On Sun, 2015-02-22 at 20:16 +0100, Helge Deller wrote: > 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. I know, I do, but that's sort of expected: most modern linux apps use mmap. It's the older stuff that uses sys-v ipc, but perhaps for us this doesn't matter. James -- 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
