mm, gup: add missing refcount overflow checks on s390
The mainline commit 8fde12ca79af ("mm: prevent get_user_pages() from overflowing page refcount") was backported to 4.19.y stable as commit d972ebbf. The backport however missed that in 4.19, there are several arch-specific gup.c versions with fast gup implementations, so these do not prevent refcount overflow. This stable-only commit fixes the s390 version, and is based on the backport in SUSE SLES/openSUSE 4.12-based kernels. The remaining architectures with own gup.c are sparc, mips, sh. It's unlikely the known overflow scenario based on FUSE, which needs 140GB of RAM, is a problem for those architectures, and I don't feel confident enough to patch them. Signed-off-by: NVlastimil Babka <vbabka@suse.cz> Signed-off-by: NSasha Levin <sashal@kernel.org>
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