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[34.171.120.74]) by smtp.gmail.com with ESMTPSA id c25-20020a02a419000000b0042b2e90ed06sm723052jal.23.2023.08.31.17.33.21 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Thu, 31 Aug 2023 17:33:21 -0700 (PDT) Date: Fri, 1 Sep 2023 00:33:21 +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: <20230901003321.GA3389909@google.com> References: <20230831171827.2625016-1-joel@joelfernandes.org> <20230901001917.GA2723108@google.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20230901001917.GA2723108@google.com> X-Rspam-User: X-Stat-Signature: sjps88483re1grht68hhid9us81b56ts X-Rspamd-Server: rspam07 X-Rspamd-Queue-Id: 4C5AB1C000A X-HE-Tag: 1693528403-647106 X-HE-Meta: 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 9LYjWGia OOhTMQy6UWqCOKVJx4hdsh4EbB+gRuqWemrX5AqCju8AcMjgP5Qf3cLsUzAlI/K0Uy2xd5po2uEZLKbHpp9UmhlHkQEXk/+6mTI5JlnTPyEI/1bNdpy1qNJThsbEz4+TXF0miC8POCzXf3n26uepb/IyD7BCktIAYUO9e5IF6yeECszDlqV68LCR2Tcz5N2O0SQTZ1sephrqs3ZcvGvJyw72VIxh0nRrhprGMCaNKEIIwmRTB7moBQSoo5sr5iph3tbYYUdz2puiKvDqBVj5+DjwE9DNXpESAQC6OUNDmnSNcyUwGVUEJAxgYLwdLEEBRYkL1ReJ/HF80PDV7QiFxdxDkvO3/DEJVisHfxLAHA+/cF0Zx9sAjgtN7sp0+/Jggankcnz7SoqS5E0rfZ8wueDpb/CveXhdfMaDzCvuZtBXjbeMfh6rtBVwThvwGykT6bOaDpEe7d3m6kQgiDOFgOhQRuW10eG5yQAllxk8fyT4gSTw2/MdCyazXNbjSIIGmuAKaugiSyn8IHObUUZXM2/z7dIhDNP2ySIGyHnbtXD1mRaRq5T/nbxf2gmrzB8MhiVKdWMlFv2A/Bi14OB8G71opPOvQRN+YxhwZFhtg7PoLnGGsGUNadM5yFZ4t4D3sSIJ+hLc+YSahnD8XVzQdB3Jpz2t++dM0a10W/TgFYsCSXKjOzuv5evolig== X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: 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