On 09/11/2015 12:39 PM, Kirill A. Shutemov wrote:
On Thu, Sep 10, 2015 at 03:27:59PM +0200, Andrey Konovalov wrote:
Can a vma be shared among a few mm's?
Define "shared".
vma can belong only to one process (mm_struct), but it can be accessed
from other process like in rmap case below.
rmap uses anon_vma_lock for anon vma and i_mmap_rwsem for file vma to make
sure that the vma will not disappear under it.
If yes, then taking current->mm->mmap_sem to protect vma is not enough.
Depends on what protection you are talking about.
In the first report below both T378 and T398 take
current->mm->mmap_sem at mm/mlock.c:650, but they turn out to be
different locks (the addresses are different).
See i_mmap_lock_read() in T398. It will guarantee that vma is there.
In the second report T309 doesn't take any locks at all, since it
assumes that after checking atomic_dec_and_test(&mm->mm_users) the mm
has no other users, but then it does a write to vma.
This one is tricky. I *assume* the mm cannot be generally accessible after
mm_users drops to zero, but I'm not entirely sure about it.
procfs? ptrace?
The VMA is still accessible via rmap at this point. And I think it can be
a problem:
CPU0 CPU1
exit_mmap()
// mmap_sem is *not* taken
munlock_vma_pages_all()
munlock_vma_pages_range()
try_to_unmap_one()
down_read_trylock(&vma->vm_mm->mmap_sem))
!!(vma->vm_flags & VM_LOCKED) == true
vma->vm_flags &= ~VM_LOCKED;
<munlock the page>
mlock_vma_page(page);
// mlocked pages is leaked.
The obvious solution is to take mmap_sem in exit path, but it would cause
performance regression.
Any comments?
Just so others don't repeat the paths that I already looked at:
- First I thought that try_to_unmap_one() has the page locked and
munlock_vma_pages_range() will also lock it... but it doesn't.
- Then I thought that exit_mmap() will revisit the page anyway doing
actual unmap. It would, if it's the one who has the page mapped, it will
clear the mlock (see page_remove_rmap()). If it's not the last one, page
will be left locked. So it won't be completely leaked, but still, it
will be mlocked when it shouldn't.
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