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[34.123.190.156]) by smtp.gmail.com with ESMTPSA id t15-20020a05663801ef00b0041cd626ea0csm713150jaq.147.2023.08.31.17.19.18 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Thu, 31 Aug 2023 17:19:18 -0700 (PDT) Date: Fri, 1 Sep 2023 00:19:17 +0000 From: Joel Fernandes To: Uladzislau Rezki Cc: 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: <20230901001917.GA2723108@google.com> References: <20230831171827.2625016-1-joel@joelfernandes.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 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 --- a/mm/vmalloc.c +++ b/mm/vmalloc.c @@ -4274,14 +4274,31 @@ void pcpu_free_vm_areas(struct vm_struct **vms, int nr_vms) #ifdef CONFIG_PRINTK bool vmalloc_dump_obj(void *object) { + void *caller, *objp = (void *)PAGE_ALIGN((unsigned long)object); struct vm_struct *vm; - void *objp = (void *)PAGE_ALIGN((unsigned long)object); + 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)addr, &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 = 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