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SubjectRe: [PATCH v7 01/22] Fix XIP fault vs truncate race
On Sun 23-03-14 15:08:27, Matthew Wilcox wrote:
> Pagecache faults recheck i_size after taking the page lock to ensure that
> the fault didn't race against a truncate. We don't have a page to lock
> in the XIP case, so use the i_mmap_mutex instead. It is locked in the
> truncate path in unmap_mapping_range() after updating i_size. So while
> we hold it in the fault path, we are guaranteed that either i_size has
> already been updated in the truncate path, or that the truncate will
> subsequently call zap_page_range_single() and so remove the mapping we
> have just inserted.
>
> There is a window of time in which i_size has been reduced and the
> thread has a mapping to a page which will be removed from the file,
> but this is harmless as the page will not be allocated to a different
> purpose before the thread's access to it is revoked.
The patch looks good. You can add:
Reviewed-by: Jan Kara <jack@suse.cz>

Honza

> Signed-off-by: Matthew Wilcox <matthew.r.wilcox@intel.com>
> ---
> mm/filemap_xip.c | 24 ++++++++++++++++++++++--
> 1 file changed, 22 insertions(+), 2 deletions(-)
>
> diff --git a/mm/filemap_xip.c b/mm/filemap_xip.c
> index d8d9fe3..c8d23e9 100644
> --- a/mm/filemap_xip.c
> +++ b/mm/filemap_xip.c
> @@ -260,8 +260,17 @@ again:
> __xip_unmap(mapping, vmf->pgoff);
>
> found:
> + /* We must recheck i_size under i_mmap_mutex */
> + mutex_lock(&mapping->i_mmap_mutex);
> + size = (i_size_read(inode) + PAGE_CACHE_SIZE - 1) >>
> + PAGE_CACHE_SHIFT;
> + if (unlikely(vmf->pgoff >= size)) {
> + mutex_unlock(&mapping->i_mmap_mutex);
> + return VM_FAULT_SIGBUS;
> + }
> err = vm_insert_mixed(vma, (unsigned long)vmf->virtual_address,
> xip_pfn);
> + mutex_unlock(&mapping->i_mmap_mutex);
> if (err == -ENOMEM)
> return VM_FAULT_OOM;
> /*
> @@ -285,16 +294,27 @@ found:
> }
> if (error != -ENODATA)
> goto out;
> +
> + /* We must recheck i_size under i_mmap_mutex */
> + mutex_lock(&mapping->i_mmap_mutex);
> + size = (i_size_read(inode) + PAGE_CACHE_SIZE - 1) >>
> + PAGE_CACHE_SHIFT;
> + if (unlikely(vmf->pgoff >= size)) {
> + ret = VM_FAULT_SIGBUS;
> + goto unlock;
> + }
> /* not shared and writable, use xip_sparse_page() */
> page = xip_sparse_page();
> if (!page)
> - goto out;
> + goto unlock;
> err = vm_insert_page(vma, (unsigned long)vmf->virtual_address,
> page);
> if (err == -ENOMEM)
> - goto out;
> + goto unlock;
>
> ret = VM_FAULT_NOPAGE;
> +unlock:
> + mutex_unlock(&mapping->i_mmap_mutex);
> out:
> write_seqcount_end(&xip_sparse_seq);
> mutex_unlock(&xip_sparse_mutex);
> --
> 1.9.0
>
> --
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--
Jan Kara <jack@suse.cz>
SUSE Labs, CR


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