Re: [PATCH v2] mm: slub: call WARN() instead of pr_err on slab_fix.
From: Vlastimil Babka
Date: Fri Feb 07 2025 - 04:08:30 EST
On 2/7/25 04:28, Hyesoo Yu wrote:
> On Thu, Feb 06, 2025 at 12:35:22PM +0100, Vlastimil Babka wrote:
>> On 2/5/25 01:46, Hyesoo Yu wrote:
>> > If a slab object is corrupted or an error occurs in its internal
>> > value, continuing after restoration may cause other side effects.
>> > At this point, it is difficult to debug because the problem occurred
>> > in the past. It is better to use WARN() instead of pr_err to catch
>> > errors at the point of issue because WARN() could trigger panic for
>> > system debugging when panic_on_warn is enabled. WARN() should be
>> > called prior to fixing the value because when a panic is triggered by WARN(),
>> > it allows us to check corrupted data.
>> >
>> > Changes in v2:
>> > - Replace direct calling with BUG_ON with the use of WARN in slab_fix.
>> >
>> > Signed-off-by: Hyesoo Yu <hyesoo.yu@xxxxxxxxxxx>
>>
>> Hi and thanks for the patch,
>>
>> I wonder if it would be better not to change slab_fix() but rather
>> slab_err() and object_err(). It wouldn't then require to rearrange the fixup
>> code. Also I think some error reporting paths don't go through slab_fix()
>> and we still would like them to become a WARN too.
>>
>> Basically it would mean the last line in slab_err() would be a WARN and we'd
>> drop the dump_stack() as that's redundant. Same in object_err() (no
>> dump_stack() there). It would be a bit noisier as a result, but hopefully
>> acceptable. The slab specific debugging info would still be printed before
>> the WARN hits (and potentially results in a panic) so anyone investigating
>> the crash dump would have that information.
>>
>> Hm but I see some places print stuff after slab_err(). slab_pad_check() and
>> list_slab_objects(). We could create slab_err_start() and slab_err_end() for
>> those, and slab_err() would just call both at once.
>>
>
> Thank you so much for your review.
>
> That's a great suggestion. Following your suggestion, it seems possible to use
> WARN on all error reporting paths. For some places print stuff after slab_err,
> print them as follows,
>
> +static void __slab_err(struct slab *slab)
> +{
> + print_slab_info(slab);
> + add_taint(TAINT_BAD_PAGE, LOCKDEP_NOW_UNRELIABLE);
> +
> + WARN_ON(1);
> +}
> +
> static __printf(3, 4) void slab_err(struct kmem_cache *s, struct slab *slab,
> const char *fmt, ...)
> {
> vsnprintf(buf, sizeof(buf), fmt, args);
> va_end(args);
> slab_bug(s, "%s", buf);
> - print_slab_info(slab);
> - dump_stack();
> - add_taint(TAINT_BAD_PAGE, LOCKDEP_NOW_UNRELIABLE);
> + __slab_err(slab);
> }
>
> @@ -1316,11 +1322,13 @@ slab_pad_check(struct kmem_cache *s, struct slab *slab)
> while (end > fault && end[-1] == POISON_INUSE)
> end--;
>
> - slab_err(s, slab, "Padding overwritten. 0x%p-0x%p @offset=%tu",
> - fault, end - 1, fault - start);
> + slab_bug(s, "Padding overwritten. 0x%p-0x%p @offset=%tu",
> + fault, end - 1, fault - start);
> print_section(KERN_ERR, "Padding ", pad, remainder);
>
> restore_bytes(s, "slab padding", POISON_INUSE, fault, end);
> +
> + __slab_err(slab);
Yeah but move that above restore_bytes()?
BTW I think we could also do this?
--- a/mm/slub.c
+++ b/mm/slub.c
@@ -1162,7 +1162,7 @@ void skip_orig_size_check(struct kmem_cache *s, const void *object)
set_orig_size(s, (void *)object, s->object_size);
}
-static void slab_bug(struct kmem_cache *s, char *fmt, ...)
+static void slab_bug(struct kmem_cache *s, const char *fmt, ...)
{
struct va_format vaf;
va_list args;
@@ -1263,15 +1263,11 @@ static __printf(3, 4) void slab_err(struct kmem_cache *s, struct slab *slab,
const char *fmt, ...)
{
va_list args;
- char buf[100];
if (slab_add_kunit_errors())
return;
- va_start(args, fmt);
- vsnprintf(buf, sizeof(buf), fmt, args);
- va_end(args);
- slab_bug(s, "%s", buf);
+ slab_bug(s, fmt, args);
> }
>
>> > ---
>> > mm/slub.c | 10 +++++-----
>> > 1 file changed, 5 insertions(+), 5 deletions(-)
>> >
>> > diff --git a/mm/slub.c b/mm/slub.c
>> > index 1f50129dcfb3..ea956cb4b8be 100644
>> > --- a/mm/slub.c
>> > +++ b/mm/slub.c
>> > @@ -1043,7 +1043,7 @@ static void slab_fix(struct kmem_cache *s, char *fmt, ...)
>> > va_start(args, fmt);
>> > vaf.fmt = fmt;
>> > vaf.va = &args;
>> > - pr_err("FIX %s: %pV\n", s->name, &vaf);
>> > + WARN(1, "FIX %s: %pV\n", s->name, &vaf);
>> > va_end(args);
>> > }
>> >
>> > @@ -1106,8 +1106,8 @@ static bool freelist_corrupted(struct kmem_cache *s, struct slab *slab,
>> > if ((s->flags & SLAB_CONSISTENCY_CHECKS) &&
>> > !check_valid_pointer(s, slab, nextfree) && freelist) {
>> > object_err(s, slab, *freelist, "Freechain corrupt");
>> > - *freelist = NULL;
>> > slab_fix(s, "Isolate corrupted freechain");
>> > + *freelist = NULL;
>> > return true;
>> > }
>> >
>> > @@ -1445,9 +1445,9 @@ static int on_freelist(struct kmem_cache *s, struct slab *slab, void *search)
>> > set_freepointer(s, object, NULL);
>> > } else {
>> > slab_err(s, slab, "Freepointer corrupt");
>> > + slab_fix(s, "Freelist cleared");
>> > slab->freelist = NULL;
>> > slab->inuse = slab->objects;
>> > - slab_fix(s, "Freelist cleared");
>> > return 0;
>> > }
>> > break;
>> > @@ -1464,14 +1464,14 @@ static int on_freelist(struct kmem_cache *s, struct slab *slab, void *search)
>> > if (slab->objects != max_objects) {
>> > slab_err(s, slab, "Wrong number of objects. Found %d but should be %d",
>> > slab->objects, max_objects);
>> > - slab->objects = max_objects;
>> > slab_fix(s, "Number of objects adjusted");
>> > + slab->objects = max_objects;
>> > }
>> > if (slab->inuse != slab->objects - nr) {
>> > slab_err(s, slab, "Wrong object count. Counter is %d but counted were %d",
>> > slab->inuse, slab->objects - nr);
>> > - slab->inuse = slab->objects - nr;
>> > slab_fix(s, "Object count adjusted");
>> > + slab->inuse = slab->objects - nr;
>> > }
>> > return search == NULL;
>> > }
>>
>>
>
>