linux-kernel - Re: [PATCH v4 4/6] io_uring: rsrc: avoid use of vmas parameter in pin_user

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Message-ID: <82f26c52-bff7-4f54-b5ea-2fe635045e37@lucifer.local>
Date:   Wed, 19 Apr 2023 18:07:10 +0100
From:   Lorenzo Stoakes <lstoakes@...il.com>
To:     Jens Axboe <axboe@...nel.dk>
Cc:     linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        Andrew Morton <akpm@...ux-foundation.org>,
        Matthew Wilcox <willy@...radead.org>,
        David Hildenbrand <david@...hat.com>,
        Pavel Begunkov <asml.silence@...il.com>,
        io-uring@...r.kernel.org
Subject: Re: [PATCH v4 4/6] io_uring: rsrc: avoid use of vmas parameter in
 pin_user_pages()

On Wed, Apr 19, 2023 at 10:35:12AM -0600, Jens Axboe wrote:
> On 4/18/23 9:49?AM, Lorenzo Stoakes wrote:
> > We are shortly to remove pin_user_pages(), and instead perform the required
> > VMA checks ourselves. In most cases there will be a single VMA so this
> > should caues no undue impact on an already slow path.
> >
> > Doing this eliminates the one instance of vmas being used by
> > pin_user_pages().
>
> First up, please don't just send single patches from a series. It's
> really annoying when you are trying to get the full picture. Just CC the
> whole series, so reviews don't have to look it up separately.
>

Sorry about that, it's hard to strike the right balance between not
spamming people and giving appropriate context, different people have
different opinions about how best to do this, in retrospect would certainly
have been a good idea to include you on all.

> So when you're doing a respin for what I'll mention below and the issue
> that David found, please don't just show us patch 4+5 of the series.

ack

>
> > diff --git a/io_uring/rsrc.c b/io_uring/rsrc.c
> > index 7a43aed8e395..3a927df9d913 100644
> > --- a/io_uring/rsrc.c
> > +++ b/io_uring/rsrc.c
> > @@ -1138,12 +1138,37 @@ static int io_buffer_account_pin(struct io_ring_ctx *ctx, struct page **pages,
> >  	return ret;
> >  }
> >
> > +static int check_vmas_locked(unsigned long addr, unsigned long len)
> > +{
> > +	struct file *file;
> > +	VMA_ITERATOR(vmi, current->mm, addr);
> > +	struct vm_area_struct *vma = vma_next(&vmi);
> > +	unsigned long end = addr + len;
> > +
> > +	if (WARN_ON_ONCE(!vma))
> > +		return -EINVAL;
> > +
> > +	file = vma->vm_file;
> > +	if (file && !is_file_hugepages(file))
> > +		return -EOPNOTSUPP;
> > +
> > +	/* don't support file backed memory */
> > +	for_each_vma_range(vmi, vma, end) {
> > +		if (vma->vm_file != file)
> > +			return -EINVAL;
> > +
> > +		if (file && !vma_is_shmem(vma))
> > +			return -EOPNOTSUPP;
> > +	}
> > +
> > +	return 0;
> > +}
>
> I really dislike this naming. There's no point to doing locked in the
> naming here, it just makes people think it's checking whether the vmas
> are locked. Which is not at all what it does. Because what else would we
> think, there's nothing else in the name that suggests what it is
> actually checking.
>
> Don't put implied locking in the naming, the way to do that is to do
> something ala:
>
> lockdep_assert_held_read(&current->mm->mmap_lock);
>
> though I don't think it's needed here at all, as there's just one caller
> and it's clearly inside. You could even just make a comment instead.
>
> So please rename this to indicate what it's ACTUALLY checking.

ack will do!

>
> --
> Jens Axboe
>