Messages in this thread | | | From | Dan Williams <> | Date | Fri, 4 Feb 2022 22:28:15 -0800 | Subject | Re: [PATCH V8 38/44] memremap_pages: Define pgmap_mk_{readwrite|noaccess}() calls |
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On Fri, Feb 4, 2022 at 9:56 PM Ira Weiny <ira.weiny@intel.com> wrote: > > On Fri, Feb 04, 2022 at 04:27:38PM -0800, Dan Williams wrote: > > On Fri, Feb 4, 2022 at 4:25 PM Dan Williams <dan.j.williams@intel.com> wrote: > > > > > > On Fri, Feb 4, 2022 at 4:19 PM Dan Williams <dan.j.williams@intel.com> wrote: > > > > > > > > On Fri, Feb 4, 2022 at 4:10 PM Ira Weiny <ira.weiny@intel.com> wrote: > > > > > > > > > > On Fri, Feb 04, 2022 at 10:35:59AM -0800, Dan Williams wrote: > > > > > > On Thu, Jan 27, 2022 at 9:55 AM <ira.weiny@intel.com> wrote: > > > > > > > > > > > > > > > > > [snip] > > > > > > > > > > I'll address the other comments later but wanted to address the idea below. > > > > > > > > > > > > diff --git a/include/linux/sched.h b/include/linux/sched.h > > > > > > > index f5b2be39a78c..5020ed7e67b7 100644 > > > > > > > --- a/include/linux/sched.h > > > > > > > +++ b/include/linux/sched.h > > > > > > > @@ -1492,6 +1492,13 @@ struct task_struct { > > > > > > > struct callback_head l1d_flush_kill; > > > > > > > #endif > > > > > > > > > > > > > > +#ifdef CONFIG_DEVMAP_ACCESS_PROTECTION > > > > > > > + /* > > > > > > > + * NOTE: pgmap_prot_count is modified within a single thread of > > > > > > > + * execution. So it does not need to be atomic_t. > > > > > > > + */ > > > > > > > + u32 pgmap_prot_count; > > > > > > > +#endif > > > > > > > > > > > > It's not at all clear why the task struct needs to be burdened with > > > > > > this accounting. Given that a devmap instance is needed to manage page > > > > > > protections, why not move the nested protection tracking to a percpu > > > > > > variable relative to an @pgmap arg? Something like: > > > > > > > > > > > > void __pgmap_mk_readwrite(struct dev_pagemap *pgmap) > > > > > > { > > > > > > migrate_disable(); > > > > > > preempt_disable(); > > > > > > > > > > Why burden threads like this? kmap_local_page() is perfectly able to migrate > > > > > or be preempted? > > > > > > > > > > I think this is way to restrictive. > > > > > > > > kmap_local_page() holds migrate_disable() over the entire mapping, so > > > > we're only talking about preempt_disable(). I tend to think that > > > > bloating task_struct for something that is rarely used "kmap on dax > > > > pmem pages" is not the right tradeoff. > > > > > > Now, I can see an argument that promoting kmap_local_page() to > > > preempt_disable() could cause problems, but I'd like help confirming > > > that before committing to extending task_struct. > > > > ...as I say that it occurs to me that the whole point of > > kmap_local_page() is to be better than kmap_atomic() and this undoes > > that. I'd at least like that documented as the reason that task_struct > > needs to carry a new field. > > I'll try and update the commit message but kmap_local_page() only disables > migrate on a highmem system.
Right, but that still means that the code in question is prepared for migrate_disable(). Instead I would justify the task_struct expansion on the observation that moving the enable tracker to percpu would require pre-empt disable and promote kmap_local_page() to kmap_atomic() level of restriction. Otherwise if there was a way to avoid task_struct expansion with only migrate_disable() and not preempt_disable I think it would be worth it, but nothing comes to mind at the moment.
> The devmap/PKS use case is specifically not > supported on highmem systems. Mainly because on highmem systems > kmap_local_page() actually creates a new mapping which is not covered by PKS > anyway. > > So for the devmap/PKS use case kmap_local_page() is defined as: > > static inline void *kmap_local_page(struct page *page) > { > + pgmap_mk_readwrite(page); > return page_address(page); > } > > ...for the linear mapping. I'll try and update the commit message with this > detail.
Yeah, add the kmap_atomic() observation, not the !highmem note.
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