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SubjectRe: [PATCH] lightnvm: pblk: extend line wp balance check
Date
> On 29 Jan 2019, at 16.49, Hans Holmberg <hans@owltronix.com> wrote:
>
> On Tue, Jan 29, 2019 at 4:03 PM Javier González <javier@javigon.com> wrote:
>>> On 29 Jan 2019, at 13.49, Hans Holmberg <hans@owltronix.com> wrote:
>>>
>>> On Tue, Jan 29, 2019 at 12:19 PM Javier González <javier@javigon.com> wrote:
>>>>> On 29 Jan 2019, at 09.47, hans@owltronix.com wrote:
>>>>>
>>>>> From: Hans Holmberg <hans.holmberg@cnexlabs.com>
>>>>>
>>>>> pblk stripes writes of minimal write size across all non-offline chunks
>>>>> in a line, which means that the maximum write pointer delta should not
>>>>> exceed the minimal write size. Extend the line write pointer balance check
>>>>> to cover this case.
>>>>>
>>>>> Signed-off-by: Hans Holmberg <hans.holmberg@cnexlabs.com>
>>>>> ---
>>>>>
>>>>> This patch applies on top of Zhoujie's V3 of
>>>>> "lightnvm: pblk: ignore bad block wp for pblk_line_wp_is_unbalanced
>>>>>
>>>>> drivers/lightnvm/pblk-recovery.c | 60 ++++++++++++++++++++------------
>>>>> 1 file changed, 37 insertions(+), 23 deletions(-)
>>>>>
>>>>> diff --git a/drivers/lightnvm/pblk-recovery.c b/drivers/lightnvm/pblk-recovery.c
>>>>> index 02d466e6925e..d86f580036d3 100644
>>>>> --- a/drivers/lightnvm/pblk-recovery.c
>>>>> +++ b/drivers/lightnvm/pblk-recovery.c
>>>>> @@ -302,41 +302,55 @@ static int pblk_pad_distance(struct pblk *pblk, struct pblk_line *line)
>>>>> return (distance > line->left_msecs) ? line->left_msecs : distance;
>>>>> }
>>>>>
>>>>> -static int pblk_line_wp_is_unbalanced(struct pblk *pblk,
>>>>> - struct pblk_line *line)
>>>>> +/* Return a chunk belonging to a line by stripe(write order) index */
>>>>> +static struct nvm_chk_meta *pblk_get_stripe_chunk(struct pblk *pblk,
>>>>> + struct pblk_line *line,
>>>>> + int index)
>>>>> {
>>>>> struct nvm_tgt_dev *dev = pblk->dev;
>>>>> struct nvm_geo *geo = &dev->geo;
>>>>> - struct pblk_line_meta *lm = &pblk->lm;
>>>>> struct pblk_lun *rlun;
>>>>> - struct nvm_chk_meta *chunk;
>>>>> struct ppa_addr ppa;
>>>>> - u64 line_wp;
>>>>> - int pos, i, bit;
>>>>> + int pos;
>>>>>
>>>>> - bit = find_first_zero_bit(line->blk_bitmap, lm->blk_per_line);
>>>>> - if (bit >= lm->blk_per_line)
>>>>> - return 0;
>>>>> - rlun = &pblk->luns[bit];
>>>>> + rlun = &pblk->luns[index];
>>>>> ppa = rlun->bppa;
>>>>> pos = pblk_ppa_to_pos(geo, ppa);
>>>>> - chunk = &line->chks[pos];
>>>>>
>>>>> - line_wp = chunk->wp;
>>>>> + return &line->chks[pos];
>>>>> +}
>>>>>
>>>>> - for (i = bit + 1; i < lm->blk_per_line; i++) {
>>>>> - rlun = &pblk->luns[i];
>>>>> - ppa = rlun->bppa;
>>>>> - pos = pblk_ppa_to_pos(geo, ppa);
>>>>> - chunk = &line->chks[pos];
>>>>> +static int pblk_line_wps_are_unbalanced(struct pblk *pblk,
>>>>> + struct pblk_line *line)
>>>>> +{
>>>>> + struct pblk_line_meta *lm = &pblk->lm;
>>>>> + int blk_in_line = lm->blk_per_line;
>>>>> + struct nvm_chk_meta *chunk;
>>>>> + u64 max_wp, min_wp;
>>>>> + int i;
>>>>>
>>>>> - if (chunk->state & NVM_CHK_ST_OFFLINE)
>>>>> - continue;
>>>>> + i = find_first_zero_bit(line->blk_bitmap, blk_in_line);
>>>>> +
>>>>> + /* If there is one or zero good chunks in the line,
>>>>> + * the write pointers can't be unbalanced.
>>>>> + */
>>>>> + if (i >= (blk_in_line - 1))
>>>>> + return 0;
>>>>>
>>>>> - if (chunk->wp > line_wp)
>>>>> + chunk = pblk_get_stripe_chunk(pblk, line, i);
>>>>> + max_wp = chunk->wp;
>>>>> + if (max_wp > pblk->max_write_pgs)
>>>>> + min_wp = max_wp - pblk->max_write_pgs;
>>>>> + else
>>>>> + min_wp = 0;
>>>>> +
>>>>> + i = find_next_zero_bit(line->blk_bitmap, blk_in_line, i + 1);
>>>>> + while (i < blk_in_line) {
>>>>> + chunk = pblk_get_stripe_chunk(pblk, line, i);
>>>>> + if (chunk->wp > max_wp || chunk->wp < min_wp)
>>>>> return 1;
>>>>> - else if (chunk->wp < line_wp)
>>>>> - line_wp = chunk->wp;
>>>>> +
>>>>> + i = find_next_zero_bit(line->blk_bitmap, blk_in_line, i + 1);
>>>>> }
>>>>>
>>>>> return 0;
>>>>> @@ -362,7 +376,7 @@ static int pblk_recov_scan_oob(struct pblk *pblk, struct pblk_line *line,
>>>>> int ret;
>>>>> u64 left_ppas = pblk_sec_in_open_line(pblk, line) - lm->smeta_sec;
>>>>>
>>>>> - if (pblk_line_wp_is_unbalanced(pblk, line))
>>>>> + if (pblk_line_wps_are_unbalanced(pblk, line))
>>>>> pblk_warn(pblk, "recovering unbalanced line (%d)\n", line->id);
>>>>>
>>>>> ppa_list = p.ppa_list;
>>>>> --
>>>>> 2.17.1
>>>>
>>>> If I am understanding correctly, you want to protect against the case
>>>> where a pfail has broken the ws_min partition of a chunk, right? I say
>>>> this because there is a guarantee that the minimal write size and pblk's
>>>> write size align with ws_min and ws_opt. Even if we have grown bad
>>>> blocks on pfail for the current line (which is a bigger problem because
>>>> we have potentially lost data), this guarantee would remain.
>>>>
>>>> If this is the case, my first reaction would be to say that the
>>>> controller is responsible for guaranteeing atomicity for both scalar and
>>>> vector I/Os. If this is not guaranteed, we have bigger problems than
>>>> this (e.g., for the write error recovery path).
>>>>
>>>> Are you thinking of a different case?
>>>
>>> The write pointer check triggers a warning if something unexpected has
>>> happened to the chunks.
>>> i.e. if something else than pblk messed with the disk, or if the user
>>> tries to recover a pblk instance with an invalid lun configuration.
>>
>> But this will only solve a very specific corner case of this, right?
>> This is, when you are writing at WP on a middle LUN exactly < ws_min.
>>> For example, If the user attempts to start pblk with a different LUN
>> configuration, the alignment is the same an pblk will actually fail
>> because it cannot find any emeta.
>
> Well, we will try to recover a line if the emeta data cannot be read
> or the data fails the crc check, and the recovery path assumes that
> the write pointers pointers have been incremented in the pblk stripe
> order. We'll get a warning now for all cases if this assumption is not
> valid.
>
>> For completion, the original wp_unbalanced patch attempted to protect
>> against the case where several outstanding I/Os to different PUs are
>> inflight and then you have a pfail. In this case, a write to a PU that
>> is not "next" in the line bitmap completes and we have a whole, meaning
>> that if we recover this case, we risk to overwrite valid data, as we
>> break the "sequentiality" the line, which allows for recovering by
>> replaying the P2L stored on the OOB.
>
> For outstanding writes, we can leave no guarantees anyway. If a
> presync was attached to a bio, that bio will complete when the write
> completes.
>
>>> This patch adds a warning if a chunk wp is too small(i.e. if a chunk
>>> was unexpectedly reset)
>>
>> I am not arguing against the implementation, I am just trying to
>> understand what you are trying to fix. If it is the case I described
>> originally, then I do not think it is possible on the Open-Channel
>> architecture. If you want to add protection against corruptions, then
>> this needs more work as many corruption cases are missing - you would
>> need something like a OOB watermark to protect open lines and something
>> like a OOB lba CRC check in emeta to validate that no data has been
>> altered.
>>
>> If you ask me, I do not think the latter belongs to pblk, and if it did,
>> I would suggest a whole new (optional) feature that adds this short of
>> integrity protection, ideally, reusing NVMe PI. In the original pblk, we
>> have followed the same assumption as block devices do; you can go an
>> write on the side to a block device that has a FS on top; the block
>> device will not complain at all - if the FS detects this then they will
>> try to fix it, otherwise you lost data.
>>
>> In this context, we have discussed about a pblk tool a la fsck, which can
>> cover all this cases instead of adding more complexity to the recovery
>> path in the kernel. Here, you patch makes sense we fail if something
>> suspicious has occurred and move the burden to the pblk tool for it to
>> do the repair. But again, if the goal s adding integrity protection, it
>> needs to cover the rest of the cases.
>>
>> Hope this makes sense.
>
> It does! I'm not trying to do anything more than warn if the write
> pointers are not what we expect them to be.
>
> You're right, we need some sort of superblock and some recovery tool
> to make pblk more robust against corruptions.
> Now, we get a warning at least :)

Sounds good! Now I understand.

It is good for me to apply the patch then.

Reviewed-by: Javier González <javier@javigon.com>


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