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

[Jann Horn]

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,
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...

> > +     /*
> > +      * Calling put_page() for each ref is unnecessarily slow. Only the last
> > +      * ref needs a put_page().
> > +      */
> > +     if (refs > 1)
> > +             page_ref_sub(page, refs - 1);
> > +     put_page(page);
> > +}
>