Re: [PATCH 4/4] Documentation/vm: Rework "Temporary Virtual Mappings"

["Fabio M. De Francesco" <fmdefrancesco@gmail.com>]

On lunedì 25 aprile 2022 02:59:48 CEST Ira Weiny wrote:
> On Thu, Apr 21, 2022 at 08:02:00PM +0200, Fabio M. De Francesco wrote:
> > Extend and rework the "Temporary Virtual Mappings" section of the 
highmem.rst
> > documentation.
> > 
> > Despite the local kmaps were introduced by Thomas Gleixner in October 
2020,
> > documentation was still missing information about them. These additions 
rely
> > largely on Gleixner's patches, Jonathan Corbet's LWN articles, comments 
by
> > Ira Weiny and Matthew Wilcox, and in-code comments from
> > ./include/linux/highmem.h.
> > 
> > 1) Add a paragraph to document kmap_local_page().
> > 2) Reorder the list of functions by decreasing order of preference of
> > use.
> > 3) Rework part of the kmap() entry in list.
> > 
> > Cc: Jonathan Corbet <corbet@lwn.net>
> > Cc: Thomas Gleixner <tglx@linutronix.de>
> > Cc: Ira Weiny <ira.weiny@intel.com>
> > Cc: Matthew Wilcox <willy@infradead.org>
> > Cc: Peter Zijlstra <peterz@infradead.org>
> > Signed-off-by: Fabio M. De Francesco <fmdefrancesco@gmail.com>
> > ---
> >  Documentation/vm/highmem.rst | 71 ++++++++++++++++++++++++++++++------
> >  1 file changed, 60 insertions(+), 11 deletions(-)
> > 
> > diff --git a/Documentation/vm/highmem.rst b/Documentation/vm/
highmem.rst
> > index e05bf5524174..960f61e7a552 100644
> > --- a/Documentation/vm/highmem.rst
> > +++ b/Documentation/vm/highmem.rst
> > @@ -50,26 +50,75 @@ space when they use mm context tags.
> >  Temporary Virtual Mappings
> >  ==========================
> >  
> > -The kernel contains several ways of creating temporary mappings:
> > +The kernel contains several ways of creating temporary mappings. The 
following
> > +list shows them in order of preference of use.
> >  
> > -* vmap().  This can be used to make a long duration mapping of 
multiple
> > -  physical pages into a contiguous virtual space.  It needs global
> > -  synchronization to unmap.
> > +* kmap_local_page().  This function is used to require short term 
mappings.
> > +  It can be invoked from any context (including interrupts) but the 
mappings
> > +  can only be used in the context which acquired them.
> > +
> > +  This function should be preferred, where feasible, over all the 
others.
> >  
> > -* kmap().  This permits a short duration mapping of a single page.  It 
needs
> > -  global synchronization, but is amortized somewhat.  It is also prone 
to
> > -  deadlocks when using in a nested fashion, and so it is not 
recommended for
> > -  new code.
> > +  These mappings are per thread, CPU local (i.e., migration from one 
CPU to
> > +  another is disabled - this is why they are called "local"), but they 
don't
> > +  disable preemption. It's valid to take pagefaults in a local kmap 
region,
> > +  unless the context in which the local mapping is acquired does not 
allow
> > +  it for other reasons.
> > +
> > +  It is assumed that kmap_local_page() always returns the virtual 
address
> 
> kmap_local_page() does return a kernel virtual address.  Why 'assume' 
this?
> 
> Do you mean it returns an address in the direct map?
> 
> > +  of the mapping, therefore they won't ever fail.
> 
> I don't think that returning a virtual address has anything to do with 
the
> assumption they will not fail.
> 
> Why do you say this?

Oh, sorry! I didn't mean to say this. What I wrote is _not_ what I meant. 
My intention was to say the same that you may read below about 
k[un]map_atomic().

This sentence should have been:

+ It always returns a valid virtual address. It is assumed that
+ k[un]map_local() won't ever fail.

Is this rewording correct?

It's not my first time I make these kinds of silly mistakes when copy-
pasting lines and then rework or merge with other text that was already 
there. Recently I've made a couple of these kinds of mistakes.

I'd better read twice (maybe thrice) what I write before sending :(

> 
> > +
> > +  If a task holding local kmaps is preempted, the maps are removed on 
context
> > +  switch and restored when the task comes back on the CPU. As the maps 
are
> > +  strictly CPU local, it is guaranteed that the task stays on the CPU 
and
> > +  that the CPU cannot be unplugged until the local kmaps are released.
> > +
> > +  Nesting kmap_local_page() and kmap_atomic() mappings is allowed to a 
certain
> > +  extent (up to KMAP_TYPE_NR) but their invocations have to be 
strictly ordered
> > +  because the map implementation is stack based.
> 
> I think I would reference the kmap_local_page()

I suppose you are talking about the kdocs comments in code. If so, please 
remember that the kmap_local_page() kdocs have already been included in  
highmem.rst.

Am I misunderstanding what you write?

> for more details on the
> ordering because there have been some conversions I've done which were
> complicated by this.
> 
> >  
> >  * kmap_atomic().  This permits a very short duration mapping of a 
single
> >    page.  Since the mapping is restricted to the CPU that issued it, it
> >    performs well, but the issuing task is therefore required to stay on 
that
> >    CPU until it has finished, lest some other task displace its 
mappings.
> >  
> > -  kmap_atomic() may also be used by interrupt contexts, since it is 
does not
> > -  sleep and the caller may not sleep until after kunmap_atomic() is 
called.
> > +  kmap_atomic() may also be used by interrupt contexts, since it does 
not
> > +  sleep and the callers too may not sleep until after kunmap_atomic() 
is
> > +  called.
> > +
> > +  Each call of kmap_atomic() in the kernel creates a non-preemptible 
section
> > +  and disable pagefaults. This could be a source of unwanted latency, 
so it
> > +  should be only used if it is absolutely required, otherwise 
kmap_local_page()
> > +  should be used where it is feasible.
> >  
> > -  It may be assumed that k[un]map_atomic() won't fail.
> > +  It is assumed that k[un]map_atomic() won't fail.
> > +
> > +* kmap().  This should be used to make short duration mapping of a 
single
> > +  page with no restrictions on preemption or migration. It comes with 
an
> > +  overhead as mapping space is restricted and protected by a global 
lock
> > +  for synchronization. When mapping is no more needed, the address 
that
>                                          ^^^^^^^^
> 					 no longer

Yes, correct. I'll fix it.

> > +  the page was mapped to must be released with kunmap().
> > +
> > +  Mapping changes must be propagated across all the CPUs. kmap() also
> > +  requires global TLB invalidation when the kmap's pool wraps and it 
might
> > +  block when the mapping space is fully utilized until a slot becomes
> > +  available. Therefore, kmap() is only callable from preemptible 
context.
> > +
> > +  All the above work is necessary if a mapping must last for a 
relatively
> > +  long time but the bulk of high-memory mappings in the kernel are
> > +  short-lived and only used in one place. This means that the cost of
> > +  kmap() is mostly wasted in such cases. kmap() was not intended for 
long
> > +  term mappings but it has morphed in that direction and its use is
> > +  strongly discouraged in newer code and the set of the preceding 
functions
> > +  should be preferred.
> 
> Nice!

Now that I have your reviews for all the four patches of this series I'll 
send next version on Monday.

Thanks you so much,

Fabio

> 
> Ira
> 
> > +
> > +  On 64-bit systems, calls to kmap_local_page(), kmap_atomic() and 
kmap() have
> > +  no real work to do because a 64-bit address space is more than 
sufficient to
> > +  address all the physical memory whose pages are permanently mapped.
> > +
> > +* vmap().  This can be used to make a long duration mapping of 
multiple
> > +  physical pages into a contiguous virtual space.  It needs global
> > +  synchronization to unmap.
> >  
> >  
> >  Cost of Temporary Mappings
> > -- 
> > 2.34.1
> > 
>