NFS: Fix a deadlock involving nfs_release_folio()

Wang Zhaolong reports a deadlock involving NFSv4.1 state recovery
waiting on kthreadd, which is attempting to reclaim memory by calling
nfs_release_folio(). The latter cannot make progress due to state
recovery being needed.

It seems that the only safe thing to do here is to kick off a writeback
of the folio, without waiting for completion, or else kicking off an
asynchronous commit.

Reported-by: Wang Zhaolong <[e-mail]>
Fixes: 96780ca55e3c ("NFS: fix up nfs_release_folio() to try to release the page")
Signed-off-by: Trond Myklebust <[e-mail]>

fs/nfs/file.c

···
 		if ((current_gfp_context(gfp) & GFP_KERNEL) != GFP_KERNEL ||
 		    current_is_kswapd() || current_is_kcompactd())
 			return false;
-		if (nfs_wb_folio(folio->mapping->host, folio) < 0)
+		if (nfs_wb_folio_reclaim(folio->mapping->host, folio) < 0 ||
+		    folio_test_private(folio))
 			return false;
 	}
 	return nfs_fscache_release_folio(folio, gfp);

fs/nfs/nfstrace.h

···
 DEFINE_NFS_FOLIO_EVENT(nfs_aop_readpage);
 DEFINE_NFS_FOLIO_EVENT_DONE(nfs_aop_readpage_done);
 
+DEFINE_NFS_FOLIO_EVENT(nfs_writeback_folio_reclaim);
+DEFINE_NFS_FOLIO_EVENT_DONE(nfs_writeback_folio_reclaim_done);
+
 DEFINE_NFS_FOLIO_EVENT(nfs_writeback_folio);
 DEFINE_NFS_FOLIO_EVENT_DONE(nfs_writeback_folio_done);
 

fs/nfs/write.c

···
 }
 
 /**
+ * nfs_wb_folio_reclaim - Write back all requests on one page
+ * @inode: pointer to page
+ * @folio: pointer to folio
+ *
+ * Assumes that the folio has been locked by the caller
+ */
+int nfs_wb_folio_reclaim(struct inode *inode, struct folio *folio)
+{
+	loff_t range_start = folio_pos(folio);
+	size_t len = folio_size(folio);
+	struct writeback_control wbc = {
+		.sync_mode = WB_SYNC_ALL,
+		.nr_to_write = 0,
+		.range_start = range_start,
+		.range_end = range_start + len - 1,
+		.for_sync = 1,
+	};
+	int ret;
+
+	if (folio_test_writeback(folio))
+		return -EBUSY;
+	if (folio_clear_dirty_for_io(folio)) {
+		trace_nfs_writeback_folio_reclaim(inode, range_start, len);
+		ret = nfs_writepage_locked(folio, &wbc);
+		trace_nfs_writeback_folio_reclaim_done(inode, range_start, len,
+						       ret);
+		return ret;
+	}
+	nfs_commit_inode(inode, 0);
+	return 0;
+}
+
+/**
  * nfs_wb_folio - Write back all requests on one page
  * @inode: pointer to page
  * @folio: pointer to folio

include/linux/nfs_fs.h

···
 extern int nfs_sync_inode(struct inode *inode);
 extern int nfs_wb_all(struct inode *inode);
 extern int nfs_wb_folio(struct inode *inode, struct folio *folio);
+extern int nfs_wb_folio_reclaim(struct inode *inode, struct folio *folio);
 int nfs_wb_folio_cancel(struct inode *inode, struct folio *folio);
 extern int  nfs_commit_inode(struct inode *, int);
 extern struct nfs_commit_data *nfs_commitdata_alloc(void);