On Mon, Dec 02, 2013 at 08:58:25AM -0700, Matthew Wilcox wrote:
> On Mon, Dec 02, 2013 at 09:33:18AM +0100, Jan Kara wrote:
> > > Indeed, XIP could make use of the structures we already
> > > have in the struct inode/address space to behave more like a normal
> > > filesystem. We already use the mapping tree to store
> > > per-page information that is used to serialise truncate vs page
> > > faults, so why not make XIP do exactly the same thing?
> > I believe that grabbing mmap_sem for writing during truncate (in case of
> > ext2 around xip_truncate_page() & truncate_setsize() calls should do the
> > trick. But I need to verify with lockdep that it doesn't introduce new
> > locking problems.
>
> Umm ... mmap_sem for write? On each of the tasks that have a file mmaped?
> I know we have double_down that orders by memory address, but I don't
> think we have an N_down_write().
Here's a simpler solution: Grab i_mmap_sem over vm_insert_mixed().
i_mmap_sem is already taken in unmap_mapping_range(), called from
truncate_pagecache(), so we know that while we hold i_mmap_sem that
i_size has already been updated and the block can't be deallocated
from under us. We know that i_mmap_sem is safe to take here because it
already is (in __xip_unmap()).
Now we need only worry about holepunch, and it's the same problem for
pagecache and XIP files alike.
diff --git a/mm/filemap_xip.c b/mm/filemap_xip.c
index d8d9fe3..f6de4c3 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;
/*
@@ -285,6 +294,15 @@ 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 out;
+ }
/* not shared and writable, use xip_sparse_page() */
page = xip_sparse_page();
if (!page)
@@ -296,6 +314,7 @@ found:
ret = VM_FAULT_NOPAGE;
out:
+ mutex_unlock(&mapping->i_mmap_mutex);
write_seqcount_end(&xip_sparse_seq);
mutex_unlock(&xip_sparse_mutex);
--
Matthew Wilcox Intel Open Source Technology Centre
"Bill, look, we understand that you're interested in selling us this
operating system, but compare it to ours. We can't possibly take such
a retrograde step."
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