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    SubjectRe: per-bdi-throttling: synchronous writepage doesn't work correctly
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    On Thu, 2007-11-01 at 20:19 +0100, Miklos Szeredi wrote:
    > > >
    > > > See the file "Locking" for more details.
    > > >
    > > >
    > > > The "should set PG_Writeback" bit threw me off I guess.
    > >
    > > Hmm, set_page_writeback() is also the one clearing the radix tree dirty
    > > tag. So if that is not called, we get in a bit of a mess, no?
    > >
    > > Which makes me think hostfs is buggy.
    >
    > Yes, looks like that sort of usage is not valid. But not clearing the
    > dirty tag won't cause any malfunction, it'll just waste some CPU when
    > looking for dirty pages to write back. This is probably why this
    > wasn't noticed earlier.

    Documentation/filesystems/Locking is also quite clear on the need to
    call set_page_writeback() and end_page_writeback().

    minimal fix for hostfs

    Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
    ---

    diff --git a/fs/hostfs/hostfs_kern.c b/fs/hostfs/hostfs_kern.c
    index 8966b05..b6c1e12 100644
    --- a/fs/hostfs/hostfs_kern.c
    +++ b/fs/hostfs/hostfs_kern.c
    @@ -415,6 +415,7 @@ int hostfs_writepage(struct page *page, struct writeback_control *wbc)
    int end_index = inode->i_size >> PAGE_CACHE_SHIFT;
    int err;

    + set_page_writeback(page);
    if (page->index >= end_index)
    count = inode->i_size & (PAGE_CACHE_SIZE-1);

    @@ -438,6 +439,7 @@ int hostfs_writepage(struct page *page, struct writeback_control *wbc)
    kunmap(page);

    unlock_page(page);
    + end_page_writeback(page);
    return err;
    }


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