Re: [PATCH 3/3] block: set REQ_PREFLUSH to the final bio from __blkdev_issue_zero_pages()

From: Tom Yan <tom.ty89@gmail.com>
To: Hannes Reinecke <hare@suse.de>
Cc: linux-block@vger.kernel.org, linux-scsi@vger.kernel.org
Subject: Re: [PATCH 3/3] block: set REQ_PREFLUSH to the final bio from __blkdev_issue_zero_pages()
Date: Tue, 8 Dec 2020 20:51:30 +0800	[thread overview]
Message-ID: <CAGnHSE=EXK9H_OzNdBEwQQuQgamCpSSQW76jTLb1ZCzq8ksLnA@mail.gmail.com> (raw)
In-Reply-To: <b18fa5df-2a18-e29c-911b-483dcb451f06@suse.de>

On Mon, 7 Dec 2020 at 00:05, Hannes Reinecke <hare@suse.de> wrote:
>
> On 12/6/20 3:14 PM, Tom Yan wrote:
> > On Sun, 6 Dec 2020 at 22:05, Hannes Reinecke <hare@suse.de> wrote:
> >>
> >> On 12/6/20 2:32 PM, Tom Yan wrote:
> >>> Why? Did you miss that it is in the condition where
> >>> __blkdev_issue_zero_pages() is called (i.e. it's not WRITE SAME but
> >>> WRITE). From what I gathered REQ_PREFLUSH triggers a write back cache
> >>> (that is on the device; not sure about dirty pages) flush, wouldn't it
> >>> be a right thing to do after we performed a series of WRITE (which is
> >>> more or less purposed to get a drive wiped clean).
> >>>
> >>
> >> But what makes 'zero_pages' special as compared to, say, WRITE_SAME?
> >> One could use WRITE SAME with '0' content, arriving at pretty much the
> >> same content than usine zeroout without unmapping. And neither of them
> >> worries about cache flushing.
> >> Nor should they, IMO.
> >
> > Because we are writing actual pages (just that they are zero and
> > "shared memory" in the system) to the device, instead of triggering a
> > special command (with a specific parameter)?
> >
>
> But these pages are ephemeral, and never visible to the user.

What do you mean by the "user"? What I meant was, since it's no
different than "normal" write operation, the zero pages will go to the
volatile write cache of the device.

>
> >>
> >> These are 'native' block layer calls, providing abstract accesses to
> >> hardware functionality. If an application wants to use them, it would be
> >> the task of the application to insert a 'flush' if it deems neccessary.
> >> (There _is_ blkdev_issue_flush(), after all).
> >
> > Well my argument would be the call has the purpose of "wiping" so it
> > should try to "atomically" guarantee that the wiping is synced. It's
> > like a complement to REQ_SYNC in the final submit_bio_wait().
> >
> That's an assumption.
>
> It would be valid if blkdev_issue_zeroout() would only allow to wipe the
> entire disk. As it stands, it doesn't, and so we shouldn't presume what
> users might want to do with it.

Whether it's an entire disk doesn't matter. It still stands when it's
only a certain range of blocks.

>
> Cheers,
>
> Hannes
> --
> Dr. Hannes Reinecke                Kernel Storage Architect
> hare@suse.de                              +49 911 74053 688
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