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

[John Hubbard]

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.

Then you could use it directly here.


thanks,

--
John Hubbard
NVIDIA