Re: [Patch 8/8] kexec: allow to shrink reserved memory

[Amerigo Wang]

Eric W. Biederman wrote:
> Amerigo Wang <amwang@xxxxxxxxxx> writes:
>
>   
> > >   
> > >       
> > > > +	ret = kexec_crash_image != NULL;
> > > > +	mutex_unlock(&kexec_mutex);
> > > > +	return ret;
> > > > +}
> > > > +
> > > > +size_t get_crash_memory_size(void)
> > > > +{
> > > > +	size_t size;
> > > > +	if (!mutex_trylock(&kexec_mutex))
> > > > +		return 1;
> > > >     
> > > >         
> > > We don't need trylock on this code path 
> > >
> > >   
> > >       
> > Hmm, crashk_res is a global struct, so other process can also
> > change it... but currently no process does that, right?
> >
> >     
>
> We still need the lock.  Just doing trylock doesn't instead
> of just sleeping doesn't seem to make any sense on these
> code paths.
>
>   

Ok, got it.

> > >   
> > >       
> > > > +	start = crashk_res.start;
> > > > +	end = crashk_res.end;
> > > > +
> > > > +	if (new_size >= end - start + 1) {
> > > > +		ret = -EINVAL;
> > > > +		if (new_size == end - start + 1)
> > > > +			ret = 0;
> > > > +		goto unlock;
> > > > +	}
> > > > +
> > > > +	start = roundup(start, PAGE_SIZE);
> > > > +	end = roundup(start + new_size, PAGE_SIZE) - 1;
> > > > +	npages = (end + 1 - start ) / PAGE_SIZE;
> > > > +
> > > > +	pages = kmalloc(sizeof(struct page *) * npages, GFP_KERNEL);
> > > > +	if (!pages) {
> > > > +		ret = -ENOMEM;
> > > > +		goto unlock;
> > > > +	}
> > > > +	for (i = 0; i < npages; i++) {
> > > > +		addr = end + 1 + i * PAGE_SIZE;
> > > > +		pages[i] = virt_to_page(addr);
> > > > +	}
> > > > +
> > > > +	vaddr = vm_map_ram(pages, npages, 0, PAGE_KERNEL);
> > > >     
> > > >         
> > > This is the wrong kernel call to use.  I expect this needs to look
> > > like a memory hotplug event.  This does not put the pages into the
> > > free page pool.
> > >   
> > >       
> > Well, I also wanted to use an memory-hotplug API, but that will make the code
> > depend on memory-hotplug, which certainly is not what we want...
> >
> > I checked the mm code, actually what I need is an API which is similar to
> > add_active_range(), but add_active_range() can't be used here since it is marked
> > as "__init".
> >
> > Do we have that kind of API in mm? I can't find one.
> >     
>
> Perhaps we will need to remove __init from add_active_range.  I know the logic
> but I'm not up to speed on the mm pieces at the moment.
>   

Not that simple, marking it as "__init" means it uses some "__init" data 
which will be dropped after initialization.

Thanks.

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