Re: [PATCH] pagemap: update documentation

Naoya Horiguchi
2015-07-21T08:36:45Z

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.
>