linux-kernel - Re: [PATCH] tpm: Detach page allocation from tpm

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Date:   Thu, 03 Oct 2019 08:50:54 -0400
From:   Mimi Zohar <zohar@...ux.ibm.com>
To:     Jarkko Sakkinen <jarkko.sakkinen@...ux.intel.com>
Cc:     James Bottomley <James.Bottomley@...senPartnership.com>,
        linux-integrity@...r.kernel.org,
        Jerry Snitselaar <jsnitsel@...hat.com>,
        Sumit Garg <sumit.garg@...aro.org>,
        Stefan Berger <stefanb@...ux.vnet.ibm.com>,
        Peter Huewe <peterhuewe@....de>,
        Jason Gunthorpe <jgg@...pe.ca>, Arnd Bergmann <arnd@...db.de>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        open list <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] tpm: Detach page allocation from tpm_buf

On Thu, 2019-10-03 at 14:35 +0300, Jarkko Sakkinen wrote:
> On Wed, Oct 02, 2019 at 08:41:45AM -0400, Mimi Zohar wrote:
> > On Fri, 2019-09-27 at 16:06 +0300, Jarkko Sakkinen wrote:
> > > On Wed, Sep 25, 2019 at 10:03:46AM -0400, James Bottomley wrote:
> > > > On Wed, 2019-09-25 at 16:48 +0300, Jarkko Sakkinen wrote:
> > > > [...]
> > > > > +	data_page = alloc_page(GFP_HIGHUSER);
> > > > > +	if (!data_page)
> > > > > +		return -ENOMEM;
> > > > > +
> > > > > +	data_ptr = kmap(data_page);
> > > > 
> > > > I don't think this is such a good idea.  On 64 bit it's no different
> > > > from GFP_KERNEL and on 32 bit where we do have highmem, kmap space is
> > > > at a premium, so doing a highmem allocation + kmap is more wasteful of
> > > > resources than simply doing GFP_KERNEL.  In general, you should only do
> > > > GFP_HIGHMEM if the page is going to be mostly used by userspace, which
> > > > really isn't the case here.
> > > 
> > > Changing that in this commit would be wrong even if you are right.
> > > After this commit has been applied it is somewhat easier to make
> > > best choices for allocation in each call site (probably most will
> > > end up using stack).
> > 
> > Agreed, but it could be a separate patch, prior to this one.  Why
> > duplicate the problem all over only to change it later?
> 
> What problem exactly it is duplicating? The existing allocation
> scheme here works correctly.

In the current code "alloc_page(GFP_HIGHUSER)" exists in a single
function.  With this patch, "alloc_page(GFP_HIGHUSER)" is duplicated
24 times.  If it is incorrect and we shouldn't be using GFP_HIGHUSER,
as James said, then why duplicate it 24 times?  Fix it as a separate
patch first, that could be backported if needed, and then make the
change.

Mimi