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Subject[patch 18/21] mlock vma pages under mmap_sem held for read
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.

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.

Signed-off-by: Lee Schermerhorn <lee.schermerhorn@hp.com>
Signed-off-by: Rik van Riel <riel@redhat.com>

Index: linux-2.6.25-rc2-mm1/mm/mlock.c
===================================================================
--- linux-2.6.25-rc2-mm1.orig/mm/mlock.c 2008-02-28 12:49:02.000000000 -0500
+++ linux-2.6.25-rc2-mm1/mm/mlock.c 2008-02-28 12:49:09.000000000 -0500
@@ -200,6 +200,37 @@ int __mlock_vma_pages_range(struct vm_ar
return ret;
}

+/**
+ * mlock_vma_pages_range
+ * @vma - vm area to mlock into memory
+ * @start - start address in @vma of range to mlock,
+ * @end - end address in @vma of range
+ *
+ * Called with current->mm->mmap_sem held write locked. Downgrade to read
+ * for faulting in pages. This can take a looong time for large segments.
+ *
+ * We need to restore the mmap_sem to write locked because our callers'
+ * callers expect this. However, because the mmap could have changed
+ * [in a multi-threaded process], we need to recheck.
+ */
+int mlock_vma_pages_range(struct vm_area_struct *vma,
+ unsigned long start, unsigned long end)
+{
+ struct mm_struct *mm = vma->vm_mm;
+
+ downgrade_write(&mm->mmap_sem);
+ __mlock_vma_pages_range(vma, start, end, 1);
+
+ 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 0;
+}
+
#else /* CONFIG_NORECLAIM_MLOCK */

/*
@@ -266,14 +297,38 @@ success:
mm->locked_vm += nr_pages;

/*
- * vm_flags is protected by the mmap_sem held in write mode.
+ * vm_flags is protected by the mmap_sem held for write.
* It's okay if try_to_unmap_one unmaps a page just after we
* set VM_LOCKED, __mlock_vma_pages_range will bring it back.
*/
vma->vm_flags = newflags;

+ /*
+ * mmap_sem is currently held for write. If we're locking pages,
+ * downgrade the write lock to a read lock so that other faults,
+ * mmap scans, ... while we fault in all pages.
+ */
+ if (lock)
+ downgrade_write(&mm->mmap_sem);
+
__mlock_vma_pages_range(vma, start, end, lock);

+ if (lock) {
+ /*
+ * 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 changes 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;
+ }
+
out:
if (ret == -ENOMEM)
ret = -EAGAIN;
Index: linux-2.6.25-rc2-mm1/mm/internal.h
===================================================================
--- linux-2.6.25-rc2-mm1.orig/mm/internal.h 2008-02-28 12:49:02.000000000 -0500
+++ linux-2.6.25-rc2-mm1/mm/internal.h 2008-02-28 12:49:09.000000000 -0500
@@ -61,24 +61,21 @@ extern int __mlock_vma_pages_range(struc
/*
* mlock all pages in this vma range. For mmap()/mremap()/...
*/
-static inline void mlock_vma_pages_range(struct vm_area_struct *vma,
- unsigned long start, unsigned long end)
-{
- __mlock_vma_pages_range(vma, start, end, 1);
-}
+extern int mlock_vma_pages_range(struct vm_area_struct *vma,
+ unsigned long start, unsigned long end);

/*
* munlock range of pages. For munmap() and exit().
* Always called to operate on a full vma that is being unmapped.
*/
-static inline void munlock_vma_pages_range(struct vm_area_struct *vma,
+static inline int munlock_vma_pages_range(struct vm_area_struct *vma,
unsigned long start, unsigned long end)
{
// TODO: verify my assumption. Should we just drop the start/end args?
VM_BUG_ON(start != vma->vm_start || end != vma->vm_end);

vma->vm_flags &= ~VM_LOCKED; /* try_to_unlock() needs this */
- __mlock_vma_pages_range(vma, start, end, 0);
+ return __mlock_vma_pages_range(vma, start, end, 0);
}

extern void clear_page_mlock(struct page *page);
@@ -90,10 +87,10 @@ static inline int is_mlocked_vma(struct
}
static inline void clear_page_mlock(struct page *page) { }
static inline void mlock_vma_page(struct page *page) { }
-static inline void mlock_vma_pages_range(struct vm_area_struct *vma,
- unsigned long start, unsigned long end) { }
-static inline void munlock_vma_pages_range(struct vm_area_struct *vma,
- unsigned long start, unsigned long end) { }
+static inline int mlock_vma_pages_range(struct vm_area_struct *vma,
+ unsigned long start, unsigned long end) { return 0; }
+static inline int munlock_vma_pages_range(struct vm_area_struct *vma,
+ unsigned long start, unsigned long end) { return 0; }

#endif /* CONFIG_NORECLAIM_MLOCK */

--
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