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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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