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

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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.

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).

Cheers,

Hannes
--
Dr. Hannes Reinecke                Kernel Storage Architect
hare@xxxxxxx                              +49 911 74053 688
SUSE Software Solutions GmbH, Maxfeldstr. 5, 90409 Nürnberg
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