linux-kernel - Re: [PATCH v2 1/3] vmalloc: Choose a better start address in vm_area_register

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Date:   Wed, 4 Aug 2021 12:14:02 +0100
From:   Catalin Marinas <catalin.marinas@....com>
To:     Kefeng Wang <wangkefeng.wang@...wei.com>
Cc:     Will Deacon <will@...nel.org>,
        Andrey Ryabinin <ryabinin.a.a@...il.com>,
        Andrey Konovalov <andreyknvl@...il.com>,
        Dmitry Vyukov <dvyukov@...gle.com>,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        kasan-dev@...glegroups.com, linux-mm@...ck.org
Subject: Re: [PATCH v2 1/3] vmalloc: Choose a better start address in
 vm_area_register_early()

On Mon, Aug 02, 2021 at 10:39:04AM +0800, Kefeng Wang wrote:
> On 2021/8/1 23:23, Catalin Marinas wrote:
> > On Tue, Jul 20, 2021 at 10:51:03AM +0800, Kefeng Wang wrote:
> > > There are some fixed locations in the vmalloc area be reserved
> > > in ARM(see iotable_init()) and ARM64(see map_kernel()), but for
> > > pcpu_page_first_chunk(), it calls vm_area_register_early() and
> > > choose VMALLOC_START as the start address of vmap area which
> > > could be conflicted with above address, then could trigger a
> > > BUG_ON in vm_area_add_early().
> > > 
> > > Let's choose the end of existing address range in vmlist as the
> > > start address instead of VMALLOC_START to avoid the BUG_ON.
> > > 
> > > Signed-off-by: Kefeng Wang <wangkefeng.wang@...wei.com>
> > > ---
> > >   mm/vmalloc.c | 8 +++++---
> > >   1 file changed, 5 insertions(+), 3 deletions(-)
> > > 
> > > diff --git a/mm/vmalloc.c b/mm/vmalloc.c
> > > index d5cd52805149..a98cf97f032f 100644
> > > --- a/mm/vmalloc.c
> > > +++ b/mm/vmalloc.c
> > > @@ -2238,12 +2238,14 @@ void __init vm_area_add_early(struct vm_struct *vm)
> > >    */
> > >   void __init vm_area_register_early(struct vm_struct *vm, size_t align)
> > >   {
> > > -	static size_t vm_init_off __initdata;
> > > +	unsigned long vm_start = VMALLOC_START;
> > > +	struct vm_struct *tmp;
> > >   	unsigned long addr;
> > > -	addr = ALIGN(VMALLOC_START + vm_init_off, align);
> > > -	vm_init_off = PFN_ALIGN(addr + vm->size) - VMALLOC_START;
> > > +	for (tmp = vmlist; tmp; tmp = tmp->next)
> > > +		vm_start = (unsigned long)tmp->addr + tmp->size;
> > > +	addr = ALIGN(vm_start, align);
> > >   	vm->addr = (void *)addr;
> > >   	vm_area_add_early(vm);
> > Is there a risk of breaking other architectures? It doesn't look like to
> > me but I thought I'd ask.
> 
> Before this patch, vm_init_off is to record the offset from VMALLOC_START,
> 
> but it use VMALLOC_START as start address on the function
> vm_area_register_early()
> 
> called firstly,  this will cause the BUG_ON.
> 
> With this patch, the most important change is that we choose the start
> address via
> 
> dynamic calculate the 'start' address by traversing the list.
> 
> [wkf@...alhost linux-next]$ git grep vm_area_register_early
> arch/alpha/mm/init.c: vm_area_register_early(&console_remap_vm, PAGE_SIZE);
> arch/x86/xen/p2m.c:     vm_area_register_early(&vm, PMD_SIZE *
> PMDS_PER_MID_PAGE);
> mm/percpu.c:    vm_area_register_early(&vm, PAGE_SIZE);
> [wkf@...alhost linux-next]$ git grep vm_area_add_early
> arch/arm/mm/ioremap.c:  vm_area_add_early(vm);
> arch/arm64/mm/mmu.c:    vm_area_add_early(vma);
> 
> x86/alpha won't call vm_area_add_early(), only arm64 could call both vm_area_add_early()
> and  vm_area_register_early() when this patchset is merged. so it won't break other architectures.

Thanks for checking.

> > Also, instead of always picking the end, could we search for a range
> > that fits?
> 
> We only need a space in vmalloc range,  using end or a range in the middle
> is not different.

I was thinking of making it more future-proof in case one registers a
vm area towards the end of the range. It's fairly easy to pick a range
in the middle now that you are adding a list traversal.

-- 
Catalin