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@xxxxxxx> > > Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx> > > Cc: Ira Weiny <ira.weiny@xxxxxxxxx> > > Cc: Matthew Wilcox <willy@xxxxxxxxxxxxx> > > Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx> > > Signed-off-by: Fabio M. De Francesco <fmdefrancesco@xxxxxxxxx> > > --- > > 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 > > >
