Re: [PATCH v5 3/8] nbd: reset write cache on disconnect

Hi,

=E5=9C=A8 2026/8/2 19:51, yu kuai =E5=86=99=E9=81=93:
> Hi,
>
> =E5=9C=A8 2026/7/30 16:20, Yang Erkun =E5=86=99=E9=81=93:
>> An inactive nbd device may refuse any I/O operations. The nbd_config_put
>> function calls invalidate_disk, which sets the device capacity to zero
>> to reject all read and write I/O. For zero-sector flush I/O requests
>> from blkdev_issue_flush, if the write cache is disabled, the zero-sector
>> flush I/O immediately returns 0 in submit_bio_noacct. However, since
>> nbd_config_put does not clear the write cache state, an inactive nbd
>> device might still have the write cache enabled. In this situation,
>> zero-sector flush I/O will return -EIO because there is no active socket=
.
>> Although this does not cause any bugs, resetting the write cache state
>> would make the behavior consistent.
> I don't think this make sense, at this point nbd->config_refs is 0, which=
 means
> there is no opener for the device, so how can there still be outstanding =
read/write
> or flush IO?

I see what you want to do after checking the next patch. However, since a d=
isconnected
nbd device can be reused later, the stale BLK_FEAT flags from the last conn=
ection looks
like a real problem. So, I think it's better to reset queue_limits to defau=
lt, the same
as add_disk().

>
>> Signed-off-by: Yang Erkun <[email protected]>
>> ---
>>    drivers/block/nbd.c | 5 +++++
>>    1 file changed, 5 insertions(+)
>>
>> diff --git a/drivers/block/nbd.c b/drivers/block/nbd.c
>> index 7ec85f94f742..da4b48ef3c79 100644
>> --- a/drivers/block/nbd.c
>> +++ b/drivers/block/nbd.c
>> @@ -1469,8 +1469,13 @@ static void nbd_config_put(struct nbd_device *nbd=
)
>>    	if (refcount_dec_and_mutex_lock(&nbd->config_refs,
>>    					&nbd->config_lock)) {
>>    		struct nbd_config *config =3D nbd->config;
>> +		struct queue_limits lim;
>>    		nbd_dev_dbg_close(nbd);
>>    		invalidate_disk(nbd->disk);
>> +		/* reset write cache */
>> +		lim =3D queue_limits_start_update(nbd->disk->queue);
>> +		lim.features &=3D ~(BLK_FEAT_WRITE_CACHE | BLK_FEAT_FUA);
>> +		queue_limits_commit_update(nbd->disk->queue, &lim);
>>    		if (nbd->config->bytesize)
>>    			kobject_uevent(&nbd_to_dev(nbd)->kobj, KOBJ_CHANGE);
>>    		if (test_and_clear_bit(NBD_RT_HAS_PID_FILE,

--=20
Thanks,
Kuai