Re: [PATCH resend] mm/gup: fix try_grab_compound_head() race with split_huge_page()

[Jann Horn]

On Sat, Jun 12, 2021 at 12:17 PM John Hubbard <jhubbard@xxxxxxxxxx> wrote:
> On 6/11/21 3:49 PM, Jann Horn wrote:
> > On Sat, Jun 12, 2021 at 12:36 AM Andrew Morton
> > <akpm@xxxxxxxxxxxxxxxxxxxx> wrote:
> >> On Fri, 11 Jun 2021 18:15:45 +0200 Jann Horn <jannh@xxxxxxxxxx> wrote:
> >>> +/* Equivalent to calling put_page() @refs times. */
> >>> +static void put_page_refs(struct page *page, int refs)
> >>> +{
> >>> +     VM_BUG_ON_PAGE(page_ref_count(page) < refs, page);
> >>
> >> I don't think there's a need to nuke the whole kernel in this case.
> >> Can we warn then simply leak the page?  That way we have a much better
> >> chance of getting a good bug report.
> >
> > Ah, yeah, I guess that makes sense. I had just copied this over from
> > put_compound_head(), and figured it was fine to keep it as-is, but I
> > guess changing it would be reasonable. I'm not quite sure what the
> > best way to do that would be though.
> >
> > I guess the check should go away in !DEBUG_VM builds?
> >
> > Should I just explicitly put the check in an ifdef block? Like so:
> >
> > #ifdef CONFIG_DEBUG_VM
> > if (VM_WARN_ON_ONCE_PAGE(...))
> >    return;
> > #endif
> >
> > Or, since inline ifdeffery looks ugly, get rid of the explicit ifdef,
>
> Agreed: VM_WARN_ON_ONCE_PAGE(), at least at the API level, seems like
> the best thing to use here. However, as you point out below, it needs a
> little something first.
>
> > and change the !DEBUG_VM definition of VM_WARN_ON_ONCE_PAGE() as
> > follows so that the branch is compiled away?
> >
> > #define VM_WARN_ON_ONCE_PAGE(cond, page)  (BUILD_BUG_ON_INVALID(cond), false)
> >
> > That would look kinda neat, but it would be different from the
> > behavior of WARN_ON(), which still returns the original condition even
> > in !BUG builds, so that could be confusing...
> >
>
> The VM_WARN_ON_ONCE_PAGE() is not implemented exactly right
> in the !CONFIG_DEBUG_VM case. IMHO it should follow the WARN*()
> behavior, and return the original condition and keep going
> in that case.

But the point of the existing definition is that the compiler can
avoid generating code for the condition in !DEBUG_VM builds, even if
it can't prove that the condition is free of side effects, right? If
VM_WARN_ON_ONCE_PAGE() was changed as you propose, then I think that
in mem_cgroup_page_lruvec(), the compiler would have to generate code
for mem_cgroup_disabled(), which calls static_branch_likely(), which
ends up in "asm volatile" statements; so the compiler probably won't
be able to eliminate the condition.

> Then you could use it directly here.

Depending on whether the intended behavior here is to skip the check
in !DEBUG_VM builds (which was the case before) or also perform the
check in DEBUG_VM builds. And if DEBUG_VM is a config option because
it might have some performance impact, isn't the cost of the check
probably quite large compared to the cost of printing the warning on a
codpath that should never execute?