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Subject: Re: [PATCH 08/17] prmem: struct page: track vmap_area
To:     Matthew Wilcox <willy@infradead.org>
Cc:     Mimi Zohar <zohar@linux.vnet.ibm.com>,
        Kees Cook <keescook@chromium.org>,
        Dave Chinner <david@fromorbit.com>,
        James Morris <jmorris@namei.org>,
        Michal Hocko <mhocko@kernel.org>,
        kernel-hardening@lists.openwall.com,
        linux-integrity@vger.kernel.org,
        linux-security-module@vger.kernel.org, igor.stoppa@huawei.com,
        Dave Hansen <dave.hansen@linux.intel.com>,
        Jonathan Corbet <corbet@lwn.net>,
        Laura Abbott <labbott@redhat.com>,
        Vlastimil Babka <vbabka@suse.cz>,
        "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Pavel Tatashin <pasha.tatashin@oracle.com>, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org
References: <20181023213504.28905-1-igor.stoppa@huawei.com>
 <20181023213504.28905-9-igor.stoppa@huawei.com>
 <20181024031200.GC25444@bombadil.infradead.org>
 <ffb887e1-2029-42d5-3a97-54546e4d28d8@gmail.com>
 <20181025021307.GH25444@bombadil.infradead.org>
From:   Igor Stoppa <igor.stoppa@gmail.com>
Message-ID: <4b1ed33c-9b3c-a61f-b919-aeed97edddac@gmail.com>
Date:   Mon, 29 Oct 2018 20:21:18 +0200
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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