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From: Ira Weiny <ira.weiny@intel.com>
To: Matthew Wilcox <willy@infradead.org>
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Subject: Re: [PATCH RFC PKS/PMEM 22/58] fs/f2fs: Utilize new kmap_thread()
Date: Mon, 12 Oct 2020 16:31:26 -0700
Message-ID: <20201012233126.GD2046448@iweiny-DESK2.sc.intel.com> (raw)
In-Reply-To: <20201012200254.GB20115@casper.infradead.org>

On Mon, Oct 12, 2020 at 09:02:54PM +0100, Matthew Wilcox wrote:
> On Mon, Oct 12, 2020 at 12:53:54PM -0700, Ira Weiny wrote:
> > On Mon, Oct 12, 2020 at 05:44:38PM +0100, Matthew Wilcox wrote:
> > > On Mon, Oct 12, 2020 at 09:28:29AM -0700, Dave Hansen wrote:
> > > > kmap_atomic() is always preferred over kmap()/kmap_thread().
> > > > kmap_atomic() is _much_ more lightweight since its TLB invalidation is
> > > > always CPU-local and never broadcast.
> > > > 
> > > > So, basically, unless you *must* sleep while the mapping is in place,
> > > > kmap_atomic() is preferred.
> > > 
> > > But kmap_atomic() disables preemption, so the _ideal_ interface would map
> > > it only locally, then on preemption make it global.  I don't even know
> > > if that _can_ be done.  But this email makes it seem like kmap_atomic()
> > > has no downsides.
> > 
> > And that is IIUC what Thomas was trying to solve.
> > 
> > Also, Linus brought up that kmap_atomic() has quirks in nesting.[1]
> > 
> > >From what I can see all of these discussions support the need to have something
> > between kmap() and kmap_atomic().
> > 
> > However, the reason behind converting call sites to kmap_thread() are different
> > between Thomas' patch set and mine.  Both require more kmap granularity.
> > However, they do so with different reasons and underlying implementations but
> > with the _same_ resulting semantics; a thread local mapping which is
> > preemptable.[2]  Therefore they each focus on changing different call sites.
> > 
> > While this patch set is huge I think it serves a valuable purpose to identify a
> > large number of call sites which are candidates for this new semantic.
> 
> Yes, I agree.  My problem with this patch-set is that it ties it to
> some Intel feature that almost nobody cares about.

I humbly disagree.  At this level the only thing this is tied to is the idea
that there are additional memory protections available which can be enabled
quickly on a per-thread basis.  PKS on Intel is but 1 implementation of that.

Even the kmap code only has knowledge that there is something which needs to be
done special on a devm page.

>
> Maybe we should
> care about it, but you didn't try very hard to make anyone care about
> it in the cover letter.

Ok my bad.  We have customers who care very much about restricting access to
the PMEM pages to prevent bugs in the kernel from causing permanent damage to
their data/file systems.  I'll reword the cover letter better.

> 
> For a future patch-set, I'd like to see you just introduce the new
> API.  Then you can optimise the Intel implementation of it afterwards.
> Those patch-sets have entirely different reviewers.

I considered doing this.  But this seemed more logical because the feature is
being driven by PMEM which is behind the kmap interface not by the users of the
API.

I can introduce a patch set with a kmap_thread() call which does nothing if
that is more palatable but it seems wrong to me to do so.

Ira

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Thread overview: 93+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-10-09 19:49 [PATCH RFC PKS/PMEM 00/58] PMEM: Introduce stray write protection for PMEM ira.weiny
2020-10-09 19:49 ` [PATCH RFC PKS/PMEM 01/58] x86/pks: Add a global pkrs option ira.weiny
2020-10-09 19:49 ` [PATCH RFC PKS/PMEM 02/58] x86/pks/test: Add testing for global option ira.weiny
2020-10-09 19:49 ` [PATCH RFC PKS/PMEM 03/58] memremap: Add zone device access protection ira.weiny
2020-10-09 19:49 ` [PATCH RFC PKS/PMEM 04/58] kmap: Add stray access protection for device pages ira.weiny
2020-10-09 19:49 ` [PATCH RFC PKS/PMEM 05/58] kmap: Introduce k[un]map_thread ira.weiny
2020-11-10  1:13   ` Thomas Gleixner
2020-11-10  4:59     ` Ira Weiny
2020-11-10  8:48       ` Thomas Gleixner
2020-10-09 19:49 ` [PATCH RFC PKS/PMEM 06/58] kmap: Introduce k[un]map_thread debugging ira.weiny
2020-10-09 19:49 ` [PATCH RFC PKS/PMEM 07/58] drivers/drbd: Utilize new kmap_thread() ira.weiny
2020-10-09 19:49 ` [PATCH RFC PKS/PMEM 08/58] drivers/firmware_loader: " ira.weiny
2020-10-09 19:49 ` [PATCH RFC PKS/PMEM 09/58] drivers/gpu: " ira.weiny
2020-10-09 22:03   ` Daniel Vetter
2020-10-10 23:01     ` Ira Weiny
2020-10-09 19:49 ` [PATCH RFC PKS/PMEM 10/58] drivers/rdma: " ira.weiny
2020-10-09 19:49 ` [PATCH RFC PKS/PMEM 11/58] drivers/net: " ira.weiny
2020-10-09 19:49 ` [PATCH RFC PKS/PMEM 12/58] fs/afs: " ira.weiny
2020-10-09 19:49 ` [PATCH RFC PKS/PMEM 13/58] fs/btrfs: " ira.weiny
2020-10-09 19:49 ` [PATCH RFC PKS/PMEM 14/58] fs/cifs: " ira.weiny
2020-10-09 19:49 ` [PATCH RFC PKS/PMEM 15/58] fs/ecryptfs: " ira.weiny
2020-10-09 19:49 ` [PATCH RFC PKS/PMEM 16/58] fs/gfs2: " ira.weiny
2020-10-09 19:49 ` [PATCH RFC PKS/PMEM 17/58] fs/nilfs2: " ira.weiny
2020-10-09 19:49 ` [PATCH RFC PKS/PMEM 18/58] fs/hfs: " ira.weiny
2020-10-09 19:49 ` [PATCH RFC PKS/PMEM 19/58] fs/hfsplus: " ira.weiny
2020-10-09 19:49 ` [PATCH RFC PKS/PMEM 20/58] fs/jffs2: " ira.weiny
2020-10-09 19:49 ` [PATCH RFC PKS/PMEM 21/58] fs/nfs: " ira.weiny
2020-10-09 19:49 ` [PATCH RFC PKS/PMEM 22/58] fs/f2fs: " ira.weiny
2020-10-09 21:34   ` Eric Biggers
2020-10-10  0:39     ` Matthew Wilcox
2020-10-10  1:30       ` Eric Biggers
2020-10-12  6:56         ` Ira Weiny
2020-10-12 16:19           ` Eric Biggers
2020-10-12 16:28             ` Dave Hansen
2020-10-12 16:44               ` Matthew Wilcox
2020-10-12 19:53                 ` Ira Weiny
2020-10-12 20:02                   ` Matthew Wilcox
2020-10-12 23:31                     ` Ira Weiny [this message]
2020-10-10  2:43     ` James Bottomley
2020-10-09 19:49 ` [PATCH RFC PKS/PMEM 23/58] fs/fuse: " ira.weiny
2020-10-09 19:49 ` [PATCH RFC PKS/PMEM 24/58] fs/freevxfs: " ira.weiny
2020-10-13 11:25   ` Christoph Hellwig
2020-10-13 20:52     ` Ira Weiny
2020-10-09 19:50 ` [PATCH RFC PKS/PMEM 25/58] fs/reiserfs: " ira.weiny
2020-10-09 19:50 ` [PATCH RFC PKS/PMEM 26/58] fs/zonefs: " ira.weiny
2020-10-12  2:30   ` Damien Le Moal
2020-10-09 19:50 ` [PATCH RFC PKS/PMEM 27/58] fs/ubifs: " ira.weiny
2020-10-09 19:50 ` [PATCH RFC PKS/PMEM 28/58] fs/cachefiles: " ira.weiny
2020-10-09 19:50 ` [PATCH RFC PKS/PMEM 29/58] fs/ntfs: " ira.weiny
2020-10-09 19:50 ` [PATCH RFC PKS/PMEM 30/58] fs/romfs: " ira.weiny
2020-10-09 19:50 ` [PATCH RFC PKS/PMEM 31/58] fs/vboxsf: " ira.weiny
2020-10-09 19:50 ` [PATCH RFC PKS/PMEM 32/58] fs/hostfs: " ira.weiny
2020-10-09 19:50 ` [PATCH RFC PKS/PMEM 33/58] fs/cramfs: " ira.weiny
2020-10-13 18:36   ` Nicolas Pitre
2020-10-13 18:44   ` Dan Williams
2020-10-13 19:36     ` Matthew Wilcox
2020-10-13 19:41       ` Dan Williams
2020-10-13 20:01       ` Al Viro
2020-10-13 20:50         ` Ira Weiny
2020-10-13 20:45       ` Ira Weiny
2020-10-09 19:50 ` [PATCH RFC PKS/PMEM 34/58] fs/erofs: " ira.weiny
2020-10-09 19:50 ` [PATCH RFC PKS/PMEM 35/58] fs: " ira.weiny
2020-10-09 19:50 ` [PATCH RFC PKS/PMEM 36/58] fs/ext2: Use ext2_put_page ira.weiny
2020-10-09 19:50 ` [PATCH RFC PKS/PMEM 37/58] fs/ext2: Utilize new kmap_thread() ira.weiny
2020-10-09 19:50 ` [PATCH RFC PKS/PMEM 38/58] fs/isofs: " ira.weiny
2020-10-09 19:50 ` [PATCH RFC PKS/PMEM 39/58] fs/jffs2: " ira.weiny
2020-10-09 19:50 ` [PATCH RFC PKS/PMEM 40/58] net: " ira.weiny
2020-10-09 19:50 ` [PATCH RFC PKS/PMEM 41/58] drivers/target: " ira.weiny
2020-10-09 19:50 ` [PATCH RFC PKS/PMEM 42/58] drivers/scsi: " ira.weiny
2020-10-09 19:50 ` [PATCH RFC PKS/PMEM 43/58] drivers/mmc: " ira.weiny
2020-10-09 19:50 ` [PATCH RFC PKS/PMEM 44/58] drivers/xen: " ira.weiny
2020-10-09 19:50 ` [PATCH RFC PKS/PMEM 45/58] drivers/firmware: " ira.weiny
2020-10-09 19:50 ` [PATCH RFC PKS/PMEM 46/58] drives/staging: " ira.weiny
2020-10-09 19:50 ` [PATCH RFC PKS/PMEM 47/58] drivers/mtd: " ira.weiny
2020-10-09 19:50 ` [PATCH RFC PKS/PMEM 48/58] drivers/md: " ira.weiny
2020-10-10  2:20   ` Coly Li
2020-10-12  5:28     ` Ira Weiny
2020-10-12  7:40       ` Coly Li
2020-10-09 19:50 ` [PATCH RFC PKS/PMEM 49/58] drivers/misc: " ira.weiny
2020-10-09 19:50 ` [PATCH RFC PKS/PMEM 50/58] drivers/android: " ira.weiny
2020-10-09 19:50 ` [PATCH RFC PKS/PMEM 51/58] kernel: " ira.weiny
2020-10-10  3:43   ` Eric W. Biederman
2020-10-09 19:50 ` [PATCH RFC PKS/PMEM 52/58] mm: " ira.weiny
2020-10-09 19:50 ` [PATCH RFC PKS/PMEM 53/58] lib: " ira.weiny
2020-10-09 19:50 ` [PATCH RFC PKS/PMEM 54/58] powerpc: " ira.weiny
2020-10-09 19:50 ` [PATCH RFC PKS/PMEM 55/58] samples: " ira.weiny
2020-10-09 19:50 ` [PATCH RFC PKS/PMEM 56/58] dax: Stray access protection for dax_direct_access() ira.weiny
2020-10-09 19:50 ` [PATCH RFC PKS/PMEM 57/58] nvdimm/pmem: Stray access protection for pmem->virt_addr ira.weiny
2020-10-10  2:53   ` John Hubbard
2020-10-12  5:52     ` Ira Weiny
2020-10-09 19:50 ` [PATCH RFC PKS/PMEM 58/58] [dax|pmem]: Enable stray access protection ira.weiny
2020-10-10 11:36 ` [PATCH RFC PKS/PMEM 10/58] drivers/rdma: Utilize new kmap_thread() Bernard Metzler
2020-10-12  4:47   ` Ira Weiny

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