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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> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20230831171827.2625016-1-joel@joelfernandes.org> Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 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", > 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. Is that a real issue or it gets triggered due to some syntetic test case? If i were you, i would go with open-coded version of trylock. Because there is only one user so far. -- Uladzislau Rezki