linux-kernel - Re: [PATCH] mm: Create the new vm_fault

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Message-Id: <20181104083611.GB7829@rapoport-lnx>
Date:   Sun, 4 Nov 2018 10:36:11 +0200
From:   Mike Rapoport <rppt@...ux.ibm.com>
To:     Matthew Wilcox <willy@...radead.org>
Cc:     Souptick Joarder <jrdr.linux@...il.com>, akpm@...ux-foundation.org,
        mhocko@...e.com, dan.j.williams@...el.com,
        kirill.shutemov@...ux.intel.com, pasha.tatashin@...cle.com,
        vbabka@...e.cz, riel@...hat.com, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] mm: Create the new vm_fault_t type

On Sat, Nov 03, 2018 at 05:02:36AM -0700, Matthew Wilcox wrote:
> On Sat, Nov 03, 2018 at 10:35:04AM +0530, Souptick Joarder wrote:
> > Page fault handlers are supposed to return VM_FAULT codes,
> > but some drivers/file systems mistakenly return error
> > numbers. Now that all drivers/file systems have been converted
> > to use the vm_fault_t return type, change the type definition
> > to no longer be compatible with 'int'. By making it an unsigned
> > int, the function prototype becomes incompatible with a function
> > which returns int. Sparse will detect any attempts to return a
> > value which is not a VM_FAULT code.
> 
> 
> > -/* Encode hstate index for a hwpoisoned large page */
> > -#define VM_FAULT_SET_HINDEX(x) ((x) << 12)
> > -#define VM_FAULT_GET_HINDEX(x) (((x) >> 12) & 0xf)
> ...
> > +/* Encode hstate index for a hwpoisoned large page */
> > +#define VM_FAULT_SET_HINDEX(x) ((__force vm_fault_t)((x) << 16))
> > +#define VM_FAULT_GET_HINDEX(x) (((x) >> 16) & 0xf)
> 
> I think it's important to mention in the changelog that these values
> have been changed to avoid conflicts with other VM_FAULT codes.
> 
> > +/**
> > + * typedef vm_fault_t -  __bitwise unsigned int
> > + *
> > + * vm_fault_t is the new unsigned int type to return VM_FAULT
> > + * code by page fault handlers of drivers/file systems. Now if
> > + * any page fault handlers returns non VM_FAULT code instead
> > + * of VM_FAULT code, it will be a mismatch with function
> > + * prototype and sparse will detect it.
> > + */
> 
> The first line should be what the typedef *means*, not repeat the
> compiler's definition.  The rest of the description should be information
> for someone coming to the type for the first time; what you've written
> here is changelog material.
> 
> /**
>  * typedef vm_fault_t - Return type for page fault handlers.
>  *
>  * Page fault handlers return a bitmask of %VM_FAULT values.
>  */
> 
> > +typedef __bitwise unsigned int vm_fault_t;
> > +
> > +/**
> > + * enum - VM_FAULT code
> 
> Can you document an anonymous enum?  I've never tried.  Did you run this
> through 'make htmldocs'?

You cannot document an anonymous enum.
 
> > + * This enum is used to track the VM_FAULT code return by page
> > + * fault handlers.
> 
>  * Page fault handlers return a bitmask of these values to tell the
>  * core VM what happened when handling the fault.
> 

-- 
Sincerely yours,
Mike.