From: Jeff Moyer <jmoyer@redhat.com>
To: Jan Kara <jack@suse.cz>
Cc: linux-nvdimm@lists.01.org, linux-api@vger.kernel.org,
linux-xfs@vger.kernel.org, linux-mm@kvack.org,
linux-fsdevel@vger.kernel.org, linux-ext4@vger.kernel.org
Subject: Re: Problems with VM_MIXEDMAP removal from /proc/<pid>/smaps
Date: Wed, 17 Oct 2018 16:23:50 -0400 [thread overview]
Message-ID: <x49h8hkfhk9.fsf@segfault.boston.devel.redhat.com> (raw)
In-Reply-To: <20181002142959.GD9127@quack2.suse.cz> (Jan Kara's message of "Tue, 2 Oct 2018 16:29:59 +0200")
Jan Kara <jack@suse.cz> writes:
> [Added ext4, xfs, and linux-api folks to CC for the interface discussion]
>
> On Tue 02-10-18 14:10:39, Johannes Thumshirn wrote:
>> On Tue, Oct 02, 2018 at 12:05:31PM +0200, Jan Kara wrote:
>> > Hello,
>> >
>> > commit e1fb4a086495 "dax: remove VM_MIXEDMAP for fsdax and device dax" has
>> > removed VM_MIXEDMAP flag from DAX VMAs. Now our testing shows that in the
>> > mean time certain customer of ours started poking into /proc/<pid>/smaps
>> > and looks at VMA flags there and if VM_MIXEDMAP is missing among the VMA
>> > flags, the application just fails to start complaining that DAX support is
>> > missing in the kernel. The question now is how do we go about this?
>>
>> OK naive question from me, how do we want an application to be able to
>> check if it is running on a DAX mapping?
>
> The question from me is: Should application really care? After all DAX is
> just a caching decision. Sure it affects performance characteristics and
> memory usage of the kernel but it is not a correctness issue (in particular
> we took care for MAP_SYNC to return EOPNOTSUPP if the feature cannot be
> supported for current mapping). And in the future the details of what we do
> with DAX mapping can change - e.g. I could imagine we might decide to cache
> writes in DRAM but do direct PMEM access on reads. And all this could be
> auto-tuned based on media properties. And we don't want to tie our hands by
> specifying too narrowly how the kernel is going to behave.
For read and write, I would expect the O_DIRECT open flag to still work,
even for dax-capable persistent memory. Is that a contentious opinion?
So, what we're really discussing is the behavior for mmap. MAP_SYNC
will certainly ensure that the page cache is not used for writes. It
would also be odd for us to decide to cache reads. The only issue I can
see is that perhaps the application doesn't want to take a performance
hit on write faults. I haven't heard that concern expressed in this
thread, though.
Just to be clear, this is my understanding of the world:
MAP_SYNC
- file system guarantees that metadata required to reach faulted-in file
data is consistent on media before a write fault is completed. A
side-effect is that the page cache will not be used for
writably-mapped pages.
and what I think Dan had proposed:
mmap flag, MAP_DIRECT
- file system guarantees that page cache will not be used to front storage.
storage MUST be directly addressable. This *almost* implies MAP_SYNC.
The subtle difference is that a write fault /may/ not result in metadata
being written back to media.
and this is what I think you were proposing, Jan:
madvise flag, MADV_DIRECT_ACCESS
- same semantics as MAP_DIRECT, but specified via the madvise system call
Cheers,
Jeff
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next prev parent reply other threads:[~2018-10-17 20:23 UTC|newest]
Thread overview: 45+ messages / expand[flat|nested] mbox.gz Atom feed top
2018-10-02 10:05 Problems with VM_MIXEDMAP removal from /proc/<pid>/smaps Jan Kara
2018-10-02 10:50 ` Michal Hocko
2018-10-02 13:32 ` Jan Kara
2018-10-02 12:10 ` Johannes Thumshirn
2018-10-02 14:20 ` Johannes Thumshirn
2018-10-02 14:45 ` Christoph Hellwig
2018-10-02 15:01 ` Johannes Thumshirn
2018-10-02 15:06 ` Christoph Hellwig
2018-10-04 10:09 ` Johannes Thumshirn
2018-10-05 6:25 ` Christoph Hellwig
2018-10-05 6:35 ` Johannes Thumshirn
2018-10-06 1:17 ` Dan Williams
2018-10-14 15:47 ` Dan Williams
2018-10-17 20:01 ` Dan Williams
2018-10-18 17:43 ` Jan Kara
2018-10-18 19:10 ` Dan Williams
2018-10-19 3:01 ` Dave Chinner
2018-10-02 14:29 ` Jan Kara
2018-10-02 14:37 ` Christoph Hellwig
2018-10-02 14:44 ` Johannes Thumshirn
2018-10-02 14:52 ` Christoph Hellwig
2018-10-02 15:31 ` Jan Kara
2018-10-02 20:18 ` Dan Williams
2018-10-03 12:50 ` Jan Kara
2018-10-03 14:38 ` Dan Williams
2018-10-03 15:06 ` Jan Kara
2018-10-03 15:13 ` Dan Williams
2018-10-03 16:44 ` Jan Kara
2018-10-03 21:13 ` Dan Williams
2018-10-04 10:04 ` Johannes Thumshirn
2018-10-02 15:07 ` Jan Kara
2018-10-17 20:23 ` Jeff Moyer [this message]
2018-10-18 0:25 ` Dave Chinner
2018-10-18 14:55 ` Jan Kara
2018-10-19 0:43 ` Dave Chinner
2018-10-30 6:30 ` Dan Williams
2018-10-30 22:49 ` Dave Chinner
2018-10-30 22:59 ` Dan Williams
2018-10-31 5:59 ` y-goto
2018-11-01 23:00 ` Dave Chinner
2018-11-02 1:43 ` y-goto
2018-10-18 21:05 ` Jeff Moyer
2018-10-09 19:43 ` Jeff Moyer
2018-10-16 8:25 ` Jan Kara
2018-10-16 12:35 ` Jeff Moyer
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