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

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On Sun, 6 Dec 2020 at 22:05, Hannes Reinecke <hare@xxxxxxx> 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)?

>
> 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().

>
> Cheers,
>
> Hannes
> --
> Dr. Hannes Reinecke                Kernel Storage Architect
> hare@xxxxxxx                              +49 911 74053 688
> SUSE Software Solutions GmbH, Maxfeldstr. 5, 90409 Nürnberg
> HRB 36809 (AG Nürnberg), Geschäftsführer: Felix Imendörffer




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