On Wed, May 4, 2011 at 5:09 PM, Michel Lespinasse <walken@xxxxxxxxxx> wrote:
>
> FYI, the attached code causes an infinite loop in kernels that have
> the 95042f9eb7 commit:
Mmm.
Yes. The atomic fault will never work, and the get_user_pages() thing
won't either, so things will just loop forever.
> Linus, I am not sure as to what would be the preferred way to fix
> this. One option could be to modify fault_in_user_writeable so that it
> passes a non-NULL page pointer, and just does a put_page on it
> afterwards. While this would work, this is kinda ugly and would slow
> down futex operations somewhat.
No, that's just ugly as hell.
> A more conservative alternative could
> be to enable the guard page special case under an new GUP flag, but
> this loses much of the elegance of your original proposal...
How about only doing that only for FOLL_MLOCK?
Also, looking at mm/mlock.c, why _do_ we call get_user_pages() even if
the vma isn't mlocked? That looks bogus. Since we have dropped the
mm_semaphore, an unlock may have happened, and afaik we should *not*
try to bring those pages back in at all. There's this whole comment
about that in the caller ("__mlock_vma_pages_range() double checks the
vma flags, so that it won't mlock pages if the vma was already
munlocked."), but despite that it would actually call
__get_user_pages() even if the VM_LOCKED bit had been cleared (it just
wouldn't call it with the FOLL_MLOCK flag).
So maybe something like the attached?
UNTESTED! And maybe there was some really subtle reason to still call
__get_user_pages() without that FOLL_MLOCK thing that I'm missing.
Linus
mm/memory.c | 2 +-
mm/mlock.c | 8 ++++----
2 files changed, 5 insertions(+), 5 deletions(-)
diff --git a/mm/memory.c b/mm/memory.c
index 607098d47e74..f7a487c908a5 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -1555,7 +1555,7 @@ int __get_user_pages(struct task_struct *tsk, struct mm_struct *mm,
* If we don't actually want the page itself,
* and it's the stack guard page, just skip it.
*/
- if (!pages && stack_guard_page(vma, start))
+ if (!pages && (gup_flags & FOLL_MLOCK) && stack_guard_page(vma, start))
goto next_page;
do {
diff --git a/mm/mlock.c b/mm/mlock.c
index 6b55e3efe0df..8ed7fd09f81c 100644
--- a/mm/mlock.c
+++ b/mm/mlock.c
@@ -162,7 +162,10 @@ static long __mlock_vma_pages_range(struct vm_area_struct *vma,
VM_BUG_ON(end > vma->vm_end);
VM_BUG_ON(!rwsem_is_locked(&mm->mmap_sem));
- gup_flags = FOLL_TOUCH;
+ if (!(vma->vm_flags & VM_LOCKED))
+ return nr_pages;
+
+ gup_flags = FOLL_TOUCH | FOLL_MLOCK;
/*
* We want to touch writable mappings with a write fault in order
* to break COW, except for shared mappings because these don't COW
@@ -178,9 +181,6 @@ static long __mlock_vma_pages_range(struct vm_area_struct *vma,
if (vma->vm_flags & (VM_READ | VM_WRITE | VM_EXEC))
gup_flags |= FOLL_FORCE;
- if (vma->vm_flags & VM_LOCKED)
- gup_flags |= FOLL_MLOCK;
-
return __get_user_pages(current, mm, addr, nr_pages, gup_flags,
NULL, NULL, nonblocking);
}