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Date: Thu, 3 Aug 2017 15:55:50 +0200
From: Michal Hocko <mhocko@...nel.org>
To: Igor Stoppa <igor.stoppa@...wei.com>
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 Thu 03-08-17 15:20:31, Igor Stoppa wrote:
> On 03/08/17 14:48, Michal Hocko wrote:
> > On Thu 03-08-17 13:11:45, Igor Stoppa wrote:
> >> On 02/08/17 20:08, Jerome Glisse wrote:
> >>> On Wed, Aug 02, 2017 at 06:14:28PM +0300, Igor Stoppa wrote:
>
> [...]
>
> >>>> from include/linux/mm_types.h:
> >>>>
> >>>> struct page {
> >>>> ...
> >>>> union {
> >>>> unsigned long private; /* Mapping-private opaque data:
> >>>> * usually used for buffer_heads
> >>>> * if PagePrivate set; used for
> >>>> * swp_entry_t if PageSwapCache;
> >>>> * indicates order in the buddy
> >>>> * system if PG_buddy is set.
> >>>> */
>
> [...]
>
> >> If the "Mapping-private" was dropped or somehow connected exclusively to
> >> the cases listed in the comment, then I think it would be more clear
> >> that the comment needs to be intended as related to mapping in certain
> >> cases only.
> >> But it is otherwise ok to use the "private" field for whatever purpose
> >> it might be suitable, as long as it is not already in use.
> >
> > I would recommend adding a new field into the enum...
>
> s/enum/union/ ?
>
> If not, I am not sure what is the enum that you are talking about.
yeah, fat fingers on my side
>
> [...]
>
> >> But, to reply more specifically to your advice, yes, I think I could add
> >> a flag to vm_struct and then retrieve its value, for the address being
> >> processed, by passing through find_vm_area().
> >
> > ... and you can store vm_struct pointer to the struct page there
>
> "there" as in the new field of the union?
> btw, what would be a meaningful name, since "private" is already taken?
>
> For simplicity, I'll use, for now, "private2"
why not explicit vm_area?
> > and you> won't need to do the slow find_vm_area. I haven't checked
> very closely
> > but this should be possible in principle. I guess other callers might
> > benefit from this as well.
>
> I am confused about this: if "private2" is a pointer, but when I get an
> address, I do not even know if the address represents a valid pmalloc
> page, how can i know when it's ok to dereference "private2"?
because you can make all pages which back vmalloc mappings have vm_area
pointer set.
--
Michal Hocko
SUSE Labs
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