On 07/21/2017 07:36 AM, Michal Hocko wrote: > On Thu 20-07-17 13:37:59, Mike Kravetz wrote: >> mremap will create a 'duplicate' mapping if old_size == 0 is >> specified. Such duplicate mappings make no sense for private >> mappings. > > sorry for the nit picking but this is not true strictly speaking. > It makes some sense, arguably (e.g. take an atomic snapshot of the > mapping). It doesn't make any sense with the _current_ implementation. > >> If duplication is attempted for a private mapping, >> mremap creates a separate private mapping unrelated to the >> original mapping and makes no modifications to the original. >> This is contrary to the purpose of mremap which should return >> a mapping which is in some way related to the original. >> >> Therefore, return EINVAL in the case where if an attempt is >> made to duplicate a private mapping. Also, print a warning >> message (once) if such an attempt is made. >> >> Signed-off-by: Mike Kravetz <[email protected]> > > I do not insist on the comment update suggested > http://lkml.kernel.org/r/[email protected] > but I would appreciate it... > > Other than that looks reasonably to me > > Acked-by: Michal Hocko <[email protected]>
My apologies. I overlooked your comment about the comment when creating the patch. Below is the patch with commit message and comment updated. >From 5c4a1602bd6a942544ed011dc0a72fd258e874b2 Mon Sep 17 00:00:00 2001 From: Mike Kravetz <[email protected]> Date: Wed, 12 Jul 2017 13:52:47 -0700 Subject: [PATCH] mm/mremap: Fail map duplication attempts for private mappings mremap will attempt to create a 'duplicate' mapping if old_size == 0 is specified. In the case of private mappings, mremap will actually create a fresh separate private mapping unrelated to the original. This does not fit with the design semantics of mremap as the intention is to create a new mapping based on the original. Therefore, return EINVAL in the case where an attempt is made to duplicate a private mapping. Also, print a warning message (once) if such an attempt is made. Signed-off-by: Mike Kravetz <[email protected]> --- mm/mremap.c | 13 +++++++++++++ 1 file changed, 13 insertions(+) diff --git a/mm/mremap.c b/mm/mremap.c index cd8a1b1..75b167d 100644 --- a/mm/mremap.c +++ b/mm/mremap.c @@ -383,6 +383,19 @@ static struct vm_area_struct *vma_to_resize(unsigned long addr, if (!vma || vma->vm_start > addr) return ERR_PTR(-EFAULT); + /* + * !old_len is a special case where an attempt is made to 'duplicate' + * a mapping. This makes no sense for private mappings as it will + * instead create a fresh/new mapping unrelated to the original. This + * is contrary to the basic idea of mremap which creates new mappings + * based on the original. There are no known use cases for this + * behavior. As a result, fail such attempts. + */ + if (!old_len && !(vma->vm_flags & (VM_SHARED | VM_MAYSHARE))) { + pr_warn_once("%s (%d): attempted to duplicate a private mapping with mremap. This is not supported.\n", current->comm, current->pid); + return ERR_PTR(-EINVAL); + } + if (is_vm_hugetlb_page(vma)) return ERR_PTR(-EINVAL); -- 2.7.5
