LKML Archive on lore.kernel.org
 help / color / Atom feed
From: "Kirill A. Shutemov" <kirill@shutemov.name>
To: Andy Lutomirski <luto@kernel.org>
Cc: Dave Hansen <dave.hansen@intel.com>,
	Konstantin Khlebnikov <khlebnikov@yandex-team.ru>,
	X86 ML <x86@kernel.org>, Borislav Petkov <bp@alien8.de>,
	Neil Berrington <neil.berrington@datacore.com>,
	LKML <linux-kernel@vger.kernel.org>,
	stable <stable@vger.kernel.org>,
	"Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>
Subject: Re: [PATCH v2 1/2] x86/mm/64: Fix vmapped stack syncing on very-large-memory 4-level systems
Date: Fri, 26 Jan 2018 21:54:51 +0300
Message-ID: <20180126185451.jdjgekr7awwhukml@node.shutemov.name> (raw)
In-Reply-To: <CALCETrVwJCL=QTRT70b8u3p8xOXUiC7_Mkz45Bi3M9-vYgXWtg@mail.gmail.com>

On Thu, Jan 25, 2018 at 02:00:22PM -0800, Andy Lutomirski wrote:
> On Thu, Jan 25, 2018 at 1:49 PM, Dave Hansen <dave.hansen@intel.com> wrote:
> > On 01/25/2018 01:12 PM, Andy Lutomirski wrote:
> >> Neil Berrington reported a double-fault on a VM with 768GB of RAM that
> >> uses large amounts of vmalloc space with PTI enabled.
> >>
> >> The cause is that load_new_mm_cr3() was never fixed to take the
> >> 5-level pgd folding code into account, so, on a 4-level kernel, the
> >> pgd synchronization logic compiles away to exactly nothing.
> >
> > You don't mention it, but we can normally handle vmalloc() faults in the
> > kernel that are due to unsynchronized page tables.  The thing that kills
> > us here is that we have an unmapped stack and we try to use that stack
> > when entering the page fault handler, which double faults.  The double
> > fault handler gets a new stack and saves us enough to get an oops out.
> >
> > Right?
> 
> Exactly.
> 
> There are two special code paths that can't use vmalloc_fault(): this
> one and switch_to().  The latter avoids explicit page table fiddling
> and just touches the new stack before loading it into rsp.
> 
> >
> >> +static void sync_current_stack_to_mm(struct mm_struct *mm)
> >> +{
> >> +     unsigned long sp = current_stack_pointer;
> >> +     pgd_t *pgd = pgd_offset(mm, sp);
> >> +
> >> +     if (CONFIG_PGTABLE_LEVELS > 4) {
> >> +             if (unlikely(pgd_none(*pgd))) {
> >> +                     pgd_t *pgd_ref = pgd_offset_k(sp);
> >> +
> >> +                     set_pgd(pgd, *pgd_ref);
> >> +             }
> >> +     } else {
> >> +             /*
> >> +              * "pgd" is faked.  The top level entries are "p4d"s, so sync
> >> +              * the p4d.  This compiles to approximately the same code as
> >> +              * the 5-level case.
> >> +              */
> >> +             p4d_t *p4d = p4d_offset(pgd, sp);
> >> +
> >> +             if (unlikely(p4d_none(*p4d))) {
> >> +                     pgd_t *pgd_ref = pgd_offset_k(sp);
> >> +                     p4d_t *p4d_ref = p4d_offset(pgd_ref, sp);
> >> +
> >> +                     set_p4d(p4d, *p4d_ref);
> >> +             }
> >> +     }
> >> +}
> >
> > We keep having to add these.  It seems like a real deficiency in the
> > mechanism that we're using for pgd folding.  Can't we get a warning or
> > something when we try to do a set_pgd() that's (silently) not doing
> > anything?  This exact same pattern bit me more than once with the
> > KPTI/KAISER patches.
> 
> Hmm, maybe.
> 
> What I'd really like to see is an entirely different API.  Maybe:
> 
> typedef struct {
>   opaque, but probably includes:
>   int depth;  /* 0 is root */
>   void *table;
> } ptbl_ptr;
> 
> ptbl_ptr root_table = mm_root_ptbl(mm);
> 
> set_ptbl_entry(root_table, pa, prot);
> 
> /* walk tables */
> ptbl_ptr pt = ...;
> ptentry_ptr entry;
> while (ptbl_has_children(pt)) {
>   pt = pt_next(pt, addr);
> }
> entry = pt_entry_at(pt, addr);
> /* do something with entry */
> 
> etc.

I thought about very similar design, but never got time to try it really.
It's not one-week-end type of project :/

-- 
 Kirill A. Shutemov

  parent reply index

Thread overview: 12+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2018-01-25 21:12 [PATCH v2 0/2] x86/mm/64: vmalloc pgd synchronization cleanups/fixes Andy Lutomirski
2018-01-25 21:12 ` [PATCH v2 1/2] x86/mm/64: Fix vmapped stack syncing on very-large-memory 4-level systems Andy Lutomirski
2018-01-25 21:49   ` Dave Hansen
2018-01-25 22:00     ` Andy Lutomirski
2018-01-26  9:30       ` Ingo Molnar
2018-01-26 18:54       ` Kirill A. Shutemov [this message]
2018-01-26 15:06   ` [tip:x86/urgent] " tip-bot for Andy Lutomirski
2018-01-26 18:51   ` [PATCH v2 1/2] " Kirill A. Shutemov
2018-01-26 19:02     ` Andy Lutomirski
2018-01-26 20:50       ` Kirill A. Shutemov
2018-01-25 21:12 ` [PATCH v2 2/2] x86/mm/64: Tighten up vmalloc_fault() sanity checks on 5-level kernels Andy Lutomirski
2018-01-26 15:07   ` [tip:x86/urgent] " tip-bot for Andy Lutomirski

Reply instructions:

You may reply publicly to this message via plain-text email
using any one of the following methods:

* Save the following mbox file, import it into your mail client,
  and reply-to-all from there: mbox

  Avoid top-posting and favor interleaved quoting:
  https://en.wikipedia.org/wiki/Posting_style#Interleaved_style

* Reply using the --to, --cc, and --in-reply-to
  switches of git-send-email(1):

  git send-email \
    --in-reply-to=20180126185451.jdjgekr7awwhukml@node.shutemov.name \
    --to=kirill@shutemov.name \
    --cc=bp@alien8.de \
    --cc=dave.hansen@intel.com \
    --cc=khlebnikov@yandex-team.ru \
    --cc=kirill.shutemov@linux.intel.com \
    --cc=linux-kernel@vger.kernel.org \
    --cc=luto@kernel.org \
    --cc=neil.berrington@datacore.com \
    --cc=stable@vger.kernel.org \
    --cc=x86@kernel.org \
    /path/to/YOUR_REPLY

  https://kernel.org/pub/software/scm/git/docs/git-send-email.html

* If your mail client supports setting the In-Reply-To header
  via mailto: links, try the mailto: link

LKML Archive on lore.kernel.org

Archives are clonable:
	git clone --mirror https://lore.kernel.org/lkml/0 lkml/git/0.git
	git clone --mirror https://lore.kernel.org/lkml/1 lkml/git/1.git
	git clone --mirror https://lore.kernel.org/lkml/2 lkml/git/2.git
	git clone --mirror https://lore.kernel.org/lkml/3 lkml/git/3.git
	git clone --mirror https://lore.kernel.org/lkml/4 lkml/git/4.git
	git clone --mirror https://lore.kernel.org/lkml/5 lkml/git/5.git
	git clone --mirror https://lore.kernel.org/lkml/6 lkml/git/6.git
	git clone --mirror https://lore.kernel.org/lkml/7 lkml/git/7.git
	git clone --mirror https://lore.kernel.org/lkml/8 lkml/git/8.git
	git clone --mirror https://lore.kernel.org/lkml/9 lkml/git/9.git

	# If you have public-inbox 1.1+ installed, you may
	# initialize and index your mirror using the following commands:
	public-inbox-init -V2 lkml lkml/ https://lore.kernel.org/lkml \
		linux-kernel@vger.kernel.org
	public-inbox-index lkml

Example config snippet for mirrors

Newsgroup available over NNTP:
	nntp://nntp.lore.kernel.org/org.kernel.vger.linux-kernel


AGPL code for this site: git clone https://public-inbox.org/public-inbox.git