Re: [RFC PATCH 1/1] mm/slub: fix endless "No data" printing for alloc/free_traces attribute

[Gerald Schaefer]

On Tue, 23 Nov 2021 15:19:49 +0100
Vlastimil Babka <vbabka@xxxxxxx> wrote:

> On 11/22/21 21:33, Gerald Schaefer wrote:
> > On Mon, 22 Nov 2021 21:14:00 +0100
> > Gerald Schaefer <gerald.schaefer@xxxxxxxxxxxxx> wrote:
> > 
> > [...]
> >> 
> >> Thanks. While testing this properly, yet another bug showed up. The idx
> >> in op->show remains 0 in all iterations, so I always see the same line
> >> printed t->count times (or infinitely, ATM). Not sure if this only shows
> >> on s390 due to endianness, but the reason is this:
> >> 
> >>   unsigned int idx = *(unsigned int *)v;
> 
> Uh, good catch. I was actually looking suspiciously at how we cast signed to
> unsigned, but didn't occur to me that shortening together with endiannes is
> the problem.
> 
> >> 
> >> IIUC, void *v is always the same as loff_t *ppos, and therefore idx also
> >> should be *ppos. De-referencing the loff_t * with an unsigned int * only
> >> gives the upper 32 bit half of the 64 bit value, which remains 0.
> >> 
> >> This would be fixed e.g. with
> >> 
> >>   unsigned int idx = (unsigned int) *(loff_t *) v;
> 
> With all this experience I'm now inclined to rather follow more the example
> in Documentation/filesystems/seq_file.rst and don't pass around the pointer
> that we got as ppos in slab_debugfs_start(), and that seq_file.c points to
> m->index.
> 
> In that example an own value is kmalloced:
> 
> loff_t *spos = kmalloc(sizeof(loff_t), GFP_KERNEL);
> 
> while we could just make this a field of loc_track?

Yes, following the example sounds good, and it would also make proper use
of *v in op->next, which might make the code more readable. It also looks
like it already does exactly what is needed here, i.e. have a simple
iterator that just counts the lines.

I don't think the iterator needs to be saved in loc_track. IIUC, it is
already passed around like in the example, and can then be simply compared
to t->count, similar to the existing code.

This is what I'm currently testing, and it seems to work fine. Will send
a new patch, if there are no objections:

---
 mm/slub.c | 21 +++++++++++++--------
 1 file changed, 13 insertions(+), 8 deletions(-)

diff --git a/mm/slub.c b/mm/slub.c
index a8626825a829..effb7b8d8f88 100644
--- a/mm/slub.c
+++ b/mm/slub.c
@@ -6052,10 +6052,9 @@ __initcall(slab_sysfs_init);
 #if defined(CONFIG_SLUB_DEBUG) && defined(CONFIG_DEBUG_FS)
 static int slab_debugfs_show(struct seq_file *seq, void *v)
 {
-
-	struct location *l;
-	unsigned int idx = *(unsigned int *)v;
 	struct loc_track *t = seq->private;
+	loff_t idx = *(loff_t *) v;
+	struct location *l;
 
 	if (idx < t->count) {
 		l = &t->loc[idx];
@@ -6099,23 +6098,29 @@ static int slab_debugfs_show(struct seq_file *seq, void *v)
 
 static void slab_debugfs_stop(struct seq_file *seq, void *v)
 {
+	kfree(v);
 }
 
 static void *slab_debugfs_next(struct seq_file *seq, void *v, loff_t *ppos)
 {
 	struct loc_track *t = seq->private;
+	loff_t *idxp = (loff_t *) v;
 
-	v = ppos;
-	++*ppos;
-	if (*ppos <= t->count)
-		return v;
+	*ppos = ++(*idxp);
+	if (*idxp <= t->count)
+		return idxp;
 
 	return NULL;
 }
 
 static void *slab_debugfs_start(struct seq_file *seq, loff_t *ppos)
 {
-	return ppos;
+	loff_t *idxp = kmalloc(sizeof(loff_t), GFP_KERNEL);
+
+	if (!idxp)
+		return NULL;
+	*idxp = *ppos;
+	return idxp;
 }
 
 static const struct seq_operations slab_debugfs_sops = {
-- 
2.32.0