linux-kernel - Re: [PATCH v41 10/24] mm: Add 'mprotect' hook to struct vm_operations

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Date:   Tue, 17 Nov 2020 21:15:40 +0200
From:   Jarkko Sakkinen <jarkko@...nel.org>
To:     Mel Gorman <mgorman@...hsingularity.net>
Cc:     Dave Hansen <dave.hansen@...el.com>,
        Matthew Wilcox <willy@...radead.org>, x86@...nel.org,
        linux-sgx@...r.kernel.org, linux-kernel@...r.kernel.org,
        Sean Christopherson <sean.j.christopherson@...el.com>,
        linux-mm@...ck.org, Andrew Morton <akpm@...ux-foundation.org>,
        Jethro Beekman <jethro@...tanix.com>,
        andriy.shevchenko@...ux.intel.com, asapek@...gle.com, bp@...en8.de,
        cedric.xing@...el.com, chenalexchen@...gle.com,
        conradparker@...gle.com, cyhanish@...gle.com,
        haitao.huang@...el.com, kai.huang@...el.com, kai.svahn@...el.com,
        kmoy@...gle.com, ludloff@...gle.com, luto@...nel.org,
        nhorman@...hat.com, npmccallum@...hat.com, puiterwijk@...hat.com,
        rientjes@...gle.com, tglx@...utronix.de, yaozhangx@...gle.com,
        mikko.ylinen@...el.com
Subject: Re: [PATCH v41 10/24] mm: Add 'mprotect' hook to struct
 vm_operations_struct

On Mon, Nov 16, 2020 at 10:09:57AM +0000, Mel Gorman wrote:
> On Sun, Nov 15, 2020 at 10:36:51AM -0800, Dave Hansen wrote:
> > On 11/15/20 9:32 AM, Matthew Wilcox wrote:
> > > On Fri, Nov 13, 2020 at 12:01:21AM +0200, Jarkko Sakkinen wrote:
> > >> +++ b/include/linux/mm.h
> > >> @@ -559,6 +559,13 @@ struct vm_operations_struct {
> > >>  	void (*close)(struct vm_area_struct * area);
> > >>  	int (*split)(struct vm_area_struct * area, unsigned long addr);
> > >>  	int (*mremap)(struct vm_area_struct * area);
> > >> +	/*
> > >> +	 * Called by mprotect() to make driver-specific permission
> > >> +	 * checks before mprotect() is finalised.   The VMA must not
> > >> +	 * be modified.  Returns 0 if eprotect() can proceed.
> > >> +	 */

Wonder if this should also document the negative case for the return
value, i.e. -EACCES is returned otherwise.

> > > 
> > > This is the wrong place for this documentation, and it's absurdly
> > > specific to your implementation.  It should be in
> > > Documentation/filesystems/locking.rst.
> > 
> > I'll let you and Mel duke that one out:
> > 
> 
> I suggested placing the comment there to make it clear what the expected
> semantics of the hook was to reduce the chances of abuse or surprises. The
> hook does not affect locking so Documentation/filesystems/locking.rst
> didn't appear appropriate other than maybe adding a note there
> that it doesn't affect locks. The hook also is not expecting any
> filesystems-specific action that I aware of but a note could be added to
> the effect that filesystems should not need to take special action for it.
> Protections on the filesystem level are for the inode, I can't imagine what
> a filesystem would do with a protection change on the page table level
> but maybe I'm not particularly imaginative today.

I try to decipher this in generic context.

In a permission check of a filesystem, truncated pages should be
encapsulated in to the permission decision. It's a just a query.

So maybe I'll add something like:

"This callback does only a permission query, and thus does never
return locked pages."

> -- 
> Mel Gorman
> SUSE Labs

/Jarkko