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




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