Messages in this thread | | | From | Jann Horn <> | Date | Sat, 12 Jun 2021 03:49:57 +0200 | Subject | Re: [PATCH resend] mm/gup: fix try_grab_compound_head() race with split_huge_page() |
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On Sat, Jun 12, 2021 at 12:36 AM Andrew Morton <akpm@linux-foundation.org> wrote: > On Fri, 11 Jun 2021 18:15:45 +0200 Jann Horn <jannh@google.com> 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); > > +} >
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