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SubjectRe: [PATCH 08/17] prmem: struct page: track vmap_area
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On 25/10/2018 03:13, Matthew Wilcox wrote:
> On Thu, Oct 25, 2018 at 02:01:02AM +0300, Igor Stoppa wrote:
>>>> @@ -1747,6 +1750,10 @@ void *__vmalloc_node_range(unsigned long size, unsigned long align,
>>>> if (!addr)
>>>> return NULL;
>>>> + va = __find_vmap_area((unsigned long)addr);
>>>> + for (i = 0; i < va->vm->nr_pages; i++)
>>>> + va->vm->pages[i]->area = va;
>>>
>>> I don't like it that you're calling this for _every_ vmalloc() caller
>>> when most of them will never use this. Perhaps have page->va be initially
>>> NULL and then cache the lookup in it when it's accessed for the first time.
>>>
>>
>> If __find_vmap_area() was part of the API, this loop could be left out from
>> __vmalloc_node_range() and the user of the allocation could initialize the
>> field, if needed.
>>
>> What is the reason for keeping __find_vmap_area() private?
>
> Well, for one, you're walking the rbtree without holding the spinlock,
> so you're going to get crashes. I don't see why we shouldn't export
> find_vmap_area() though.

Argh, yes, sorry. But find_vmap_area() would be enough for what I need.

> Another way we could approach this is to embed the vmap_area in the
> vm_struct. It'd require a bit of juggling of the alloc/free paths in
> vmalloc, but it might be worthwhile.

I have a feeling of unease about the whole vmap_area / vm_struct
duality. They clearly are different types, with different purposes, but
here and there there are functions that are named after some "area", yet
they actually refer to vm_struct pointers.

I wouldn't mind spending some time understanding the reason for this
apparently bizarre choice, but after I have emerged from the prmem swamp.

For now I'd stick to find_vmap_area().

--
igor

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