linux-kernel - Re: Odd NFS related SIGBUS (& possible fix)

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Date:	Fri, 01 Oct 2010 14:35:04 -0400
From:	Trond Myklebust <Trond.Myklebust@...app.com>
To:	Benjamin Herrenschmidt <benh@...nel.crashing.org>
Cc:	Nick Piggin <npiggin@...nel.dk>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Al Viro <viro@...IV.linux.org.uk>,
	linux-fsdevel@...r.kernel.org
Subject: Re: Odd NFS related SIGBUS (& possible fix)

On Fri, 2010-10-01 at 14:12 -0400, Trond Myklebust wrote:
> However, it looks to me as if the right thing to do when the
> page->mapping has changed would be to do the same thing as
> block_page_mkwrite(), and just return VM_FAULT_NOPAGE so that the VM can
> retry the fault.
> IMO: We should only SIGBUS if the calls to nfs_flush_incompatible()
> and/or nfs_updatepage() fail.
> 
> Cheers
>   Trond

IOW: Something like the following patch.

Cheers
  Trond

---------------------------------------------------------------------------
NFS: Don't SIGBUS if nfs_vm_page_mkwrite races with a cache invalidation

From: Trond Myklebust <Trond.Myklebust@...app.com>

In the case where we lock the page, and then find out that the page has
been thrown out of the page cache, we should just return VM_FAULT_NOPAGE.
This is what block_page_mkwrite() does in these situations.

Signed-off-by: Trond Myklebust <Trond.Myklebust@...app.com>
---

 fs/nfs/file.c |   17 ++++++++---------
 1 files changed, 8 insertions(+), 9 deletions(-)


diff --git a/fs/nfs/file.c b/fs/nfs/file.c
index 05bf3c0..6d95e24 100644
--- a/fs/nfs/file.c
+++ b/fs/nfs/file.c
@@ -551,7 +551,7 @@ static int nfs_vm_page_mkwrite(struct vm_area_struct *vma, struct vm_fault *vmf)
 	struct file *filp = vma->vm_file;
 	struct dentry *dentry = filp->f_path.dentry;
 	unsigned pagelen;
-	int ret = -EINVAL;
+	int ret = VM_FAULT_NOPAGE;
 	struct address_space *mapping;
 
 	dfprintk(PAGECACHE, "NFS: vm_page_mkwrite(%s/%s(%ld), offset %lld)\n",
@@ -567,21 +567,20 @@ static int nfs_vm_page_mkwrite(struct vm_area_struct *vma, struct vm_fault *vmf)
 	if (mapping != dentry->d_inode->i_mapping)
 		goto out_unlock;
 
-	ret = 0;
 	pagelen = nfs_page_length(page);
 	if (pagelen == 0)
 		goto out_unlock;
 
-	ret = nfs_flush_incompatible(filp, page);
-	if (ret != 0)
-		goto out_unlock;
+	ret = VM_FAULT_LOCKED;
+	if (nfs_flush_incompatible(filp, page) == 0 &&
+	    nfs_updatepage(filp, page, 0, pagelen) == 0)
+		goto out;
 
-	ret = nfs_updatepage(filp, page, 0, pagelen);
+	ret = VM_FAULT_SIGBUS;
 out_unlock:
-	if (!ret)
-		return VM_FAULT_LOCKED;
 	unlock_page(page);
-	return VM_FAULT_SIGBUS;
+out:
+	return ret;
 }
 
 static const struct vm_operations_struct nfs_file_vm_ops = {

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