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

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Date:   Sat, 3 Nov 2018 05:02:36 -0700
From:   Matthew Wilcox <willy@...radead.org>
To:     Souptick Joarder <jrdr.linux@...il.com>
Cc:     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 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'?

> + * 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.