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    SubjectRe: [PATCH 04/14] mm,migration: Allow the migration of PageSwapCache pages
    On Thu, Apr 22, 2010 at 04:40:04PM +0100, Mel Gorman wrote:
    > On Thu, Apr 22, 2010 at 11:18:14PM +0900, Minchan Kim wrote:
    > > On Thu, Apr 22, 2010 at 11:14 PM, Mel Gorman <mel@csn.ul.ie> wrote:
    > > > On Thu, Apr 22, 2010 at 07:51:53PM +0900, KAMEZAWA Hiroyuki wrote:
    > > >> On Thu, 22 Apr 2010 19:31:06 +0900
    > > >> KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com> wrote:
    > > >>
    > > >> > On Thu, 22 Apr 2010 19:13:12 +0900
    > > >> > Minchan Kim <minchan.kim@gmail.com> wrote:
    > > >> >
    > > >> > > On Thu, Apr 22, 2010 at 6:46 PM, KAMEZAWA Hiroyuki
    > > >> > > <kamezawa.hiroyu@jp.fujitsu.com> wrote:
    > > >> >
    > > >> > > > Hmm..in my test, the case was.
    > > >> > > >
    > > >> > > > Before try_to_unmap:
    > > >> > > >        mapcount=1, SwapCache, remap_swapcache=1
    > > >> > > > After remap
    > > >> > > >        mapcount=0, SwapCache, rc=0.
    > > >> > > >
    > > >> > > > So, I think there may be some race in rmap_walk() and vma handling or
    > > >> > > > anon_vma handling. migration_entry isn't found by rmap_walk.
    > > >> > > >
    > > >> > > > Hmm..it seems this kind patch will be required for debug.
    > > >> > >
    > > >>
    > > >> Ok, here is my patch for _fix_. But still testing...
    > > >> Running well at least for 30 minutes, where I can see bug in 10minutes.
    > > >> But this patch is too naive. please think about something better fix.
    > > >>
    > > >> ==
    > > >> From: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
    > > >>
    > > >> At adjust_vma(), vma's start address and pgoff is updated under
    > > >> write lock of mmap_sem. This means the vma's rmap information
    > > >> update is atoimic only under read lock of mmap_sem.
    > > >>
    > > >>
    > > >> Even if it's not atomic, in usual case, try_to_ummap() etc...
    > > >> just fails to decrease mapcount to be 0. no problem.
    > > >>
    > > >> But at page migration's rmap_walk(), it requires to know all
    > > >> migration_entry in page tables and recover mapcount.
    > > >>
    > > >> So, this race in vma's address is critical. When rmap_walk meet
    > > >> the race, rmap_walk will mistakenly get -EFAULT and don't call
    > > >> rmap_one(). This patch adds a lock for vma's rmap information.
    > > >> But, this is _very slow_.
    > > >
    > > > Ok wow. That is exceptionally well-spotted. This looks like a proper bug
    > > > that compaction exposes as opposed to a bug that compaction introduces.
    > > >
    > > >> We need something sophisitcated, light-weight update for this..
    > > >>
    > > >
    > > > In the event the VMA is backed by a file, the mapping i_mmap_lock is taken for
    > > > the duration of the update and is  taken elsewhere where the VMA information
    > > > is read such as rmap_walk_file()
    > > >
    > > > In the event the VMA is anon, vma_adjust currently talks no locks and your
    > > > patch introduces a new one but why not use the anon_vma lock here? Am I
    > > > missing something that requires the new lock?
    > >
    > > rmap_walk_anon doesn't hold vma's anon_vma->lock.
    > > It holds page->anon_vma->lock.
    > >
    >
    > Of course, thank you for pointing out my error. With multiple
    > anon_vma's, the locking is a bit of a mess. We cannot hold spinlocks on
    > two vma's in the same list at the same time without potentially causing
    > a livelock. The problem becomes how we can safely drop one anon_vma and
    > acquire the other without them disappearing from under us.
    >
    > See the XXX mark in the following incomplete patch for example. It's
    > incomplete because the list traversal is also not safe once the lock has
    > been dropped and -EFAULT is returned by vma_address.
    >

    There is a simplier alternative I guess. When the vma->anon_vma is difference,
    try and lock it. If the lock is uncontended, continue. If not, release the
    pages anon_vma lock and start from the beginning - repeat until the list is
    walked uncontended. This should avoid livelocking against other walkers.
    I have the test running now for 30 minutes with no problems but will
    leave it overnight and see what happens.

    I tried the approach of having vma_adjust and rmap_walk always seeing
    the same anon_vma but I couldn't devise a method. It doesn't seem
    possible but I'm still getting to grips with the anon_vma_chain stuff.
    Maybe Rik can spot a better way of doing this.

    ==== CUT HERE ====
    mm: Take the vma anon_vma lock in vma_adjust and during rmap_walk

    vma_adjust() is updating anon VMA information without any locks taken.
    In constract, file-backed mappings use the i_mmap_lock. This lack of
    locking can result in races with page migration. During rmap_walk(),
    vma_address() can return -EFAULT for an address that will soon be valid.
    This leaves a dangling migration PTE behind which can later cause a
    BUG_ON to trigger when the page is faulted in.

    This patch has vma_adjust() take the vma->anon_vma lock. When
    rmap_walk_anon() is walking the anon_vma_chain list, it can encounter a
    VMA a different anon_vma. It cannot just take this lock because
    depending on the order of traversal of anon_vma_chains in other
    processes, it could cause a livelock. Instead, it releases the pages
    anon_vma lock it has and starts again from scratch until it can traverse
    the full list uncontended.

    Signed-off-by: Mel Gorman <mel@csn.ul.ie>
    ---
    mm/mmap.c | 6 ++++++
    mm/rmap.c | 21 ++++++++++++++++++++-
    2 files changed, 26 insertions(+), 1 deletions(-)

    diff --git a/mm/mmap.c b/mm/mmap.c
    index f90ea92..61d6f1d 100644
    --- a/mm/mmap.c
    +++ b/mm/mmap.c
    @@ -578,6 +578,9 @@ again: remove_next = 1 + (end > next->vm_end);
    }
    }

    + if (vma->anon_vma)
    + spin_lock(&vma->anon_vma->lock);
    +
    if (root) {
    flush_dcache_mmap_lock(mapping);
    vma_prio_tree_remove(vma, root);
    @@ -620,6 +623,9 @@ again: remove_next = 1 + (end > next->vm_end);
    if (mapping)
    spin_unlock(&mapping->i_mmap_lock);

    + if (vma->anon_vma)
    + spin_unlock(&vma->anon_vma->lock);
    +
    if (remove_next) {
    if (file) {
    fput(file);
    diff --git a/mm/rmap.c b/mm/rmap.c
    index 85f203e..59d5553 100644
    --- a/mm/rmap.c
    +++ b/mm/rmap.c
    @@ -1368,13 +1368,32 @@ static int rmap_walk_anon(struct page *page, int (*rmap_one)(struct page *,
    * are holding mmap_sem. Users without mmap_sem are required to
    * take a reference count to prevent the anon_vma disappearing
    */
    +retry:
    anon_vma = page_anon_vma(page);
    if (!anon_vma)
    return ret;
    spin_lock(&anon_vma->lock);
    list_for_each_entry(avc, &anon_vma->head, same_anon_vma) {
    struct vm_area_struct *vma = avc->vma;
    - unsigned long address = vma_address(page, vma);
    + unsigned long address;
    +
    + /*
    + * Guard against livelocks by not spinning against
    + * vma->anon_vma->lock. If contention is found, release our lock and
    + * try again until VMA list can be traversed without worrying about
    + * the details of the VMA changing underneath us.
    + */
    + if (anon_vma != vma->anon_vma) {
    + if (!spin_trylock(&vma->anon_vma->lock)) {
    + spin_unlock(&anon_vma->lock);
    + goto retry;
    + }
    + }
    +
    + address = vma_address(page, vma);
    + if (anon_vma != vma->anon_vma)
    + spin_unlock(&vma->anon_vma->lock);
    +
    if (address == -EFAULT)
    continue;
    ret = rmap_one(page, vma, address, arg);
    --
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