Re: [bug report] dead loop in generic_perform_write() //Re: [PATCH v7 07/12] iov_iter: Convert iterate*() to inline funcs

[Tong Tiangen]

在 2024/3/4 19:56, David Howells 写道:
> Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx> wrote:
>
> > Actually, I think the right model is to get rid of that horrendous
> > .copy_mc field entirely.
> >
> > We only have one single place that uses it - that nasty core dumping
> > code. And that code is *not* performance critical.
> >
> > And not only isn't it performance-critical, it already does all the
> > core dumping one page at a time because it doesn't want to write pages
> > that were never mapped into user space.
> >
> > So what we can do is
> >
> >   (a) make the core dumping code *copy* the page to a good location
> > with copy_mc_to_kernel() first
> >
> >   (b) remove this horrendous .copy_mc crap entirely from iov_iter
> >
> > This is slightly complicated by the fact that copy_mc_to_kernel() may
> > not even exist, and architectures that don't have it don't want the
> > silly extra copy. So we need to abstract the "copy to temporary page"
> > code a bit. But that's probably a good thing anyway in that it forces
> > us to have nice interfaces.
> >
> > End result: something like the attached.
> >
> > AGAIN: THIS IS ENTIRELY UNTESTED.
> >
> > But hey, so was clearly all the .copy_mc code too that this removes, so...
>
> I like it:-)
>
> I've tested it by SIGQUIT'ing a number of processes and using gdb to examine
> the coredumps - which seems to work - at least without the production of any
> MCEs.  I'm not sure how I could test it with MCEs.

I'm going to test the coredump with the MCE.

> Feel free to add:
>
> Reviewed-by: David Howells <dhowells@xxxxxxxxxx>
> Tested-by: David Howells <dhowells@xxxxxxxxxx>
>
> That said, I wonder if:
>
> 	#ifdef copy_mc_to_kernel
>
> should be:
>
> 	#ifdef CONFIG_ARCH_HAS_COPY_MC
>
> and whether it's possible to find out dynamically if MCEs can occur at all.

MCE can occur during the use of a page. So i think it occurs
dynamically.

Thanks,
Tong

> David
>
> .