linux-kernel - Re: [RFC] Tagging of vmalloc pages for supporting the pmalloc allocator

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Date:   Fri, 4 Aug 2017 11:02:46 +0300
From:   Igor Stoppa <igor.stoppa@...wei.com>
To:     Michal Hocko <mhocko@...nel.org>
CC:     Jerome Glisse <jglisse@...hat.com>, Linux-MM <linux-mm@...ck.org>,
        LKML <linux-kernel@...r.kernel.org>,
        <linux-security-module@...r.kernel.org>,
        "kernel-hardening@...ts.openwall.com" 
        <kernel-hardening@...ts.openwall.com>,
        Kees Cook <keescook@...gle.com>
Subject: Re: [RFC] Tagging of vmalloc pages for supporting the pmalloc
 allocator



On 03/08/17 18:15, Michal Hocko wrote:

> I would check the one where we have mapping. It is rather unlikely
> vmalloc users would touch this one.

That was also the initial recommendation from Jerome Glisse, but it
seemed unusable, because of the related comment.

I should have asked for clarifications back then :-(

But it's never too late ...


struct page {
  /* First double word block */
  unsigned long flags;		/* Atomic flags, some possibly
				 * updated asynchronously */
union {
	struct address_space *mapping;	/* If low bit clear, points to
					 * inode address_space, or NULL.
					 * If page mapped as anonymous
					 * memory, low bit is set, and
					 * it points to anon_vma object:
					 * see PAGE_MAPPING_ANON below.
					 */
...
}

mapping seems to be used exclusively in 2 ways, based on the value of
its lower bit.

Therefore I discarded it as valid option ("private", otoh was far more
alluring), but maybe I could wrap it inside a union, together with vm_area?

---
thanks, igor