On 22.02.2015 20:13, James Bottomley wrote:
On Sun, 2015-02-22 at 19:07 +0100, Helge Deller wrote:
SHMLBA is 4096 /* (1 << PGSHIFT) */ on hpux.
The following is in <asm-generic/shmparam.h>:
#define SHMLBA PAGE_SIZE /* attach addr a multiple of this */
Shared mappings are handled with
asm/shmparam.h:#define SHM_COLOUR 0x00400000 /* shared mappings colouring */
So how is the sys-v ipc problem fixed? There the user is told to select
an address which is a multiple of SHMLBA. Programs that do this today
will start to break on writeable mappings if we set SHMLBA to PAGE_SIZE
because the colour will be wrong.
The code returns -EINVAL. See arch_get_unmapped_area.
But that's not a solution. Let me try to illustrate: I have an existing
application, it uses sys-v ipc and selects a shmat address based on the
multiple of SHMLBA for a writeable mapping. Today it works.
It will work as well with SHMBLA=4096, if you just use SHM_RND too
(and most applications do have SHM_RND).
man shmat says:
* If shmaddr isn't NULL and SHM_RND is specified in shmflg, the attach occurs at the address equal to shmaddr rounded down to the nearest multiple of SHMLBA.
* Otherwise, shmaddr must be a page-aligned address at which the attach occurs.
So, even here shmaddr is mentioned to be page-aligned (4k), not
SHMLBA-aligned (4M in your case).
I think that part is x86. All the other VI architectures impose their
VI colour constrainst in SHMLBA. We're the odd one out because we have
a huge stride (everyone else is small multiples of pages).
But agree if no applications are affected, we can make the ABI change.
Tomorrow
when you make this change, it fails with -EINVAL. That's breaking an
existing application because chances are the app will just report the
failure and exit.
Tomorrow? This change is *already* implemented in eglibc since a year or
so and I don't see any applications which break because of SHMBLA=4096...
Is eglibc a big enough sample to make the claim that no applications
will be broken?
Try yourself:
https://parisc.wiki.kernel.org/index.php/Debian_Ports_Installation
Just install debian 8.0 (aka unstable for hppa). KDE, Xfce, libreoffice,
and a all others packages just work out of the box.
Helge
--
To unsubscribe from this list: send the line "unsubscribe linux-parisc" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at http://vger.kernel.org/majordomo-info.html
