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);+}