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    SubjectRe: [PATCH] writeback: skip balance_dirty_pages() for in-memory fs
    On Fri, 17 Dec 2010, Wu Fengguang wrote:

    > This avoids unnecessary checks and dirty throttling on tmpfs/ramfs.
    >
    > It also prevents
    >
    > [ 388.126563] BUG: unable to handle kernel NULL pointer dereference at 0000000000000050
    >
    > in the balance_dirty_pages tracepoint, which will call
    >
    > dev_name(mapping->backing_dev_info->dev)
    >
    > but shmem_backing_dev_info.dev is NULL.
    >
    > CC: Hugh Dickins <hughd@google.com>
    > Signed-off-by: Wu Fengguang <fengguang.wu@intel.com>

    Whilst I do like this change, and I do think it's the right thing to do
    (given that the bdi has explicitly opted out of what it then got into),
    I've a sneaking feeling that something somewhere may show a regression
    from it. IIRC, there were circumstances in which it actually did
    (inadvertently) end up throttling the tmpfs writing - if there were
    too many dirty non-tmpfs pages around??

    What am I saying?! I think I'm asking you to look more closely at what
    actually used to happen, and be more explicit about the behavior you're
    stopping here - although the patch is mainly code optimization, there
    is some functional change I think. (You do mention throttling on
    tmpfs/ramfs, but the way it worked out wasn't straightforward.)

    I'd better not burble on for a third paragraph!

    Hugh

    > ---
    > mm/page-writeback.c | 3 +++
    > 1 file changed, 3 insertions(+)
    >
    > --- linux-next.orig/mm/page-writeback.c 2010-12-17 19:09:19.000000000 +0800
    > +++ linux-next/mm/page-writeback.c 2010-12-17 19:09:22.000000000 +0800
    > @@ -899,6 +899,9 @@ void balance_dirty_pages_ratelimited_nr(
    > {
    > struct backing_dev_info *bdi = mapping->backing_dev_info;
    >
    > + if (!bdi_cap_account_dirty(bdi))
    > + return;
    > +
    > current->nr_dirtied += nr_pages_dirtied;
    >
    > if (unlikely(!current->nr_dirtied_pause))


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