sg: remove ->sg_magic member

This was introduced more than a decade ago when sg chaining was
added, but we never really caught anything with it. The scatterlist
entry size can be critical, since drivers allocate it, so remove
the magic member. Recently it's been triggering allocation stalls
and failures in NVMe.
Tested-by: Jordan Glover <Golden_Miller83@protonmail.ch>
Acked-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

drivers/gpu/drm/i915/i915_drv.h

@@ -2238,9 +2238,6 @@ static inline struct scatterlist *____sg_next(struct scatterlist *sg)
  **/
 static inline struct scatterlist *__sg_next(struct scatterlist *sg)
 {
-#ifdef CONFIG_DEBUG_SG
-	BUG_ON(sg->sg_magic != SG_MAGIC);
-#endif
 	return sg_is_last(sg) ? NULL : ____sg_next(sg);
 }
 

include/linux/scatterlist.h

@@ -9,9 +9,6 @@
 #include <asm/io.h>
 
 struct scatterlist {
-#ifdef CONFIG_DEBUG_SG
-	unsigned long	sg_magic;
-#endif
 	unsigned long	page_link;
 	unsigned int	offset;
 	unsigned int	length;
@@ -64,7 +61,6 @@ struct sg_table {
  *
  */
 
-#define SG_MAGIC	0x87654321
 #define SG_CHAIN	0x01UL
 #define SG_END		0x02UL
 
@@ -98,7 +94,6 @@ static inline void sg_assign_page(struct scatterlist *sg, struct page *page)
 	 */
 	BUG_ON((unsigned long) page & (SG_CHAIN | SG_END));
 #ifdef CONFIG_DEBUG_SG
-	BUG_ON(sg->sg_magic != SG_MAGIC);
 	BUG_ON(sg_is_chain(sg));
 #endif
 	sg->page_link = page_link | (unsigned long) page;
@@ -129,7 +124,6 @@ static inline void sg_set_page(struct scatterlist *sg, struct page *page,
 static inline struct page *sg_page(struct scatterlist *sg)
 {
 #ifdef CONFIG_DEBUG_SG
-	BUG_ON(sg->sg_magic != SG_MAGIC);
 	BUG_ON(sg_is_chain(sg));
 #endif
 	return (struct page *)((sg)->page_link & ~(SG_CHAIN | SG_END));
@@ -195,9 +189,6 @@ static inline void sg_chain(struct scatterlist *prv, unsigned int prv_nents,
  **/
 static inline void sg_mark_end(struct scatterlist *sg)
 {
-#ifdef CONFIG_DEBUG_SG
-	BUG_ON(sg->sg_magic != SG_MAGIC);
-#endif
 	/*
 	 * Set termination bit, clear potential chain bit
 	 */
@@ -215,9 +206,6 @@ static inline void sg_mark_end(struct scatterlist *sg)
  **/
 static inline void sg_unmark_end(struct scatterlist *sg)
 {
-#ifdef CONFIG_DEBUG_SG
-	BUG_ON(sg->sg_magic != SG_MAGIC);
-#endif
 	sg->page_link &= ~SG_END;
 }
 
@@ -260,12 +248,6 @@ static inline void *sg_virt(struct scatterlist *sg)
 static inline void sg_init_marker(struct scatterlist *sgl,
 				  unsigned int nents)
 {
-#ifdef CONFIG_DEBUG_SG
-	unsigned int i;
-
-	for (i = 0; i < nents; i++)
-		sgl[i].sg_magic = SG_MAGIC;
-#endif
 	sg_mark_end(&sgl[nents - 1]);
 }
 

lib/scatterlist.c

@@ -24,9 +24,6 @@
  **/
 struct scatterlist *sg_next(struct scatterlist *sg)
 {
-#ifdef CONFIG_DEBUG_SG
-	BUG_ON(sg->sg_magic != SG_MAGIC);
-#endif
 	if (sg_is_last(sg))
 		return NULL;
 
@@ -111,10 +108,7 @@ struct scatterlist *sg_last(struct scatterlist *sgl, unsigned int nents)
 	for_each_sg(sgl, sg, nents, i)
 		ret = sg;
 
-#ifdef CONFIG_DEBUG_SG
-	BUG_ON(sgl[0].sg_magic != SG_MAGIC);
 	BUG_ON(!sg_is_last(ret));
-#endif
 	return ret;
 }
 EXPORT_SYMBOL(sg_last);

tools/virtio/linux/scatterlist.h

@@ -36,7 +36,6 @@ static inline void sg_assign_page(struct scatterlist *sg, struct page *page)
 	 */
 	BUG_ON((unsigned long) page & 0x03);
 #ifdef CONFIG_DEBUG_SG
-	BUG_ON(sg->sg_magic != SG_MAGIC);
 	BUG_ON(sg_is_chain(sg));
 #endif
 	sg->page_link = page_link | (unsigned long) page;
@@ -67,7 +66,6 @@ static inline void sg_set_page(struct scatterlist *sg, struct page *page,
 static inline struct page *sg_page(struct scatterlist *sg)
 {
 #ifdef CONFIG_DEBUG_SG
-	BUG_ON(sg->sg_magic != SG_MAGIC);
 	BUG_ON(sg_is_chain(sg));
 #endif
 	return (struct page *)((sg)->page_link & ~0x3);
@@ -116,9 +114,6 @@ static inline void sg_chain(struct scatterlist *prv, unsigned int prv_nents,
  **/
 static inline void sg_mark_end(struct scatterlist *sg)
 {
-#ifdef CONFIG_DEBUG_SG
-	BUG_ON(sg->sg_magic != SG_MAGIC);
-#endif
 	/*
 	 * Set termination bit, clear potential chain bit
 	 */
@@ -136,17 +131,11 @@ static inline void sg_mark_end(struct scatterlist *sg)
  **/
 static inline void sg_unmark_end(struct scatterlist *sg)
 {
-#ifdef CONFIG_DEBUG_SG
-	BUG_ON(sg->sg_magic != SG_MAGIC);
-#endif
 	sg->page_link &= ~0x02;
 }
 
 static inline struct scatterlist *sg_next(struct scatterlist *sg)
 {
-#ifdef CONFIG_DEBUG_SG
-	BUG_ON(sg->sg_magic != SG_MAGIC);
-#endif
 	if (sg_is_last(sg))
 		return NULL;
 
@@ -160,13 +149,6 @@ static inline struct scatterlist *sg_next(struct scatterlist *sg)
 static inline void sg_init_table(struct scatterlist *sgl, unsigned int nents)
 {
 	memset(sgl, 0, sizeof(*sgl) * nents);
-#ifdef CONFIG_DEBUG_SG
-	{
-		unsigned int i;
-		for (i = 0; i < nents; i++)
-			sgl[i].sg_magic = SG_MAGIC;
-	}
-#endif
 	sg_mark_end(&sgl[nents - 1]);
 }