linux-kernel - Re: [PATCH] mm: add account_locked

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Date:   Mon, 20 May 2019 11:30:20 -0400
From:   Daniel Jordan <daniel.m.jordan@...cle.com>
To:     Alexey Kardashevskiy <aik@...abs.ru>
Cc:     Daniel Jordan <daniel.m.jordan@...cle.com>,
        akpm@...ux-foundation.org, Alan Tull <atull@...nel.org>,
        Alex Williamson <alex.williamson@...hat.com>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Christoph Lameter <cl@...ux.com>,
        Christophe Leroy <christophe.leroy@....fr>,
        Davidlohr Bueso <dave@...olabs.net>,
        Jason Gunthorpe <jgg@...lanox.com>,
        Mark Rutland <mark.rutland@....com>,
        Michael Ellerman <mpe@...erman.id.au>,
        Moritz Fischer <mdf@...nel.org>,
        Paul Mackerras <paulus@...abs.org>,
        Steve Sistare <steven.sistare@...cle.com>,
        Wu Hao <hao.wu@...el.com>, linux-mm@...ck.org,
        kvm@...r.kernel.org, kvm-ppc@...r.kernel.org,
        linuxppc-dev@...ts.ozlabs.org, linux-fpga@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] mm: add account_locked_vm utility function

On Mon, May 20, 2019 at 04:19:34PM +1000, Alexey Kardashevskiy wrote:
> On 04/05/2019 06:16, Daniel Jordan wrote:
> > locked_vm accounting is done roughly the same way in five places, so
> > unify them in a helper.  Standardize the debug prints, which vary
> > slightly.
> 
> And I rather liked that prints were different and tell precisely which
> one of three each printk is.

I'm not following.  One of three...callsites?  But there were five callsites.

Anyway, I added a _RET_IP_ to the debug print so you can differentiate.

> I commented below but in general this seems working.
> 
> Tested-by: Alexey Kardashevskiy <aik@...abs.ru>

Thanks!  And for the review as well.

> > diff --git a/drivers/vfio/vfio_iommu_spapr_tce.c b/drivers/vfio/vfio_iommu_spapr_tce.c
> > index 6b64e45a5269..d39a1b830d82 100644
> > --- a/drivers/vfio/vfio_iommu_spapr_tce.c
> > +++ b/drivers/vfio/vfio_iommu_spapr_tce.c
> > @@ -34,49 +35,13 @@
> >  static void tce_iommu_detach_group(void *iommu_data,
> >  		struct iommu_group *iommu_group);
> >  
> > -static long try_increment_locked_vm(struct mm_struct *mm, long npages)
> > +static int tce_account_locked_vm(struct mm_struct *mm, unsigned long npages,
> > +				 bool inc)
> >  {
> > -	long ret = 0, locked, lock_limit;
> > -
> >  	if (WARN_ON_ONCE(!mm))
> >  		return -EPERM;
> 
> 
> If this WARN_ON is the only reason for having tce_account_locked_vm()
> instead of calling account_locked_vm() directly, you can then ditch the
> check as I have never ever seen this triggered.

Great, will do.

> > diff --git a/drivers/vfio/vfio_iommu_type1.c b/drivers/vfio/vfio_iommu_type1.c
> > index d0f731c9920a..15ac76171ccd 100644
> > --- a/drivers/vfio/vfio_iommu_type1.c
> > +++ b/drivers/vfio/vfio_iommu_type1.c
> > @@ -273,25 +273,14 @@ static int vfio_lock_acct(struct vfio_dma *dma, long npage, bool async)
> >  		return -ESRCH; /* process exited */
> >  
> >  	ret = down_write_killable(&mm->mmap_sem);
> > -	if (!ret) {
> > -		if (npage > 0) {
> > -			if (!dma->lock_cap) {
> > -				unsigned long limit;
> > -
> > -				limit = task_rlimit(dma->task,
> > -						RLIMIT_MEMLOCK) >> PAGE_SHIFT;
> > -
> > -				if (mm->locked_vm + npage > limit)
> > -					ret = -ENOMEM;
> > -			}
> > -		}
> > +	if (ret)
> > +		goto out;
> 
> 
> A single "goto" to jump just 3 lines below seems unnecessary.

No strong preference here, I'll take out the goto.

> > +int __account_locked_vm(struct mm_struct *mm, unsigned long pages, bool inc,
> > +			struct task_struct *task, bool bypass_rlim)
> > +{
> > +	unsigned long locked_vm, limit;
> > +	int ret = 0;
> > +
> > +	locked_vm = mm->locked_vm;
> > +	if (inc) {
> > +		if (!bypass_rlim) {
> > +			limit = task_rlimit(task, RLIMIT_MEMLOCK) >> PAGE_SHIFT;
> > +			if (locked_vm + pages > limit) {
> > +				ret = -ENOMEM;
> > +				goto out;
> > +			}
> > +		}
> 
> Nit:
> 
> if (!ret)
> 
> and then you don't need "goto out".

Ok, sure.

> > +		mm->locked_vm = locked_vm + pages;
> > +	} else {
> > +		WARN_ON_ONCE(pages > locked_vm);
> > +		mm->locked_vm = locked_vm - pages;
> 
> 
> Can go negative here. Not a huge deal but inaccurate imo.

I hear you, but setting a negative value to zero, as we had done previously,
doesn't make much sense to me.