Hi lianbo,

On Wed, Nov 19, 2025 at 10:31 PM lijiang <lijiang@redhat.com> wrote:
>
> On Wed, Nov 19, 2025 at 12:51 PM <devel-request@lists.crash-utility.osci.io> wrote:
>>
>> Date: Wed, 19 Nov 2025 15:36:29 +1300
>> From: Tao Liu <ltao@redhat.com>
>> Subject: [Crash-utility] [PATCH] Add blk_mq_tag_set shared_tags check
>> To: devel@lists.crash-utility.osci.io
>> Cc: Tao Liu <ltao@redhat.com>
>> Message-ID: <20251119023628.15998-2-ltao@redhat.com>
>> Content-Type: text/plain; charset="US-ASCII"; x-default=true
>>
>> A regression of cmd dev -d/-D noticed on some vmcores as:
>>
>>     dev: invalid structure member offset: blk_mq_tag_set_shared_tags
>>          FILE: dev.c  LINE: 4468  FUNCTION: blk_mq_queue_tag_busy_iter()
>>
>>     MAJOR GENDISK            NAME       REQUEST_QUEUE      TOTAL ASYNC  SYNC
>>     [crash] error trace: 91aa85 => 91a552 => 94d6ba => 94d647
>>
>>       94d647: OFFSET_verify.part.0+55
>>       94d6ba: OFFSET_verify+42
>>       91a552: get_diskio_1+2194
>>       91aa85: display_all_diskio+1253
>>
>> The root cause is, though upstream kernel have defined BLK_MQ_F_TAG_HCTX_SHARED
>> as (1 << 3), the value might be set different, e.g. [1]. So a single
>
>
> We should be able to get the value of  BLK_MQ_F_TAG_HCTX_SHARED via the dump_enumerator_list(), and then test it in the following function:

Please see [1], I removed (1<<3) and resolved it at runtime.
dump_enumerator_list() is not appropriate, but I used
arg_to_datatype().

In dump_enumerator_list("BLK_MQ_F_TAG_HCTX_SHARED"),

if (!arg_to_datatype(e, dm, RETURN_ON_ERROR) ||
(dm->size < 0) || (dm->type != ENUM) || dm->tagname)

dm->tagname is not NULL and will make dump_enumerator_list return false:

(gdb) p dm
$1 = {name = 0xe4911b "BLK_MQ_F_TAG_HCTX_SHARED", member = 0x0, type =
8192, size = 0, member_offset = 0, member_size = 0,
  member_typecode = 0, flags = 0, tagname = 0xd41dfd "(unknown)",
value = 8, vaddr = 0}

[1]: https://www.mail-archive.com/devel@lists.crash-utility.osci.io/msg01841.html

>
> static bool blk_mq_is_shared_tags(unsigned int flags)
> {
>         return flags & BLK_MQ_F_TAG_HCTX_SHARED;
> }
>
> That can follow the changes once the upstream kernel modifies the value.
>
>> blk_mq_is_shared_tags() check is not sufficient. This patch will add shared_tagsblk_mq_is_shared_tags() check is not sufficient. This patch will add shared_tags
>>  member check within blk_mq_tag_set struct as an enhancment.
>
>
> Anyway, this change looks good, so: Ack
>
> Lianbo
>
>> [1]: https://gitlab.com/redhat/centos-stream/src/kernel/centos-stream-8/-/blob/c8s/include/linux/blk-mq.h?ref_type=heads#L321
>>
>> Fixes: 62486400d35b ("Add blk_mq shared tags support for dev -d/-D")
>> Signed-off-by: Tao Liu <ltao@redhat.com>
>> ---
>>  dev.c | 2 +-
>>  1 file changed, 1 insertion(+), 1 deletion(-)
>>
>> diff --git a/dev.c b/dev.c
>> index 8391d71..27318e8 100644
>> --- a/dev.c
>> +++ b/dev.c
>> @@ -4464,7 +4464,7 @@ static void blk_mq_queue_tag_busy_iter(ulong q, ulong *hctx, uint cnt,
>>         readmem(addr + OFFSET(blk_mq_tag_set_flags), KVADDR,
>>                 &flags, sizeof(uint), "blk_mq_tag_set.flags", RETURN_ON_ERROR);
>>
>> -       if (blk_mq_is_shared_tags(flags)) {
>> +       if (blk_mq_is_shared_tags(flags) && VALID_MEMBER(blk_mq_tag_set_shared_tags)) {
>>                 addr = addr + OFFSET(blk_mq_tag_set_shared_tags);
>>                 queue_for_each_hw_ctx(q, addr, bitmap_tags_is_ptr, dio);
>>                 return;
>> --
>> 2.47.0