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? 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
