[PATCH 1/7] ubi: block: Refactor sg list processing for highmem

[PATCH 1/7] ubi: block: Refactor sg list processing for highmem

[Richard Weinberger]

Currently sg_virt() is used while filling the sg list from LEB data.
This approach cannot work with highmem.

Refactor ubi_eba_read_leb_sg() to use kmap_atomic() for sg list
access.
Since kmap_atomic() disables preempt a bounce buffer is needed.
kmap_local_page() is not used to allow easy backporting of this patch
to older kernels.

The followup patches in this series will switch to kmap_sg()
and we can remove our own helper and the bounce buffer.

Cc: stable@vger.kernel.org
Fixes: 9ff08979e1742 ("UBI: Add initial support for scatter gather")
Reported-by: Stephan Wurm <stephan.wurm@a-eberle.de>
Signed-off-by: Richard Weinberger <richard@nod.at>
---
 drivers/mtd/ubi/block.c | 11 ++---
 drivers/mtd/ubi/eba.c   | 95 ++++++++++++++++++++++++++++-------------
 include/linux/mtd/ubi.h | 12 +++---
 3 files changed, 76 insertions(+), 42 deletions(-)

diff --git a/drivers/mtd/ubi/block.c b/drivers/mtd/ubi/block.c
index 437c5b83ffe51..5b2e6c74ac5a8 100644
--- a/drivers/mtd/ubi/block.c
+++ b/drivers/mtd/ubi/block.c
@@ -193,13 +193,10 @@ static blk_status_t ubiblock_read(struct request *req)
 
 	blk_mq_start_request(req);
 
-	/*
-	 * It is safe to ignore the return value of blk_rq_map_sg() because
-	 * the number of sg entries is limited to UBI_MAX_SG_COUNT
-	 * and ubi_read_sg() will check that limit.
-	 */
 	ubi_sgl_init(&pdu->usgl);
-	blk_rq_map_sg(req->q, req, pdu->usgl.sg);
+	ret = blk_rq_map_sg(req->q, req, pdu->usgl.sg);
+	ubi_assert(ret > 0 && ret < UBI_MAX_SG_COUNT);
+	pdu->usgl.len = ret;
 
 	while (bytes_left) {
 		/*
@@ -212,7 +209,7 @@ static blk_status_t ubiblock_read(struct request *req)
 		ret = ubi_read_sg(dev->desc, leb, &pdu->usgl, offset, to_read);
 		if (ret < 0)
 			break;
-
+		pdu->usgl.tot_offset += to_read;
 		bytes_left -= to_read;
 		to_read = bytes_left;
 		leb += 1;
diff --git a/drivers/mtd/ubi/eba.c b/drivers/mtd/ubi/eba.c
index 655ff41863e2b..82c54bf7c2067 100644
--- a/drivers/mtd/ubi/eba.c
+++ b/drivers/mtd/ubi/eba.c
@@ -31,6 +31,7 @@
 #include <linux/slab.h>
 #include <linux/crc32.h>
 #include <linux/err.h>
+#include <linux/highmem.h>
 #include "ubi.h"
 
 /* Number of physical eraseblocks reserved for atomic LEB change operation */
@@ -730,6 +731,44 @@ int ubi_eba_read_leb(struct ubi_device *ubi, struct ubi_volume *vol, int lnum,
 	return err;
 }
 
+/*
+ * Basically a copy of scsi_kmap_atomic_sg().
+ * As long scsi_kmap_atomic_sg() is not part of lib/scatterlist.c have
+ * our own version to avoid a dependency on CONFIG_SCSI.
+ */
+static void *ubi_kmap_atomic_sg(struct scatterlist *sgl, int sg_count,
+			  size_t *offset, size_t *len)
+{
+	int i;
+	size_t sg_len = 0, len_complete = 0;
+	struct scatterlist *sg;
+	struct page *page;
+
+	for_each_sg(sgl, sg, sg_count, i) {
+		len_complete = sg_len; /* Complete sg-entries */
+		sg_len += sg->length;
+		if (sg_len > *offset)
+			break;
+	}
+
+	if (WARN_ON_ONCE(i == sg_count))
+		return NULL;
+
+	/* Offset starting from the beginning of first page in this sg-entry */
+	*offset = *offset - len_complete + sg->offset;
+
+	/* Assumption: contiguous pages can be accessed as "page + i" */
+	page = nth_page(sg_page(sg), (*offset >> PAGE_SHIFT));
+	*offset &= ~PAGE_MASK;
+
+	/* Bytes in this sg-entry from *offset to the end of the page */
+	sg_len = PAGE_SIZE - *offset;
+	if (*len > sg_len)
+		*len = sg_len;
+
+	return kmap_atomic(page);
+}
+
 /**
  * ubi_eba_read_leb_sg - read data into a scatter gather list.
  * @ubi: UBI device description object
@@ -748,40 +787,38 @@ int ubi_eba_read_leb_sg(struct ubi_device *ubi, struct ubi_volume *vol,
 			struct ubi_sgl *sgl, int lnum, int offset, int len,
 			int check)
 {
-	int to_read;
-	int ret;
-	struct scatterlist *sg;
+	size_t map_len, map_offset, cur_offset;
+	int ret, to_read = len;
+	char *bounce_buf;
 
-	for (;;) {
-		ubi_assert(sgl->list_pos < UBI_MAX_SG_COUNT);
-		sg = &sgl->sg[sgl->list_pos];
-		if (len < sg->length - sgl->page_pos)
-			to_read = len;
-		else
-			to_read = sg->length - sgl->page_pos;
-
-		ret = ubi_eba_read_leb(ubi, vol, lnum,
-				       sg_virt(sg) + sgl->page_pos, offset,
-				       to_read, check);
-		if (ret < 0)
-			return ret;
-
-		offset += to_read;
-		len -= to_read;
-		if (!len) {
-			sgl->page_pos += to_read;
-			if (sgl->page_pos == sg->length) {
-				sgl->list_pos++;
-				sgl->page_pos = 0;
-			}
+	bounce_buf = kvmalloc(to_read, GFP_KERNEL);
+	if (!bounce_buf) {
+		ret = -ENOMEM;
+		goto out;
+	}
 
-			break;
-		}
+	ret = ubi_eba_read_leb(ubi, vol, lnum, bounce_buf, offset, to_read, check);
+	if (ret < 0)
+		goto out;
+
+	cur_offset = 0;
+	while (to_read > 0) {
+		char *dst;
 
-		sgl->list_pos++;
-		sgl->page_pos = 0;
+		map_len = to_read;
+		map_offset = cur_offset + sgl->tot_offset;
+
+		dst = ubi_kmap_atomic_sg(sgl->sg, sgl->len, &map_offset, &map_len);
+		memcpy(dst + map_offset, bounce_buf + cur_offset, map_len);
+		kunmap_atomic(dst);
+
+		cur_offset += map_len;
+		to_read -= map_len;
 	}
 
+	ret = 0;
+out:
+	kvfree(bounce_buf);
 	return ret;
 }
 
diff --git a/include/linux/mtd/ubi.h b/include/linux/mtd/ubi.h
index a529347fd75b2..521e0e8b3ede3 100644
--- a/include/linux/mtd/ubi.h
+++ b/include/linux/mtd/ubi.h
@@ -115,8 +115,8 @@ struct ubi_volume_info {
 
 /**
  * struct ubi_sgl - UBI scatter gather list data structure.
- * @list_pos: current position in @sg[]
- * @page_pos: current position in @sg[@list_pos]
+ * @list_len: number of elemtns in @sg[]
+ * @tot_offset: current position the scatter gather list
  * @sg: the scatter gather list itself
  *
  * ubi_sgl is a wrapper around a scatter list which keeps track of the
@@ -124,8 +124,8 @@ struct ubi_volume_info {
  * it can be used across multiple ubi_leb_read_sg() calls.
  */
 struct ubi_sgl {
-	int list_pos;
-	int page_pos;
+	int len;
+	int tot_offset;
 	struct scatterlist sg[UBI_MAX_SG_COUNT];
 };
 
@@ -138,8 +138,8 @@ struct ubi_sgl {
  */
 static inline void ubi_sgl_init(struct ubi_sgl *usgl)
 {
-	usgl->list_pos = 0;
-	usgl->page_pos = 0;
+	usgl->len = 0;
+	usgl->tot_offset = 0;
 }
 
 /**
-- 
2.35.3

Re: [PATCH 1/7] ubi: block: Refactor sg list processing for highmem

[Christoph Hellwig]

On Thu, Aug 10, 2023 at 06:00:12PM +0200, Richard Weinberger wrote:
> Currently sg_virt() is used while filling the sg list from LEB data.
> This approach cannot work with highmem.
> 
> Refactor ubi_eba_read_leb_sg() to use kmap_atomic() for sg list
> access.
> Since kmap_atomic() disables preempt a bounce buffer is needed.
> kmap_local_page() is not used to allow easy backporting of this patch
> to older kernels.
> 
> The followup patches in this series will switch to kmap_sg()
> and we can remove our own helper and the bounce buffer.

Please just use kmap_local and avoid the bounce buffering.

Re: [PATCH 1/7] ubi: block: Refactor sg list processing for highmem

[Richard Weinberger]

----- Ursprüngliche Mail -----
> Von: "Christoph Hellwig" <hch@infradead.org>
>> Refactor ubi_eba_read_leb_sg() to use kmap_atomic() for sg list
>> access.
>> Since kmap_atomic() disables preempt a bounce buffer is needed.
>> kmap_local_page() is not used to allow easy backporting of this patch
>> to older kernels.
>> 
>> The followup patches in this series will switch to kmap_sg()
>> and we can remove our own helper and the bounce buffer.
> 
> Please just use kmap_local and avoid the bounce buffering.

Patch 6 does this.

Thanks,
//richard

Re: [PATCH 1/7] ubi: block: Refactor sg list processing for highmem

[Christoph Hellwig]

On Thu, Aug 10, 2023 at 06:15:36PM +0200, Richard Weinberger wrote:
> >> The followup patches in this series will switch to kmap_sg()
> >> and we can remove our own helper and the bounce buffer.
> > 
> > Please just use kmap_local and avoid the bounce buffering.
> 
> Patch 6 does this.

But why add the bounce buffering first if you can avoid it from the
very beginning by just using kmap_local instead of adding a new
caller for the deprecate kmap_atomic?

Re: [PATCH 1/7] ubi: block: Refactor sg list processing for highmem

[Richard Weinberger]

----- Ursprüngliche Mail -----
> Von: "Christoph Hellwig" <hch@infradead.org>
> An: "richard" <richard@nod.at>
> On Thu, Aug 10, 2023 at 06:15:36PM +0200, Richard Weinberger wrote:
>> >> The followup patches in this series will switch to kmap_sg()
>> >> and we can remove our own helper and the bounce buffer.
>> > 
>> > Please just use kmap_local and avoid the bounce buffering.
>> 
>> Patch 6 does this.
> 
> But why add the bounce buffering first if you can avoid it from the
> very beginning by just using kmap_local instead of adding a new
> caller for the deprecate kmap_atomic?

Because I want this fix also in all stable trees. kmap_local() is rather
new. When back porting patch 1/7, bounce buffers and kmap_atomic()
are needed anyway.
By doing this in patch 1/7 I avoid backport troubles and keep the
delta between upstream and stable trees minimal.

Thanks,
//richard

Re: [PATCH 1/7] ubi: block: Refactor sg list processing for highmem

[Christoph Hellwig]

On Thu, Aug 10, 2023 at 09:54:46PM +0200, Richard Weinberger wrote:
> > But why add the bounce buffering first if you can avoid it from the
> > very beginning by just using kmap_local instead of adding a new
> > caller for the deprecate kmap_atomic?
> 
> Because I want this fix also in all stable trees. kmap_local() is rather
> new. When back porting patch 1/7, bounce buffers and kmap_atomic()
> are needed anyway.
> By doing this in patch 1/7 I avoid backport troubles and keep the
> delta between upstream and stable trees minimal.

Just use plain kmap for the historic backports.

Re: [PATCH 1/7] ubi: block: Refactor sg list processing for highmem

[Richard Weinberger]

----- Ursprüngliche Mail -----
> Von: "Christoph Hellwig" <hch@infradead.org>
> On Thu, Aug 10, 2023 at 09:54:46PM +0200, Richard Weinberger wrote:
>> > But why add the bounce buffering first if you can avoid it from the
>> > very beginning by just using kmap_local instead of adding a new
>> > caller for the deprecate kmap_atomic?
>> 
>> Because I want this fix also in all stable trees. kmap_local() is rather
>> new. When back porting patch 1/7, bounce buffers and kmap_atomic()
>> are needed anyway.
>> By doing this in patch 1/7 I avoid backport troubles and keep the
>> delta between upstream and stable trees minimal.
> 
> Just use plain kmap for the historic backports.

Hm, yes. For UBIblock kmap should be too expensive.

Thanks,
//richard

Re: [PATCH 1/7] ubi: block: Refactor sg list processing for highmem

[Christoph Hellwig]

On Fri, Aug 11, 2023 at 10:34:52AM +0200, Richard Weinberger wrote:
> ----- Ursprüngliche Mail -----
> > Von: "Christoph Hellwig" <hch@infradead.org>
> > On Thu, Aug 10, 2023 at 09:54:46PM +0200, Richard Weinberger wrote:
> >> > But why add the bounce buffering first if you can avoid it from the
> >> > very beginning by just using kmap_local instead of adding a new
> >> > caller for the deprecate kmap_atomic?
> >> 
> >> Because I want this fix also in all stable trees. kmap_local() is rather
> >> new. When back porting patch 1/7, bounce buffers and kmap_atomic()
> >> are needed anyway.
> >> By doing this in patch 1/7 I avoid backport troubles and keep the
> >> delta between upstream and stable trees minimal.
> > 
> > Just use plain kmap for the historic backports.
> 
> Hm, yes. For UBIblock kmap should be too expensive.

?  kmap is a no-op for !highmem.  And it will always be way cheaper
than bounce buffering for the case where you are actually fed highmem
pages.