Re: [EXT] Re: [PATCH v1 1/2] mmc: Support kmsg dumper based on pstore/blk

[Ulf Hansson]

+ Christoph

On Tue, 24 Nov 2020 at 05:09, Bhaskara Budiredla <bbudiredla@xxxxxxxxxxx> wrote:
>
>
>
> >-----Original Message-----
> >From: Ulf Hansson <ulf.hansson@xxxxxxxxxx>
> >Sent: Monday, November 23, 2020 5:49 PM
> >To: Bhaskara Budiredla <bbudiredla@xxxxxxxxxxx>
> >Cc: Kees Cook <keescook@xxxxxxxxxxxx>; Colin Cross
> ><ccross@xxxxxxxxxxx>; Tony Luck <tony.luck@xxxxxxxxx>; Sunil Kovvuri
> >Goutham <sgoutham@xxxxxxxxxxx>; linux-mmc@xxxxxxxxxxxxxxx; Linux
> >Kernel Mailing List <linux-kernel@xxxxxxxxxxxxxxx>
> >Subject: Re: [EXT] Re: [PATCH v1 1/2] mmc: Support kmsg dumper based on
> >pstore/blk
> >
> >[...]
> >
> >> >
> >> >As I said above, I would like to avoid host specific deployments from
> >> >being needed. Is there a way we can avoid this?
> >> >
> >>
> >> I don't see an alternative.
> >
> >Well, if not, can you please explain why?
> >
>
> The solution has to be polling based as panic write runs with interrupts disabled.

Right, that's a good reason. :-) Please clarify that in the commit
message and add some information around the definition of the new host
ops callbacks should be used.

> I am not sure if there is a way to write a polling function that works of all kinds
> of host/dma drivers. That’s the reason I have provided hooks to define host
> specific deployments. If you have better ideas, please help.

No, at this point I don't, sorry. I will think more about it.

>
> >[...]
> >
> >> >> +
> >> >> +void mmcpstore_card_set(struct mmc_card *card, const char
> >> >> +*disk_name) {
> >> >> +       struct mmcpstore_context *cxt = &oops_cxt;
> >> >> +       struct pstore_blk_config *conf = &cxt->conf;
> >> >> +       struct pstore_device_info *dev = &cxt->dev;
> >> >> +       struct block_device *bdev;
> >> >> +       struct mmc_command *stop;
> >> >> +       struct mmc_command *cmd;
> >> >> +       struct mmc_request *mrq;
> >> >> +       struct mmc_data *data;
> >> >> +       int ret;
> >> >> +
> >> >> +       if (!conf->device[0])
> >> >> +               return;
> >> >> +
> >> >> +       /* Multiple backend devices not allowed */
> >> >> +       if (cxt->dev_name[0])
> >> >> +               return;
> >> >> +
> >> >> +       bdev =  mmcpstore_open_backend(conf->device);
> >> >> +       if (IS_ERR(bdev)) {
> >> >> +               pr_err("%s failed to open with %ld\n",
> >> >> +                               conf->device, PTR_ERR(bdev));
> >> >> +               return;
> >> >> +       }
> >> >> +
> >> >> +       bdevname(bdev, cxt->dev_name);
> >> >> +       cxt->partno = bdev->bd_part->partno;
> >> >> +       mmcpstore_close_backend(bdev);
> >> >> +
> >> >> +       if (strncmp(cxt->dev_name, disk_name, strlen(disk_name)))
> >> >> +               return;
> >> >> +
> >> >> +       cxt->start_sect = mmc_blk_get_part(card, cxt->partno, &cxt->size);
> >> >> +       if (!cxt->start_sect) {
> >> >> +               pr_err("Non-existent partition %d selected\n", cxt->partno);
> >> >> +               return;
> >> >> +       }
> >> >> +
> >> >> +       /* Check for host mmc panic write polling function definitions */
> >> >> +       if (!card->host->ops->req_cleanup_pending ||
> >> >> +                       !card->host->ops->req_completion_poll)
> >> >> +               return;
> >> >> +
> >> >> +       cxt->card = card;
> >> >> +
> >> >> +       cxt->sub = kmalloc(conf->kmsg_size, GFP_KERNEL);
> >> >> +       if (!cxt->sub)
> >> >> +               goto out;
> >> >> +
> >> >> +       mrq = kzalloc(sizeof(struct mmc_request), GFP_KERNEL);
> >> >> +       if (!mrq)
> >> >> +               goto free_sub;
> >> >> +
> >> >> +       cmd = kzalloc(sizeof(struct mmc_command), GFP_KERNEL);
> >> >> +       if (!cmd)
> >> >> +               goto free_mrq;
> >> >> +
> >> >> +       stop = kzalloc(sizeof(struct mmc_command), GFP_KERNEL);
> >> >> +       if (!stop)
> >> >> +               goto free_cmd;
> >> >> +
> >> >> +       data = kzalloc(sizeof(struct mmc_data), GFP_KERNEL);
> >> >> +       if (!data)
> >> >> +               goto free_stop;
> >> >> +
> >> >> +       mrq->cmd = cmd;
> >> >> +       mrq->data = data;
> >> >> +       mrq->stop = stop;
> >> >> +       cxt->mrq = mrq;
> >> >> +
> >> >> +       dev->total_size = cxt->size;
> >> >> +       dev->flags = PSTORE_FLAGS_DMESG;
> >> >> +       dev->read = mmcpstore_read;
> >> >> +       dev->write = mmcpstore_write;
> >> >> +       dev->erase = NULL;
> >> >> +       dev->panic_write = mmcpstore_panic_write;
> >> >> +
> >> >> +       ret = register_pstore_device(&cxt->dev);
> >> >
> >> >By looking at all of the code above, lots are duplicated from the mmc
> >> >block device implementation. Isn't there a way to make the pstore
> >> >block device to push a request through the regular blk-mq path instead?
> >> >
> >> The regular path has pre, post processing’s and locking semantics that
> >> are not suitable for panic write scenario. Further, the locking
> >> mechanisms are implemented in host drivers. This is preferred to
> >> quickly complete the write before the kernel dies.
> >
> >I am sorry, but this doesn't make sense to me.
>
> It seems there was some confusion. My comments were specific to
> mmcpstore_panic_write() as it runs with interrupts disabled.
> mmcpstore_read()/mmcpstore_write() indeed go through regular
> blk-mq path.
>
> >
> >When it comes to complete the data write, the regular block I/O path is
> >supposed to be optimized. If there is a problem with this path, then we should
> >fix it, rather than adding a new path along the side (unless there are very good
> >reasons not to).
> >
> >>
> >> >That said, I wonder why you don't call register_pstore_blk(), as I
> >> >thought that was the interface to be used for regular block devices, no?
> >> >
> >> register_pstore_blk() is for arbitrary block devices for which best effort is not
> >defined.
> >
> >Exactly why isn't "best effort" good enough for mmc?
> >
>
> register_pstore_blk() definitely does the work. If you prefer to take that route,
> it should be fine.

It looks like Christoph is planning for some rewrite of the pstore
code, so let's see what that means in regards to this.

[...]

Kind regards
Uffe