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[90.235.3.170]) by smtp.gmail.com with ESMTPSA id q4-20020ac25a04000000b004fdfd931888sm640741lfn.293.2023.09.01.05.48.15 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Fri, 01 Sep 2023 05:48:16 -0700 (PDT) From: Uladzislau Rezki X-Google-Original-From: Uladzislau Rezki Date: Fri, 1 Sep 2023 14:48:14 +0200 To: Joel Fernandes Cc: Uladzislau Rezki , linux-kernel@vger.kernel.org, Andrew Morton , Christoph Hellwig , Lorenzo Stoakes , Zhen Lei , "Paul E . McKenney" , rcu@vger.kernel.org, Zqiang , Matthew Wilcox , linux-mm@kvack.org Subject: Re: [PATCH v2 1/2] mm/vmalloc: Add a safer version of find_vm_area() for debug Message-ID: References: <20230831171827.2625016-1-joel@joelfernandes.org> <20230901001917.GA2723108@google.com> <20230901003321.GA3389909@google.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20230901003321.GA3389909@google.com> Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 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 > > > > Cc: Paul E. McKenney > > > > Cc: rcu@vger.kernel.org > > > > Cc: Zqiang > > > > Reviewed-by: Matthew Wilcox (Oracle) > > > > Signed-off-by: Joel Fernandes (Google) > > > > --- > > > > 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