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Message-ID: <ZPHdjvFwvtzXO/6z@pc636>
Date: Fri, 1 Sep 2023 14:48:14 +0200
From: Uladzislau Rezki <urezki@...il.com>
To: Joel Fernandes <joel@...lfernandes.org>
Cc: Uladzislau Rezki <urezki@...il.com>, linux-kernel@...r.kernel.org,
Andrew Morton <akpm@...ux-foundation.org>,
Christoph Hellwig <hch@...radead.org>,
Lorenzo Stoakes <lstoakes@...il.com>,
Zhen Lei <thunder.leizhen@...weicloud.com>,
"Paul E . McKenney" <paulmck@...nel.org>, rcu@...r.kernel.org,
Zqiang <qiang.zhang1211@...il.com>,
Matthew Wilcox <willy@...radead.org>, linux-mm@...ck.org
Subject: Re: [PATCH v2 1/2] mm/vmalloc: Add a safer version of find_vm_area()
for debug
On Fri, Sep 01, 2023 at 12:33:21AM +0000, Joel Fernandes wrote:
> On Fri, Sep 01, 2023 at 12:19:17AM +0000, Joel Fernandes wrote:
> > On Thu, Aug 31, 2023 at 09:47:52PM +0200, Uladzislau Rezki wrote:
> > > On Thu, Aug 31, 2023 at 05:18:25PM +0000, Joel Fernandes (Google) wrote:
> > > > It is unsafe to dump vmalloc area information when trying to do so from
> > > > some contexts. Add a safer trylock version of the same function to do a
> > > > best-effort VMA finding and use it from vmalloc_dump_obj().
> > > >
> > > > [apply test robot feedback on unused function fix.]
> > > >
> > > > Reported-by: Zhen Lei <thunder.leizhen@...weicloud.com>
> > > > Cc: Paul E. McKenney <paulmck@...nel.org>
> > > > Cc: rcu@...r.kernel.org
> > > > Cc: Zqiang <qiang.zhang1211@...il.com>
> > > > Reviewed-by: Matthew Wilcox (Oracle) <willy@...radead.org>
> > > > Signed-off-by: Joel Fernandes (Google) <joel@...lfernandes.org>
> > > > ---
> > > > v1->v2: Apply review tags and test robot feedback.
> > > >
> > > > mm/vmalloc.c | 39 ++++++++++++++++++++++++++++++++++++++-
> > > > 1 file changed, 38 insertions(+), 1 deletion(-)
> > > >
> > > > diff --git a/mm/vmalloc.c b/mm/vmalloc.c
> > > > index 93cf99aba335..f09e882ae3b8 100644
> > > > --- a/mm/vmalloc.c
> > > > +++ b/mm/vmalloc.c
> > > > @@ -1865,6 +1865,20 @@ struct vmap_area *find_vmap_area(unsigned long addr)
> > > > return va;
> > > > }
> > > >
> > > > +#ifdef CONFIG_PRINTK
> > > > +static struct vmap_area *find_vmap_area_trylock(unsigned long addr)
> > > > +{
> > > > + struct vmap_area *va;
> > > > +
> > > > + if (!spin_trylock(&vmap_area_lock))
> > > > + return NULL;
> > > > + va = __find_vmap_area(addr, &vmap_area_root);
> > > > + spin_unlock(&vmap_area_lock);
> > > > +
> > > > + return va;
> > > > +}
> > > > +#endif
> > > > +
> > > > static struct vmap_area *find_unlink_vmap_area(unsigned long addr)
> > > > {
> > > > struct vmap_area *va;
> > > > @@ -2671,6 +2685,29 @@ struct vm_struct *find_vm_area(const void *addr)
> > > > return va->vm;
> > > > }
> > > >
> > > > +/**
> > > > + * try_to_find_vm_area - find a continuous kernel virtual area
> > > > + * @addr: base address
> > > > + *
> > > > + * This function is the same as find_vm_area() except that it is
> > > > + * safe to call if vmap_area_lock is already held and returns NULL
> > > > + * if it is. See comments in find_vmap_area() for other details.
> > > > + *
> > > > + * Return: the area descriptor on success or %NULL on failure.
> > > > + */
> > > > +#ifdef CONFIG_PRINTK
> > > > +static struct vm_struct *try_to_find_vm_area(const void *addr)
> > > > +{
> > > > + struct vmap_area *va;
> > > > +
> > > > + va = find_vmap_area_trylock((unsigned long)addr);
> > > > + if (!va)
> > > > + return NULL;
> > > > +
> > > > + return va->vm;
> > > > +}
> > > > +#endif
> > > > +
> > > > /**
> > > > * remove_vm_area - find and remove a continuous kernel virtual area
> > > > * @addr: base address
> > > > @@ -4277,7 +4314,7 @@ bool vmalloc_dump_obj(void *object)
> > > > struct vm_struct *vm;
> > > > void *objp = (void *)PAGE_ALIGN((unsigned long)object);
> > > >
> > > > - vm = find_vm_area(objp);
> > > > + vm = try_to_find_vm_area(objp);
> > > > if (!vm)
> > > > return false;
> > > > pr_cont(" %u-page vmalloc region starting at %#lx allocated at %pS\n",
> >
> > Hi Vlad,
> > Thanks for taking a look.
> >
> > > I am not sure if this patch makes a lot of sense. I agree, this is a
> > > problem and it mitigates it. But it is broken in terms of once you drop
> > > the lock, the VA should not be accessed.
> >
> > Just to note the lockless-access issue you are referring to is not introduced
> > by this patch but is rather in the existing code. Also just to note this is
> > debug code.
> >
> > > Is that a real issue or it gets triggered due to some syntetic test case?
> >
> > It is a real issue. See 2/2.
> >
> > > If i were you, i would go with open-coded version of trylock. Because
> > > there is only one user so far.
> >
> > Taking your open coding and locking suggestions, I came up with the below
> > which actually results in a smaller patch. Does it look good to you?
> >
> > diff --git a/mm/vmalloc.c b/mm/vmalloc.c
> > index 93cf99aba335..aaf6bad997a7 100644
>
> And with some trivial compiler errors fixed (sorry should have build tested
> but wanted to just share the idea earlier):
>
> diff --git a/mm/vmalloc.c b/mm/vmalloc.c
> index 93cf99aba335..2c6a0e2ff404 100644
> --- a/mm/vmalloc.c
> +++ b/mm/vmalloc.c
> @@ -4274,14 +4274,32 @@ void pcpu_free_vm_areas(struct vm_struct **vms, int nr_vms)
> #ifdef CONFIG_PRINTK
> bool vmalloc_dump_obj(void *object)
> {
> - struct vm_struct *vm;
> void *objp = (void *)PAGE_ALIGN((unsigned long)object);
> + const void *caller;
> + struct vm_struct *vm;
> + struct vmap_area *va;
> + unsigned long addr;
> + unsigned int nr_pages;
>
> - vm = find_vm_area(objp);
> - if (!vm)
> + if (!spin_trylock(&vmap_area_lock))
> + return false;
> + va = __find_vmap_area((unsigned long)objp, &vmap_area_root);
> + if (!va) {
> + spin_unlock(&vmap_area_lock);
> return false;
> + }
> +
> + vm = va->vm;
> + if (!vm) {
> + spin_unlock(&vmap_area_lock);
> + return false;
> + }
> + addr = (unsigned long)vm->addr;
> + caller = vm->caller;
> + nr_pages = vm->nr_pages;
> + spin_unlock(&vmap_area_lock);
> pr_cont(" %u-page vmalloc region starting at %#lx allocated at %pS\n",
> - vm->nr_pages, (unsigned long)vm->addr, vm->caller);
> + nr_pages, addr, caller);
> return true;
> }
> #endif
>
Looks good to me and thank you for fixing a locking issue :)
I think you will re-spin and resend it one more time?
--
Uladzislau Rezki
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