block: restore two stage elevator switch while running nr_hw_queue update

The kmemleak reports memory leaks related to elevator resources that
were originally allocated in the ->init_hctx() method. The following
leak traces are observed after running blktests block/040:

unreferenced object 0xffff8881b82f7400 (size 512):
  comm "check", pid 68454, jiffies 4310588881
  hex dump (first 32 bytes):
    00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00  ................
    00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00  ................
  backtrace (crc 5bac8b34):
    __kvmalloc_node_noprof+0x55d/0x7a0
    sbitmap_init_node+0x15a/0x6a0
    kyber_init_hctx+0x316/0xb90
    blk_mq_init_sched+0x419/0x580
    elevator_switch+0x18b/0x630
    elv_update_nr_hw_queues+0x219/0x2c0
    __blk_mq_update_nr_hw_queues+0x36a/0x6f0
    blk_mq_update_nr_hw_queues+0x3a/0x60
    0xffffffffc09ceb80
    0xffffffffc09d7e0b
    configfs_write_iter+0x2b1/0x470
    vfs_write+0x527/0xe70
    ksys_write+0xff/0x200
    do_syscall_64+0x98/0x3c0
    entry_SYSCALL_64_after_hwframe+0x76/0x7e
unreferenced object 0xffff8881b82f6000 (size 512):
  comm "check", pid 68454, jiffies 4310588881
  hex dump (first 32 bytes):
    00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00  ................
    00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00  ................
  backtrace (crc 5bac8b34):
    __kvmalloc_node_noprof+0x55d/0x7a0
    sbitmap_init_node+0x15a/0x6a0
    kyber_init_hctx+0x316/0xb90
    blk_mq_init_sched+0x419/0x580
    elevator_switch+0x18b/0x630
    elv_update_nr_hw_queues+0x219/0x2c0
    __blk_mq_update_nr_hw_queues+0x36a/0x6f0
    blk_mq_update_nr_hw_queues+0x3a/0x60
    0xffffffffc09ceb80
    0xffffffffc09d7e0b
    configfs_write_iter+0x2b1/0x470
    vfs_write+0x527/0xe70
    ksys_write+0xff/0x200
    do_syscall_64+0x98/0x3c0
    entry_SYSCALL_64_after_hwframe+0x76/0x7e
unreferenced object 0xffff8881b82f5800 (size 512):
  comm "check", pid 68454, jiffies 4310588881
  hex dump (first 32 bytes):
    00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00  ................
    00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00  ................
  backtrace (crc 5bac8b34):
    __kvmalloc_node_noprof+0x55d/0x7a0
    sbitmap_init_node+0x15a/0x6a0
    kyber_init_hctx+0x316/0xb90
    blk_mq_init_sched+0x419/0x580
    elevator_switch+0x18b/0x630
    elv_update_nr_hw_queues+0x219/0x2c0
    __blk_mq_update_nr_hw_queues+0x36a/0x6f0
    blk_mq_update_nr_hw_queues+0x3a/0x60
    0xffffffffc09ceb80
    0xffffffffc09d7e0b
    configfs_write_iter+0x2b1/0x470
    vfs_write+0x527/0xe70

    ksys_write+0xff/0x200
    do_syscall_64+0x98/0x3c0
    entry_SYSCALL_64_after_hwframe+0x76/0x7e

The issue arises while we run nr_hw_queue update,  Specifically, we first
reallocate hardware contexts (hctx) via __blk_mq_realloc_hw_ctxs(), and
then later invoke elevator_switch() (assuming q->elevator is not NULL).
The elevator switch code would first exit old elevator (elevator_exit)
and then switches to the new elevator. The elevator_exit loops through
each hctx and invokes the elevator’s per-hctx exit method ->exit_hctx(),
which releases resources allocated during ->init_hctx().

This memleak manifests when we reduce the num of h/w queues - for example,
when the initial update sets the number of queues to X, and a later update
reduces it to Y, where Y < X. In this case, we'd loose the access to old
hctxs while we get to elevator exit code because __blk_mq_realloc_hw_ctxs
would have already released the old hctxs. As we don't now have any
reference left to the old hctxs, we don't have any way to free the
scheduler resources (which are allocate in ->init_hctx()) and kmemleak
complains about it.

This issue was caused due to the commit 596dce110b7d ("block: simplify
elevator reattachment for updating nr_hw_queues"). That change unified
the two-stage elevator teardown and reattachment into a single call that
occurs after __blk_mq_realloc_hw_ctxs() has already freed the hctxs.

This patch restores the previous two-stage elevator switch logic during
nr_hw_queues updates. First, the elevator is switched to 'none', which
ensures all scheduler resources are properly freed. Then, the hardware
contexts (hctxs) are reallocated, and the software-to-hardware queue
mappings are updated. Finally, the original elevator is reattached. This
sequence prevents loss of references to old hctxs and avoids the scheduler
resource leaks reported by kmemleak.

Reported-by : Yi Zhang <yi.zhang@redhat.com>

Fixes: 596dce110b7d ("block: simplify elevator reattachment for updating nr_hw_queues")
Closes: https://lore.kernel.org/all/CAHj4cs8oJFvz=daCvjHM5dYCNQH4UXwSySPPU4v-WHce_kZXZA@mail.gmail.com/
Signed-off-by: Nilay Shroff <nilay@linux.ibm.com>
Reviewed-by: Yu Kuai <yukuai3@huawei.com>
Link: https://lore.kernel.org/r/20250724102540.1366308-1-nilay@linux.ibm.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>

0 files changed, 0 insertions, 0 deletions