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    SubjectRe: [PATCH] Fix vmtruncate race and distributed filesystem race
    Dave McCracken <dmccr@us.ibm.com> wrote:
    >
    >
    > Paul McKenney and I sat down today and hashed out just what the races are
    > for both vmtruncate and the distributed filesystems. We took Andrea's
    > idea of using seqlocks and came up with a simple solution that definitely
    > fixes the race in vmtruncate, as well as most likely the invalidate race in
    > distributed filesystems. Paul is going to discuss it with the DFS folks to
    > verify that it's a complete fix for them, but neither of us can see a hole.
    >

    > + seqlock_init(&(mtd_rawdevice->as.truncate_lock));

    Why cannot this just be an atomic_t?

    > + /* Protect against page fault */
    > + write_seqlock(&mapping->truncate_lock);
    > + write_sequnlock(&mapping->truncate_lock);

    See, this just does foo++.

    > + /*
    > + * If someone invalidated the page, serialize against the inode,
    > + * then go try again.
    > + */

    This comment is inaccurate. "If this vma is file-backed and someone has
    truncated that file, this page may have been invalidated".

    > + if (unlikely(read_seqretry(&mapping->truncate_lock, sequence))) {
    > + spin_unlock(&mm->page_table_lock);
    > + down(&inode->i_sem);
    > + up(&inode->i_sem);
    > + goto retry;
    > + }
    > +

    mm/memory.c shouldn't know about inodes (ok, vmtruncate() should be in
    filemap.c).

    How about you do this:

    do_no_page()
    {
    int sequence = 0;
    ...

    retry:
    new_page = vma->vm_ops->nopage(vma, address & PAGE_MASK, &sequence);
    ....
    if (vma->vm_ops->revalidate && vma->vm_opa->revalidate(vma, sequence))
    goto retry;
    }


    filemap_nopage(..., int *sequence)
    {
    ...
    *sequence = atomic_read(&mapping->truncate_sequence);
    ...
    }

    int filemap_revalidate(vma, sequence)
    {
    struct address_space *mapping;

    mapping = vma->vm_file->f_dentry->d_inode->i_mapping;
    return sequence - atomic_read(&mapping->truncate_sequence);
    }


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