linux-kernel - Re: [PATCH] IB/mlx5: Convert to use vm_map_pages

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Date:   Wed, 28 Aug 2019 08:00:09 +0300
From:   Leon Romanovsky <leon@...nel.org>
To:     Souptick Joarder <jrdr.linux@...il.com>
Cc:     Jason Gunthorpe <jgg@...pe.ca>, Doug Ledford <dledford@...hat.com>,
        linux-rdma@...r.kernel.org, linux-kernel@...r.kernel.org,
        Matthew Wilcox <willy@...radead.org>
Subject: Re: [PATCH] IB/mlx5: Convert to use vm_map_pages_zero()

On Wed, Aug 28, 2019 at 08:57:19AM +0530, Souptick Joarder wrote:
> On Tue, Aug 27, 2019 at 9:19 PM Jason Gunthorpe <jgg@...pe.ca> wrote:
> >
> > On Tue, Aug 27, 2019 at 01:48:57AM +0530, Souptick Joarder wrote:
> > > On Mon, Aug 26, 2019 at 5:50 PM Jason Gunthorpe <jgg@...pe.ca> wrote:
> > > >
> > > > On Mon, Aug 26, 2019 at 01:32:09AM +0530, Souptick Joarder wrote:
> > > > > On Mon, Aug 26, 2019 at 1:13 AM Jason Gunthorpe <jgg@...pe.ca> wrote:
> > > > > >
> > > > > > On Sun, Aug 25, 2019 at 11:37:27AM +0530, Souptick Joarder wrote:
> > > > > > > First, length passed to mmap is checked explicitly against
> > > > > > > PAGE_SIZE.
> > > > > > >
> > > > > > > Second, if vma->vm_pgoff is passed as non zero, it would return
> > > > > > > error. It appears like driver is expecting vma->vm_pgoff to
> > > > > > > be passed as 0 always.
> > > > > >
> > > > > > ? pg_off is not zero
> > > > >
> > > > > Sorry, I mean, driver has a check against non zero to return error -EOPNOTSUPP
> > > > > which means in true scenario driver is expecting vma->vm_pgoff should be passed
> > > > > as 0.
> > > >
> > > > get_index is masking vm_pgoff, it is not 0
> > >
> > > Sorry, I missed this part. Further looking into code,
> > > in mlx5_ib_mmap(), vma_vm_pgoff is used to get command and
> > > inside mlx5_ib_mmap_clock_info_page() entire *dev->mdev->clock_info*
> > > is mapped.
> > >
> > > Consider that, the below modification will only take care of vma length
> > > error check inside vm_map_pages_zero() and an extra check for vma
> > > length is not needed.
> >
> > What is the point of vm_map_pages_zero() Is there some reason we should
> > prefer it for mapping a single page?
>
> vm_map_pages_zero() can be used to map single/ multiple pages both.
> There were drivers previously which either check length and pg_off explicitly
> or didn't check for incorrect value of length /pg_off passed to it at
> all. Calling
> vm_map_pages_zero() in those places were more appropriate as it has
> internal check for both.
>
> Now considering this patch, avoiding an extra check for length
> explicitly is the only
> part which can be avoided if converted to use vm_map_pages_zero()
> because pg_off
> is used in different context ( to identify command). So yes,
> improvement wise convert
> to use vm_map_pages_zero() is not making much difference here.

So let's drop it, please.

Thanks