Re: [PATCH] md: fix soft lockup during resync when sync is repeatedly skipped

[Yunye Zhao]

Hi Kuai,

On 2026/7/17 14:27, Yu Kuai wrote:
>> md_do_sync()'s main loop advances io_sectors only when I/O is actually
>> issued (skipped == 0).  When sync_request() keeps returning skipped == 1,
>> io_sectors never increases, [...]
>
> That's not expected, io_sectors should always increase in the skip case.

Sorry, my description was not accurate. The problem is not that io_sectors
stays 0. md_do_sync()'s loop exit condition is j == max_sectors, and in
raid10_sync_request()'s recovery path sectors is always 128, so for a very
large max_sectors the loop iterates a huge number of times.

raid10_sync_request(), recovery branch:

    max_sync = RESYNC_PAGES << (PAGE_SHIFT-9);      /* 128 */
    must_sync = md_bitmap_start_sync(mddev, sect, &sync_blocks, true);
    if (sync_blocks < max_sync)   /* sync_blocks huge, never true */
        max_sync = sync_blocks;   /* so max_sync stays 128 */
    ...
    if (biolist == NULL) {
        *skipped = 1;
        return max_sync;          /* 128; the large span is discarded */
    }

Back in md_do_sync()'s main loop, j then crawls forward 128 sectors per
call until it reaches max_sectors:

    while (j < max_sectors) {
        sectors = mddev->pers->sync_request(mddev, j, max_sectors, &skipped);
        if (!skipped)             /* skipped == 1 -> io_sectors stays 0 */
            io_sectors += sectors;
        j += sectors;             /* j += 128 only */
        if (last_check + window > io_sectors || j == max_sectors)
            continue;
    }

>From the vmcore:

    sync_blocks returned = 0x5ED03680 (~1.59e8 sectors), clamped to 128
    recovery max_sectors = dev_sectors = 2^40
    iterations = 2^40 / 128 = 2^33 (~8.6e9)

> What kernel version you're testing?

6.6.102. I also tested mainline and hit the same problem.

> If this is latest kernel, bitmap_start_sync() need to be fixed. It can't
> return skip while setting skipping sectors to 0.

bitmap_start_sync() is actually fine -- it returns a large clean span
(0x5ED03680 above). The recovery path just discards it: max_sync is only
ever clamped down, so it returns 128 regardless.

> And since this is dead loop, a cond_resched() will not fix anything.

Agreed -- cond_resched() only stops the watchdog; the thread still spins
~8.6e9 no-op iterations and pins a CPU for ~313s.

For v2 I'd fix this at the source: make the recovery path honour the clean
span reported by the bitmap instead of capping the skip at 128. Does that
direction look right to you?

Thanks,
Yunye