[PATCH] jffs2: Don't add summary entry when MTD write fails

[PATCH] jffs2: Don't add summary entry when MTD write fails

[Thomas Betker]

From: Thomas Betker <thomas.betker@rohde-schwarz.com>

jffs2_flash_direct_writev() always invokes jffs2_sum_add_kvec(), even
if mtd_writev() fails. Usually, this results in an extra summary entry
pointing to dirty node space, which should be ignored -- it is a bit of
a waste, but harmless.

When mtd_writev() returns *retlen == 0, though, the node space is not
reserved as dirty, but re-used; the extra summary entry then points
into the space of the next node. After the erase block has been closed,
we get the following messages on remount:

    jffs2: error: (79) jffs2_link_node_ref:
        Adding new ref c3048d18 at (0x00ec5b88-0x00ec6bcc)
        not immediately after previous (0x00ec5b88-0x00ec5b88)
    ...
    jffs2: Checked all inodes but still 0x2088 bytes of unchecked space?
    jffs2: No space for garbage collection. Aborting GC thread

The extra summary entries amount to "unchecked space", so that
jffs2_garbage_collect_pass() returns -ENOSPC. And without garbage
collection, the filesystem becomes unuseable over time as the erase
blocks fill up.

Fix this by skipping jffs2_sum_add_kvec() when the MTD write fails. We
don't need the summary entry anyway, and the behaviour matches that of
jffs2_flash_writev() in wbuf.c (with write buffering enabled).

Signed-off-by: Thomas Betker <thomas.betker@rohde-schwarz.com>
---
 fs/jffs2/writev.c | 16 +++++++++++++++-
 1 file changed, 15 insertions(+), 1 deletion(-)

diff --git a/fs/jffs2/writev.c b/fs/jffs2/writev.c
index a1bda9d..eec4197 100644
--- a/fs/jffs2/writev.c
+++ b/fs/jffs2/writev.c
@@ -16,9 +16,18 @@
 int jffs2_flash_direct_writev(struct jffs2_sb_info *c, const struct kvec *vecs,
 			      unsigned long count, loff_t to, size_t *retlen)
 {
+	int ret;
+
+	ret = mtd_writev(c->mtd, vecs, count, to, retlen);
+
 	if (!jffs2_is_writebuffered(c)) {
 		if (jffs2_sum_active()) {
 			int res;
+
+			if (ret ||
+			    *retlen != iov_length((struct iovec *) vecs, count))
+				return ret;
+
 			res = jffs2_sum_add_kvec(c, vecs, count, (uint32_t) to);
 			if (res) {
 				return res;
@@ -26,19 +35,23 @@ int jffs2_flash_direct_writev(struct jffs2_sb_info *c, const struct kvec *vecs,
 		}
 	}
 
-	return mtd_writev(c->mtd, vecs, count, to, retlen);
+	return ret;
 }
 
 int jffs2_flash_direct_write(struct jffs2_sb_info *c, loff_t ofs, size_t len,
 			size_t *retlen, const u_char *buf)
 {
 	int ret;
+
 	ret = mtd_write(c->mtd, ofs, len, retlen, buf);
 
 	if (jffs2_sum_active()) {
 		struct kvec vecs[1];
 		int res;
 
+		if (ret || *retlen != len)
+			return ret;
+
 		vecs[0].iov_base = (unsigned char *) buf;
 		vecs[0].iov_len = len;
 
@@ -47,5 +60,6 @@ int jffs2_flash_direct_write(struct jffs2_sb_info *c, loff_t ofs, size_t len,
 			return res;
 		}
 	}
+
 	return ret;
 }
-- 
2.6.3

Re: [PATCH] jffs2: Don't add summary entry when MTD write fails

[David Woodhouse]

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On Wed, 2015-11-11 at 21:27 +0100, Thomas Betker wrote:
> 
>  int jffs2_flash_direct_writev(struct jffs2_sb_info *c, const struct kvec *vecs,
>                               unsigned long count, loff_t to, size_t *retlen)
>  {
> +       int ret;
> +
> +       ret = mtd_writev(c->mtd, vecs, count, to, retlen);
> +
>         if (!jffs2_is_writebuffered(c)) {
>                 if (jffs2_sum_active()) {
>                         int res;
> +
> +                       if (ret ||
> +                           *retlen != iov_length((struct iovec *) vecs, count))
> +                               return ret;
> +
>                         res = jffs2_sum_add_kvec(c, vecs, count, (uint32_t) to);
>                         if (res) {
>                                 return res;

OK... but perhaps we can dispense with the separate 'ret' and 'res'
variables and the rats nest of conditions, and do something like:

   int ret;

   ret = mtd_writev(…);

   if (!ret && *retlen == iov_length(…) &&
       !jffs2_is_writebuffered(c) && jffs2_sum_active()) 
          ret = jffs2_sum_add_kvec(…);
   
   return ret;

-- 
David Woodhouse                            Open Source Technology Centre
David.Woodhouse@intel.com                              Intel Corporation


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Re: [PATCH] jffs2: Don't add summary entry when MTD write fails

[Thomas.Betker]

Hello David:

> > +       int ret;
> > +
> > +       ret = mtd_writev(c->mtd, vecs, count, to, retlen);
> > +
> >         if (!jffs2_is_writebuffered(c)) {
> >                 if (jffs2_sum_active()) {
> >                         int res;
> > +
> > +                       if (ret ||
> > +                           *retlen != iov_length((struct iovec *)
> vecs, count))
> > +                               return ret;
> > +
> >                         res = jffs2_sum_add_kvec(c, vecs, count, 
> (uint32_t) to);
> >                         if (res) {
> >                                 return res;
> 
> OK... but perhaps we can dispense with the separate 'ret' and 'res'
> variables and the rats nest of conditions, and do something like:
> 
>    int ret;
> 
>    ret = mtd_writev(…);
> 
>    if (!ret && *retlen == iov_length(…) &&
>        !jffs2_is_writebuffered(c) && jffs2_sum_active()) 
>           ret = jffs2_sum_add_kvec(…);
>    
>    return ret;

While the logic is the same, will the compiler generate the same code? 
When CONFIG_JFFS2_SUMMARY is not set, "if (jffs2_sum_active())" means "if 
(0)", and I would assume that the compiler removes the whole clause, "if" 
and all. However, I am not sure what happens with "if (!ret && whatever && 
0)".

That's why I was taking pains to keep the original control flow intact, 
even if it's a rat's nest (it is). If I remember correctly, 
jffs2_flash_direct_writev() is called quite often, and I didn't want 
performance to suffer. I may be completely wrong here, of course, but then 
why wasn't the original source code "if (!jffs2_is_writebuffered(c) && 
jffs2_sum_active())"?

Best regards,
Thomas