Re: Buffered I/O broken on s390x with page faults disabled (gfs2)

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On Wed, Mar 9, 2022 at 9:19 PM Linus Torvalds
<torvalds@xxxxxxxxxxxxxxxxxxxx> wrote:
> On Wed, Mar 9, 2022 at 11:35 AM Andreas Gruenbacher <agruenba@xxxxxxxxxx> wrote:
> >
> > That's better, thanks.
>
> Ok, can you give this one more test?
>
> It has that simplified loop, but it also replaced the
> FAULT_FLAG_KILLABLE with just passing in 'unlocked'.
>
> I thought I didn't need to do that, but the "retry" loop inside
> fixup_user_fault() will actually set that 'unlocked' thing even if the
> caller doesn't care whether the mmap_sem was unlocked during the call,
> so we have to pass in that pointer just to get that to work.
>
> And we don't care if mmap_sem was dropped, because this loop doesn't
> cache any vma information or anything like that, but we don't want to
> get a NULL pointer oops just because fixup_user_fault() tries to
> inform us about something we don't care about ;)

It's a moot point now, but I don't think handle_mm_fault would have
returned VM_FAULT_RETRY without FAULT_FLAG_ALLOW_RETRY, so there
wouldn't have been any NULL pointer accesses.

> That incidentally gets us FAULT_FLAG_ALLOW_RETRY too, which is
> probably a good thing anyway - it means that the mmap_sem will be
> dropped if we wait for IO. Not likely a huge deal, but it's the
> RightThing(tm) to do.

Looking good.

> So this has some other changes there too, but on the whole the
> function is now really quite simple. But it would be good to have one
> final round of testing considering how many small details changed..

Sure, we'll put it through all our tests.

Thanks,
Andreas




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