Re: [PATCH v1 11/11] mtd: new support oops logger based on pstore/blk

Miquel Raynal
2020-01-22T17:41:51Z

Hello,

> >>>> +/*
> >>>> + * All zones will be read as pstore/blk will read zone one by one when do
> >>>> + * recover.
> >>>> + */
> >>>> +static ssize_t mtdpstore_read(char *buf, size_t size, loff_t off)
> >>>> +{
> >>>> + struct mtdpstore_context *cxt = &oops_cxt;
> >>>> + size_t retlen;
> >>>> + int ret;
> >>>> +
> >>>> + if (mtdpstore_block_isbad(cxt, off))
> >>>> + return -ENEXT;
> >>>> +
> >>>> + pr_debug("try to read off 0x%llx size %zu\n", off, size);
> >>>> + ret = mtd_read(cxt->mtd, off, size, &retlen, (u_char *)buf);
> >>>> + if ((ret < 0 && !mtd_is_bitflip(ret)) || size != retlen) {
> >>>
> >>> IIRC size != retlen does not mean it failed, but that you should
> >>> continue reading after retlen bytes, no?
> >>> >>
> >> Yes, you are right. I will fix it. Thanks.
> >>
> >>> Also, mtd_is_bitflip() does not mean that you are reading a false
> >>> buffer, but that the data has been corrected as it contained bitflips.
> >>> mtd_is_eccerr() however, would be meaningful.
> >>> >>
> >> Sure I know mtd_is_bitflip() does not mean failure, but I do not think
> >> mtd_is_eccerr() should be here since the codes are ret < 0 and NOT
> >> mtd_is_bitflip().
> >
> > Yes, just drop this check, only keep ret < 0.
> >
>
> If I don't get it wrong, it should not be dropped here. Like your words,
> "mtd_is_bitflip() does not mean that you are reading a false buffer,
> but that the data has been corrected as it contained bitflips.", the
> data I get are valid even if mtd_is_bitflip() return true. It's correct
> data and it's no need to go to handle error. To me, the codes
> should be:
> if (ret < 0 && !mit_is_bitflip())
> [error handling]

Please check the implementation of mtd_is_bitflip(). You'll probably
figure out what I am saying.

https://elixir.bootlin.com/linux/latest/source/include/linux/mtd/mtd.h#L585

|...]

> >>>> + return;
> >>>> + }
> >>>> + if (unlikely(info->dmesg_size % mtd->writesize)) {
> >>>> + pr_err("record size %lu KB must align to write size %d KB\n",
> >>>> + info->dmesg_size / 1024,
> >>>> + mtd->writesize / 1024);
> >>>
> >>> This condition is weird, why would you check this?
> >>> >>
> >> pstore/blk will write 'record_size' dmesg log at one time.
> >> Since each write data must be aligned to 'writesize' for flash, I am not
> >> sure
> >> all flash drivers are compatible with misaligned data, that's why i
> >> check this.
> >
> > I think you should enforce this alignment instead of checking it.
> >
>
> Do you mean that mtdpstore should enforce this alignment while running?
> The way I can think of is to handle a buffer aligned to writesize and
> write to flash with this aligned buffer.
>
> That causes some error. The MTD device will be divided into mutil
> chunks accroding to dmesg_size. Each chunk stores a individual
> OOPS/Panic log. If dmesg_size is misaligned to writesize, the last
> write results in next write failure because the page of flash can only
> be programed once before next erase and the page shared by two chunks
> has been used by the last write. Besides, we can not read to buffer,
> ersae and write back as there is no read/erase for panic case.

I mean: what is the usual size of dmesg? I don't get why you need it to
be ie. a multiple of 2k. It probably is actually, I don't know if there
is a standard. But if dmesg_size is eg 3k, just skip the end of the
partially written page and start writing at the next page?

>
> >>
> >>>> + return;
> >>>> + }
> >>>> + if (unlikely(mtd->size > MTDPSTORE_MAX_MTD_SIZE)) {
> >>>> + pr_err("mtd%d is too large (limit is %d MiB)\n",
> >>>> + mtd->index,
> >>>> + MTDPSTORE_MAX_MTD_SIZE / 1024 / 1024);
> >>>
> >>> Same question? I could understand that it is easier to manage blocks
> >>> knowing their maximum number though.
> >>> >>
> >> It refers to mtdoops.
> >
> > What do you mean?
> >
>
> To me, it's unnecessary to check at all, however it is really there
> on codes of mtdoops. I refer to module mtdoops when I design mtdpstore.
> It may be helpfull for some cases out of my think, that's why I keep it.

Why not.

[...]

> >>
> >> In case of repeated erase when users remove several log files, mtdpstore
> >> do remove jobs when exit.
> >>
> >> Besides, mtdpstore do not check the return code to ensure write back valid
> >> log as much as possible.
> >
> > You are not in a critical path, I don't understand why you don't check
> > it? If it returns an error, it means the data is not written. IMHO it
> > is best to alert the user than to silently fail.
> >
>
> This function will be called only when mtd device is removing. It's
> useless to alert the user but try to flush the other valid data to

It is useful to alert the user! It means something went wrong while
everything seems fine.

> flash as mush as possible by which reduce losses. If it's just
> because of busy, what happens next time?

Just because of busy? I don't get it.

I'm okay with the idea of trying to write the other chunks though:

while (remaining_chunk) {
ret = mtd_write()
if (ret) {
alert-user;
continue;
}
}

>
> >>
> >>>> +. >>>> + off += zonesize;
> >>>> + size -= min_t(unsigned int, zonesize, size);
> >>>> + }
> >>>> +
> >>>> +free:
> >>>> + kfree(buf);
> >>>> + return ret;
> >>>> +}
> >>>> +
> >
> >
> > [...]
> >
> >>>
> >>> Thanks,
> >>> Miquèl
> >>> >>
> >> I will collect more suggestions and submit the new version at one time.
> >>
> >
> > Sure, no hurry.
> >
>
> I am on holiday, please forgive me for my slow response.

Take your time, as I said, no hurry.

>
> >
> > Thanks,
> > Miquèl
> >

Thanks,
Miquèl