[patch v2 3/5] fs: add nofail variant of alloc_buffer_head

[David Rientjes]

Add alloc_buffer_head_nofail().  This function is equivalent to
alloc_buffer_head(), except that it will never return NULL and instead
loop forever trying to allocate memory.

If the first allocation attempt fails because the page allocator doesn't
implicitly loop, a warning will be emitted, including a call trace.
Subsequent failures will suppress this warning.

This was added as a helper function for documentation and auditability.
No future callers should be added.

Signed-off-by: David Rientjes <rientjes@xxxxxxxxxx>
---
 fs/buffer.c                 |   17 +++++++++++++++++
 fs/gfs2/log.c               |    2 +-
 fs/jbd/journal.c            |    2 +-
 include/linux/buffer_head.h |    1 +
 4 files changed, 20 insertions(+), 2 deletions(-)

diff --git a/fs/buffer.c b/fs/buffer.c
--- a/fs/buffer.c
+++ b/fs/buffer.c
@@ -3238,6 +3238,23 @@ struct buffer_head *alloc_buffer_head(gfp_t gfp_flags)
 }
 EXPORT_SYMBOL(alloc_buffer_head);
 
+/*
+ * NOTE: no new callers of this function should be implemented!
+ * All memory allocations should be failable whenever possible.
+ */
+struct buffer_head *alloc_buffer_head_nofail(gfp_t gfp_flags)
+{
+	struct buffer_head *ret;
+
+	for (;;) {
+		ret = alloc_buffer_head(gfp_flags);
+		if (ret)
+			return ret;
+		WARN_ON_ONCE(get_order(sizeof(struct buffer_head)) >
+						PAGE_ALLOC_COSTLY_ORDER);
+	}
+}
+
 void free_buffer_head(struct buffer_head *bh)
 {
 	BUG_ON(!list_empty(&bh->b_assoc_buffers));
diff --git a/fs/gfs2/log.c b/fs/gfs2/log.c
--- a/fs/gfs2/log.c
+++ b/fs/gfs2/log.c
@@ -523,7 +523,7 @@ struct buffer_head *gfs2_log_fake_buf(struct gfs2_sbd *sdp,
 	u64 blkno = log_bmap(sdp, sdp->sd_log_flush_head);
 	struct buffer_head *bh;
 
-	bh = alloc_buffer_head(GFP_NOFS | __GFP_NOFAIL);
+	bh = alloc_buffer_head_nofail(GFP_NOFS);
 	atomic_set(&bh->b_count, 1);
 	bh->b_state = (1 << BH_Mapped) | (1 << BH_Uptodate) | (1 << BH_Lock);
 	set_bh_page(bh, real->b_page, bh_offset(real));
diff --git a/fs/jbd/journal.c b/fs/jbd/journal.c
--- a/fs/jbd/journal.c
+++ b/fs/jbd/journal.c
@@ -301,7 +301,7 @@ int journal_write_metadata_buffer(transaction_t *transaction,
 	 */
 	J_ASSERT_BH(bh_in, buffer_jbddirty(bh_in));
 
-	new_bh = alloc_buffer_head(GFP_NOFS|__GFP_NOFAIL);
+	new_bh = alloc_buffer_head_nofail(GFP_NOFS);
 	/* keep subsequent assertions sane */
 	new_bh->b_state = 0;
 	init_buffer(new_bh, NULL, NULL);
diff --git a/include/linux/buffer_head.h b/include/linux/buffer_head.h
--- a/include/linux/buffer_head.h
+++ b/include/linux/buffer_head.h
@@ -176,6 +176,7 @@ void __breadahead(struct block_device *, sector_t block, unsigned int size);
 struct buffer_head *__bread(struct block_device *, sector_t block, unsigned size);
 void invalidate_bh_lrus(void);
 struct buffer_head *alloc_buffer_head(gfp_t gfp_flags);
+struct buffer_head *alloc_buffer_head_nofail(gfp_t gfp_flags);
 void free_buffer_head(struct buffer_head * bh);
 void unlock_buffer(struct buffer_head *bh);
 void __lock_buffer(struct buffer_head *bh);
--
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