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From: Milan Broz <gmazyland@gmail.com>
To: Christoph Hellwig <hch@infradead.org>,
	Linus Torvalds <torvalds@linux-foundation.org>
Cc: Mikulas Patocka <mpatocka@redhat.com>,
	Mike Snitzer <snitzer@redhat.com>,
	dm-devel@redhat.com, linux-block <linux-block@vger.kernel.org>,
	Alasdair G Kergon <agk@redhat.com>,
	AliOS system security <alios_sys_security@linux.alibaba.com>,
	Colin Ian King <colin.king@canonical.com>,
	Eric Biggers <ebiggers@google.com>,
	Heinz Mauelshagen <heinzm@redhat.com>,
	Jaegeuk Kim <jaegeuk@kernel.org>,
	Milan Broz <gmazyland@gmail.com>,
	Nikos Tsironis <ntsironis@arrikto.com>,
	Shenghui Wang <shhuiw@foxmail.com>,
	Sweet Tea <sweettea@redhat.com>,
	wuzhouhui <wuzhouhui14@mails.ucas.ac.cn>
Subject: Re: [git pull] device mapper changes for 4.21
Date: Thu, 3 Jan 2019 08:45:27 +0100	[thread overview]
Message-ID: <b527fb75-340e-c3ba-7cc7-97dd466e6780@gmail.com> (raw)
In-Reply-To: <20190103072754.GA2667@infradead.org>

On 03/01/2019 08:27, Christoph Hellwig wrote:
> On Sun, Dec 30, 2018 at 04:25:46PM -0800, Linus Torvalds wrote:
>> On Sun, Dec 30, 2018 at 11:15 AM Mikulas Patocka <mpatocka@redhat.com> wrote:
>>>
>>> But you're right that 2TiB devices are common and that perhaps this option
>>> should go away.
>>
>> 2TiB devices are definitely not common in the one situation where this
>> option might matter: small embedded devices.
>>
>> I don't think the cost of 64 bit is in the arithmetic, but it might be
>> in some of the data structures.
>>
>> But my gut feel is that it probably doesn't much matter, and we could
>> get rid of the config option without anybody ever noticing. I don't
>> think we have that many data structures with 'sector_t' in them.
>>
>> We might try to first just force the option on, and see if anybody even cares.
> 
> Our smallest embedded devices use raw flash using the MTD subsystem,
> and even that is using 64-bit size types everywhere.  So I'd be really
> surprised if it is an issue.

I agree with Christoph here.

(I fixed some CONFIG_LBDAF problems in DM in this pull request because
the code was apparently wrong, but it was a pain to see all these possible
sector overflow checks...)

If it helps anything, we require 64-bit calculations for cryptsetup userspace for >5 years
(you cannot compile it with 32bit support; everything uses 64bit, including these
DM table sector calculations for kernel) and NOBODY complained since we enforced it.
(Ok, it is not a hot path, but....)

Please, if possible, go with 64-bit sector size types by default in future.

Thanks,
Milan

WARNING: multiple messages have this Message-ID (diff)
From: Milan Broz <gmazyland@gmail.com>
To: Christoph Hellwig <hch@infradead.org>,
	Linus Torvalds <torvalds@linux-foundation.org>
Cc: Sweet Tea <sweettea@redhat.com>,
	wuzhouhui <wuzhouhui14@mails.ucas.ac.cn>,
	AliOS system security <alios_sys_security@linux.alibaba.com>,
	Nikos Tsironis <ntsironis@arrikto.com>,
	Mike Snitzer <snitzer@redhat.com>,
	Eric Biggers <ebiggers@google.com>,
	Heinz Mauelshagen <heinzm@redhat.com>,
	Shenghui Wang <shhuiw@foxmail.com>,
	linux-block <linux-block@vger.kernel.org>,
	dm-devel@redhat.com, Mikulas Patocka <mpatocka@redhat.com>,
	Jaegeuk Kim <jaegeuk@kernel.org>,
	Colin Ian King <colin.king@canonical.com>,
	Milan Broz <gmazyland@gmail.com>,
	Alasdair G Kergon <agk@redhat.com>
Subject: Re: [git pull] device mapper changes for 4.21
Date: Thu, 3 Jan 2019 08:45:27 +0100	[thread overview]
Message-ID: <b527fb75-340e-c3ba-7cc7-97dd466e6780@gmail.com> (raw)
In-Reply-To: <20190103072754.GA2667@infradead.org>

On 03/01/2019 08:27, Christoph Hellwig wrote:
> On Sun, Dec 30, 2018 at 04:25:46PM -0800, Linus Torvalds wrote:
>> On Sun, Dec 30, 2018 at 11:15 AM Mikulas Patocka <mpatocka@redhat.com> wrote:
>>>
>>> But you're right that 2TiB devices are common and that perhaps this option
>>> should go away.
>>
>> 2TiB devices are definitely not common in the one situation where this
>> option might matter: small embedded devices.
>>
>> I don't think the cost of 64 bit is in the arithmetic, but it might be
>> in some of the data structures.
>>
>> But my gut feel is that it probably doesn't much matter, and we could
>> get rid of the config option without anybody ever noticing. I don't
>> think we have that many data structures with 'sector_t' in them.
>>
>> We might try to first just force the option on, and see if anybody even cares.
> 
> Our smallest embedded devices use raw flash using the MTD subsystem,
> and even that is using 64-bit size types everywhere.  So I'd be really
> surprised if it is an issue.

I agree with Christoph here.

(I fixed some CONFIG_LBDAF problems in DM in this pull request because
the code was apparently wrong, but it was a pain to see all these possible
sector overflow checks...)

If it helps anything, we require 64-bit calculations for cryptsetup userspace for >5 years
(you cannot compile it with 32bit support; everything uses 64bit, including these
DM table sector calculations for kernel) and NOBODY complained since we enforced it.
(Ok, it is not a hot path, but....)

Please, if possible, go with 64-bit sector size types by default in future.

Thanks,
Milan

  reply	other threads:[~2019-01-03  7:45 UTC|newest]

Thread overview: 18+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2018-12-27 16:09 [git pull] device mapper changes for 4.21 Mike Snitzer
2018-12-27 16:09 ` Mike Snitzer
2018-12-29  1:30 ` pr-tracker-bot
2018-12-29  1:30   ` pr-tracker-bot
2018-12-30  9:06 ` Christoph Hellwig
2018-12-30  9:06   ` Christoph Hellwig
2018-12-30 19:15   ` Mikulas Patocka
2018-12-30 19:15     ` Mikulas Patocka
2018-12-31  0:25     ` Linus Torvalds
2018-12-31  0:25       ` Linus Torvalds
2019-01-03  7:27       ` Christoph Hellwig
2019-01-03  7:27         ` Christoph Hellwig
2019-01-03  7:45         ` Milan Broz [this message]
2019-01-03  7:45           ` Milan Broz
2018-12-30 19:40   ` James Bottomley
2018-12-30 19:40     ` James Bottomley
2018-12-31 20:10   ` Jens Axboe
2018-12-31 20:10     ` Jens Axboe

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