linux-kernel - Re: [PATCH 0/2] "big hammer" for DAX msync/fsync correctness

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Message-ID: <6BAC06FA-C855-4897-95C2-02BCA01C28A6@zytor.com>
Date:	Fri, 06 Nov 2015 12:25:15 -0800
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Dan Williams <dan.j.williams@...el.com>,
	Ross Zwisler <ross.zwisler@...ux.intel.com>
CC:	Jeff Moyer <jmoyer@...hat.com>,
	linux-nvdimm <linux-nvdimm@...1.01.org>, X86 ML <x86@...nel.org>,
	Dave Chinner <david@...morbit.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Ingo Molnar <mingo@...hat.com>,
	Thomas Gleixner <tglx@...utronix.de>, Jan Kara <jack@...e.com>
Subject: Re: [PATCH 0/2] "big hammer" for DAX msync/fsync correctness

On November 5, 2015 3:59:46 PM PST, Dan Williams <dan.j.williams@...el.com> wrote:
>On Wed, Oct 28, 2015 at 3:51 PM, Ross Zwisler
><ross.zwisler@...ux.intel.com> wrote:
>> On Wed, Oct 28, 2015 at 06:24:29PM -0400, Jeff Moyer wrote:
>>> Ross Zwisler <ross.zwisler@...ux.intel.com> writes:
>>>
>>> > This series implements the very slow but correct handling for
>>> > blkdev_issue_flush() with DAX mappings, as discussed here:
>>> >
>>> > https://lkml.org/lkml/2015/10/26/116
>>> >
>>> > I don't think that we can actually do the
>>> >
>>> >     on_each_cpu(sync_cache, ...);
>>> >
>>> > ...where sync_cache is something like:
>>> >
>>> >     cache_disable();
>>> >     wbinvd();
>>> >     pcommit();
>>> >     cache_enable();
>>> >
>>> > solution as proposed by Dan because WBINVD + PCOMMIT doesn't
>guarantee that
>>> > your writes actually make it durably onto the DIMMs.  I believe
>you really do
>>> > need to loop through the cache lines, flush them with CLWB, then
>fence and
>>> > PCOMMIT.
>>>
>>> *blink*
>>> *blink*
>>>
>>> So much for not violating the principal of least surprise.  I
>suppose
>>> you've asked the hardware folks, and they've sent you down this
>path?
>>
>> Sadly, yes, this was the guidance from the hardware folks.
>
>So it turns out we weren't asking the right question.  wbinvd may
>indeed be viable... we're still working through the caveats.

Do not disable the caches here.
-- 
Sent from my Android device with K-9 Mail. Please excuse my brevity.
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