Commit 8edb08ca authored Oct 18, 2008 by Lee Schermerhorn Committed by Linus Torvalds Oct 20, 2008

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>

parent fa07e787

mm/mlock.c

+43 −3

Original line number	Diff line number	Diff line
		@@ -318,6 +318,7 @@ static void __munlock_vma_pages_range(struct vm_area_struct *vma,
		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 @@ int mlock_vma_pages_range(struct vm_area_struct *vma,

		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 @@ 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;

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