Re: [PATCH] pagemap: update documentation

[Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>]

On Thu, Jul 16, 2015 at 09:47:42PM +0300, Konstantin Khlebnikov wrote:
> Notes about recent changes.
> 
> Signed-off-by: Konstantin Khlebnikov <khlebnikov@yandex-team.ru>
> ---
>  Documentation/vm/pagemap.txt |   14 ++++++++++++--
>  1 file changed, 12 insertions(+), 2 deletions(-)
> 
> diff --git a/Documentation/vm/pagemap.txt b/Documentation/vm/pagemap.txt
> index 3cfbbb333ea1..aab39aa7dd8f 100644
> --- a/Documentation/vm/pagemap.txt
> +++ b/Documentation/vm/pagemap.txt
> @@ -16,12 +16,17 @@ There are three components to pagemap:
>      * Bits 0-4   swap type if swapped
>      * Bits 5-54  swap offset if swapped
>      * Bit  55    pte is soft-dirty (see Documentation/vm/soft-dirty.txt)
> -    * Bit  56    page exlusively mapped
> +    * Bit  56    page exclusively mapped (since 4.2)
>      * Bits 57-60 zero
> -    * Bit  61    page is file-page or shared-anon
> +    * Bit  61    page is file-page or shared-anon (since 3.5)
>      * Bit  62    page swapped
>      * Bit  63    page present
>  
> +   Since Linux 4.0 only users with the CAP_SYS_ADMIN capability can get PFNs:
> +   for unprivileged users from 4.0 till 4.2 open fails with -EPERM, starting

I'm expecting that this patch will be merged before 4.2 is released, so if that's
right, stating "till 4.2" might be incorrect.

> +   from from 4.2 PFN field is zeroed if user has no CAP_SYS_ADMIN capability.

"from" duplicates ...

Thanks,
Naoya Horiguchi

> +   Reason: information about PFNs helps in exploiting Rowhammer vulnerability.
> +
>     If the page is not present but in swap, then the PFN contains an
>     encoding of the swap file number and the page's offset into the
>     swap. Unmapped pages return a null PFN. This allows determining
> @@ -160,3 +165,8 @@ Other notes:
>  Reading from any of the files will return -EINVAL if you are not starting
>  the read on an 8-byte boundary (e.g., if you sought an odd number of bytes
>  into the file), or if the size of the read is not a multiple of 8 bytes.
> +
> +Before Linux 3.11 pagemap bits 55-60 were used for "page-shift" (which is
> +always 12 at most architectures). Since Linux 3.11 their meaning changes
> +after first clear of soft-dirty bits. Since Linux 4.2 they are used for
> +flags unconditionally.
> 
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