Re: [PATCH v2] zram: fix idle age_sec underflow in idle_store()

[Hao Jia <jiahao.kernel@gmail.com>]

On 2026/8/31 12:15, Sergey Senozhatsky wrote:
> On (26/08/28 10:24), Andrew Morton wrote:
>> On Fri, 28 Aug 2026 16:31:49 +0800 Hao Jia <jiahao.kernel@gmail.com> wrote:
> [..]
>>
>> Thanks.  Sashiko asked a couple of questions about this change:
>> 	https://sashiko.dev/#/patchset/20260828083149.45760-1-jiahao.kernel@gmail.com
> 
> 
>> Does this early return prevent marking valid boot-time pages as idle?
>> When a page is accessed during the first second of system boot, its ac_time
>> would be 0.
> 
> There is no possibility for zram to hold pages during first second of
> system boot, regardless of whether zram was configured as a swap device
> or as a block device (mount-ed with real filesystem).
> 
> 
>> Can the early return above bypass this zram device initialization check?
> 
> We already do that, e.g. when kstrtouint(buf, 0, &age_sec) fails.  Apart
> from that, that's not how one checks if device was initialized.  We may
> want to consolidate those checks, just for symmetry.

Agreed on both points -- the early return is not a new "bypass", and
the write() return value was never a way to probe init state.

How about moving the init_done() check to the top, so all the early
returns sit behind the same device-state check?

static ssize_t idle_store(...)
{
    struct zram *zram = dev_to_zram(dev);
    time64_t cutoff = 0;

    guard(rwsem_read)(&zram->dev_lock);
    if (!init_done(zram))
            return -EINVAL;

    if (!sysfs_streq(buf, "all")) {
            ...
    }

    mark_idle(zram, cutoff);
    return len;
}

The read lock then also covers the parsing, but that is non-sleeping
and its cost is negligible.

Thanks,
Hao