lkml.org 
[lkml]   [2008]   [Jun]   [11]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
Patch in this message
/
Date
From
Subject[PATCH -mm 17/24] mlock: downgrade mmap sem while populating mlocked regions
From: Lee Schermerhorn <lee.schermerhorn@hp.com>

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>

---
V2 -> V3:
+ rebase to 23-mm1 atop RvR's split lru series [no change]
+ fix function return types [void -> int] to fix build when
not configured.
New in V2.

mm/mlock.c | 46 +++++++++++++++++++++++++++++++++++++++++++---
1 file changed, 43 insertions(+), 3 deletions(-)
Index: linux-2.6.26-rc5-mm2/mm/mlock.c
===================================================================
--- linux-2.6.26-rc5-mm2.orig/mm/mlock.c 2008-06-10 22:28:01.000000000 -0400
+++ linux-2.6.26-rc5-mm2/mm/mlock.c 2008-06-10 22:28:05.000000000 -0400
@@ -308,6 +308,7 @@ static void __munlock_vma_pages_range(st
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));

@@ -319,8 +320,19 @@ int mlock_vma_pages_range(struct vm_area

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:
@@ -414,13 +426,41 @@ success:
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;
--
All Rights Reversed



\
 
 \ /
  Last update: 2008-06-11 20:53    [from the cache]
©2003-2011 Jasper Spaans