On 12/6/20 3:14 PM, Tom Yan wrote:
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)?
But these pages are ephemeral, and never visible to the user.
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.
Cheers,
Hannes
--
Dr. Hannes Reinecke Kernel Storage Architect
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