tree 4064635cb1fdb10668227084cb68c2c4be2fb0f8
parent 1c163f4c7b3f621efff9b28a47abb36f7378d783
author Linus Torvalds <torvalds@linux-foundation.org> 1555002380 -0700
committer Linus Torvalds <torvalds@linux-foundation.org> 1555261204 -0700

mm: make page ref count overflow check tighter and more explicit

We have a VM_BUG_ON() to check that the page reference count doesn't
underflow (or get close to overflow) by checking the sign of the count.

That's all fine, but we actually want to allow people to use a "get page
ref unless it's already very high" helper function, and we want that one
to use the sign of the page ref (without triggering this VM_BUG_ON).

Change the VM_BUG_ON to only check for small underflows (or _very_ close
to overflowing), and ignore overflows which have strayed into negative
territory.

Acked-by: Matthew Wilcox <willy@infradead.org>
Cc: Jann Horn <jannh@google.com>
Cc: stable@kernel.org
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>