linux-kernel - RE: [External] Re: [RFC PATCH v1 0/6] use mm to manage NVDIMM (pmem) zone

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Date:   Wed, 16 May 2018 02:05:05 +0000
From:   Huaisheng HS1 Ye <yehs1@...ovo.com>
To:     Matthew Wilcox <willy@...radead.org>
CC:     Jeff Moyer <jmoyer@...hat.com>,
        Dan Williams <dan.j.williams@...el.com>,
        Michal Hocko <mhocko@...e.com>,
        linux-nvdimm <linux-nvdimm@...ts.01.org>,
        Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>,
        NingTing Cheng <chengnt@...ovo.com>,
        Dave Hansen <dave.hansen@...el.com>,
        "Linux Kernel Mailing List" <linux-kernel@...r.kernel.org>,
        "pasha.tatashin@...cle.com" <pasha.tatashin@...cle.com>,
        Linux MM <linux-mm@...ck.org>,
        "colyli@...e.de" <colyli@...e.de>,
        Johannes Weiner <hannes@...xchg.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Sasha Levin <alexander.levin@...izon.com>,
        "Mel Gorman" <mgorman@...hsingularity.net>,
        Vlastimil Babka <vbabka@...e.cz>,
        "Ocean HY1 He" <hehy1@...ovo.com>
Subject: RE: [External]  Re: [RFC PATCH v1 0/6] use mm to manage NVDIMM (pmem)
 zone

> From: Matthew Wilcox [mailto:willy@...radead.org]
> Sent: Wednesday, May 16, 2018 12:20 AM> 
> > > > > Then there's the problem of reconnecting the page cache (which is
> > > > > pointed to by ephemeral data structures like inodes and dentries) to
> > > > > the new inodes.
> > > > Yes, it is not easy.
> > >
> > > Right ... and until we have that ability, there's no point in this patch.
> > We are focusing to realize this ability.
> 
> But is it the right approach?  So far we have (I think) two parallel
> activities.  The first is for local storage, using DAX to store files
> directly on the pmem.  The second is a physical block cache for network
> filesystems (both NAS and SAN).  You seem to be wanting to supplant the
> second effort, but I think it's much harder to reconnect the logical cache
> (ie the page cache) than it is the physical cache (ie the block cache).

Dear Matthew,

Thanks for correcting my idea with cache line.
But I have questions about that, assuming NVDIMM works with pmem mode, even we
used it as physical block cache, like dm-cache, there is potential risk with
this cache line issue, because NVDIMMs are bytes-address storage, right?
If system crash happens, that means CPU doesn't have opportunity to flush all dirty
data from cache lines to NVDIMM, during copying data pointed by bio_vec.bv_page to
NVDIMM. 
I know there is btt which is used to guarantee sector atomic with block mode,
but for pmem mode that will likely cause mix of new and old data in one page
of NVDIMM.
Correct me if anything wrong.

Another question, if we used NVDIMMs as physical block cache for network filesystems,
Does industry have existing implementation to bypass Page Cache similarly like DAX way,
that is to say, directly storing data to NVDIMMs from userspace, rather than copying
data from kernel space memory to NVDIMMs.

BRs,
Huaisheng