hard time
explaining to myself what the code does and why, and why it's so full
of this pattern:

> - if ((fault & VM_FAULT_RETRY) && fatal_signal_pending(current))
> + if ((fault & VM_FAULT_RETRY) &&
> + fault_should_check_signal(user_mode(regs)))
> return;

which isn't all that pretty.

Why isn't this just

static bool fault_signal_pending(unsigned int fault_flags, struct
pt_regs *regs)
{
return (fault_flags & VM_FAULT_RETRY) &&
(fatal_signal_pending(current) ||
(user_mode(regs) && signal_pending(current)));
}

and then most of the users would be something like

if (fault_signal_pending(fault, regs))
return;

and the exceptions could do their own thing.

Now the code is prettier and more understandable, I feel.

And if something doesn't follow this pattern, maybe it either _should_
follow that pattern or it should just not use the helper but explain
why it has an unusual pattern.

Linus

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