mlock: downgrade mmap sem while populating mlocked regions
We need to hold the mmap_sem for write to initiatate mlock()/munlock()
because we may need to merge/split vmas. However, this can lead to very
long lock hold times attempting to fault in a large memory region to mlock
it into memory. This can hold off other faults against the mm
[multithreaded tasks] and other scans of the mm, such as via /proc. To
alleviate this, downgrade the mmap_sem to read mode during the population
of the region for locking. This is especially the case if we need to
reclaim memory to lock down the region. We [probably?] don't need to do
this for unlocking as all of the pages should be resident--they're already
mlocked.
Now, the caller's of the mlock functions [mlock_fixup() and
mlock_vma_pages_range()] expect the mmap_sem to be returned in write mode.
Changing all callers appears to be way too much effort at this point.
So, restore write mode before returning. Note that this opens a window
where the mmap list could change in a multithreaded process. So, at least
for mlock_fixup(), where we could be called in a loop over multiple vmas,
we check that a vma still exists at the start address and that vma still
covers the page range [start,end). If not, we return an error, -EAGAIN,
and let the caller deal with it.
Return -EAGAIN from mlock_vma_pages_range() function and mlock_fixup() if
the vma at 'start' disappears or changes so that the page range
[start,end) is no longer contained in the vma. Again, let the caller deal
with it. Looks like only sys_remap_file_pages() [via mmap_region()]
should actually care.
With this patch, I no longer see processes like ps(1) blocked for seconds
or minutes at a time waiting for a large [multiple gigabyte] region to be
locked down. However, I occassionally see delays while unlocking or
unmapping a large mlocked region. Should we also downgrade the mmap_sem
for the unlock path?
Signed-off-by: Lee Schermerhorn <lee.schermerhorn@hp.com>
Signed-off-by: Rik van Riel <riel@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
diff --git a/mm/mlock.c b/mm/mlock.c
index 8746fe3..c83896a 100644
--- a/mm/mlock.c
+++ b/mm/mlock.c
@@ -318,6 +318,7 @@
int mlock_vma_pages_range(struct vm_area_struct *vma,
unsigned long start, unsigned long end)
{
+ struct mm_struct *mm = vma->vm_mm;
int nr_pages = (end - start) / PAGE_SIZE;
BUG_ON(!(vma->vm_flags & VM_LOCKED));
@@ -329,8 +330,19 @@
if (!((vma->vm_flags & (VM_DONTEXPAND | VM_RESERVED)) ||
is_vm_hugetlb_page(vma) ||
- vma == get_gate_vma(current)))
- return __mlock_vma_pages_range(vma, start, end);
+ vma == get_gate_vma(current))) {
+ downgrade_write(&mm->mmap_sem);
+ nr_pages = __mlock_vma_pages_range(vma, start, end);
+
+ up_read(&mm->mmap_sem);
+ /* vma can change or disappear */
+ down_write(&mm->mmap_sem);
+ vma = find_vma(mm, start);
+ /* non-NULL vma must contain @start, but need to check @end */
+ if (!vma || end > vma->vm_end)
+ return -EAGAIN;
+ return nr_pages;
+ }
/*
* User mapped kernel pages or huge pages:
@@ -424,13 +436,41 @@
vma->vm_flags = newflags;
if (lock) {
+ /*
+ * mmap_sem is currently held for write. Downgrade the write
+ * lock to a read lock so that other faults, mmap scans, ...
+ * while we fault in all pages.
+ */
+ downgrade_write(&mm->mmap_sem);
+
ret = __mlock_vma_pages_range(vma, start, end);
if (ret > 0) {
mm->locked_vm -= ret;
ret = 0;
}
- } else
+ /*
+ * Need to reacquire mmap sem in write mode, as our callers
+ * expect this. We have no support for atomically upgrading
+ * a sem to write, so we need to check for ranges while sem
+ * is unlocked.
+ */
+ up_read(&mm->mmap_sem);
+ /* vma can change or disappear */
+ down_write(&mm->mmap_sem);
+ *prev = find_vma(mm, start);
+ /* non-NULL *prev must contain @start, but need to check @end */
+ if (!(*prev) || end > (*prev)->vm_end)
+ ret = -EAGAIN;
+ } else {
+ /*
+ * TODO: for unlocking, pages will already be resident, so
+ * we don't need to wait for allocations/reclaim/pagein, ...
+ * However, unlocking a very large region can still take a
+ * while. Should we downgrade the semaphore for both lock
+ * AND unlock ?
+ */
__munlock_vma_pages_range(vma, start, end);
+ }
out:
*prev = vma;