* IPC SHM alignment on ARMv7
2013-01-31 13:47 ` Russell King - ARM Linux
@ 2013-01-31 14:08 ` Alexander Kartashov
2013-01-31 15:35 ` Alexander Kartashov
1 sibling, 0 replies; 4+ messages in thread
From: Alexander Kartashov @ 2013-01-31 14:08 UTC (permalink / raw)
To: linux-arm-kernel
On 01/31/2013 05:47 PM, Russell King - ARM Linux wrote:
> Err, no. Try again - this is the mainline kernel:
I'm sorry, I forgot to note that it's very old code,
it's from the tglx/history.git that you may find here:
http://git.kernel.org/?p=linux/kernel/git/tglx/history.git;a=commit;h=4197692eef113eeb8e3e413cc70993a5e667e5b8
The piece of code I'm telling about hasn't changed
very much since then, it looks in the following way
in that commit:
25 unsigned long
26 arch_get_unmapped_area(struct file *filp, unsigned long addr,
27 unsigned long len, unsigned long pgoff, unsigned long flags)
28 {
29 struct mm_struct *mm = current->mm;
30 struct vm_area_struct *vma;
31 unsigned long start_addr;
32 #ifdef CONFIG_CPU_V6
33 unsigned int cache_type;
34 int do_align = 0, aliasing = 0;
35
36 /*
37 * We only need to do colour alignment if either the I or D
38 * caches alias. This is indicated by bits 9 and 21 of the
39 * cache type register.
40 */
41 cache_type = read_cpuid(CPUID_CACHETYPE);
42 if (cache_type != read_cpuid(CPUID_ID)) {
43 aliasing = (cache_type | cache_type >> 12) & (1 << 9);
44 if (aliasing)
45 do_align = filp || flags & MAP_SHARED;
46 }
The last commit that touches the file is
394ef6403abc36900d9303395a49a72d32666f2a
(http://git.kernel.org/?p=linux/kernel/git/torvalds/linux.git;a=commit;h=394ef6403abc36900d9303395a49a72d32666f2a)
and currently the lines look in the following way:
[arch/arm/mm/mmap.c]
[...]
54 unsigned long
55 arch_get_unmapped_area(struct file *filp, unsigned long addr,
56 unsigned long len, unsigned long pgoff, unsigned long
flags)
57 {
58 struct mm_struct *mm = current->mm;
59 struct vm_area_struct *vma;
60 int do_align = 0;
61 int aliasing = cache_is_vipt_aliasing();
62 struct vm_unmapped_area_info info;
63
64 /*
65 * We only need to do colour alignment if either the I or D
66 * caches alias.
67 */
68 if (aliasing)
69 do_align = filp || (flags & MAP_SHARED);
Particularly, the lines
68 if (aliasing)
69 do_align = filp || (flags & MAP_SHARED);
hasn't changed since then.
I intentionally provided a reference to a historical branch
to point out that this code hasn't changed.
--
Sincerely yours,
Alexander Kartashov
Intern
Core team
www.parallels.com
Skype: aleksandr.kartashov
Email: alekskartashov at parallels.com
^ permalink raw reply [flat|nested] 4+ messages in thread* IPC SHM alignment on ARMv7
2013-01-31 13:47 ` Russell King - ARM Linux
2013-01-31 14:08 ` Alexander Kartashov
@ 2013-01-31 15:35 ` Alexander Kartashov
1 sibling, 0 replies; 4+ messages in thread
From: Alexander Kartashov @ 2013-01-31 15:35 UTC (permalink / raw)
To: linux-arm-kernel
On 01/31/2013 05:47 PM, Russell King - ARM Linux wrote:
> Err, no. Try again
OK, let me be more specific. The following code is a simplified
version of the test in the CRIU test suite that checks IPC SHM
checkpoint/restoration work correctly:
#include <stdio.h>
#include <sys/ipc.h>
int main()
{
int loop;
int shmid = shmget(0x94646337, 40960, IPC_CREAT);
if (shmid < 0) {
perror("Failed to get a SHM descriptor");
return 1;
}
void *addr1, *addr2;
addr1 = shmat(shmid, 0, 0);
if (addr1 == (void*)-1) {
perror("Failed to attach the SHM segment");
return 2;
}
printf("Attached to %p\n", addr1);
*((int*)addr1) = 1;
shmdt(addr1);
addr2 = shmat(shmid, addr1, 0);
if (addr2 == (void*)-1) {
perror("Failed to re-attach the SHM");
printf("The SHM segment was formerly attached to %p\n", addr1);
return 3;
}
if (addr2 != addr1) {
printf("The SHM segment address changed: was %p, now %p\n",
addr1, addr2);
return 4;
}
return 0;
}
I'm running this code in the Linux 3.7.5:
root at crtest:~/ipc-fail# cat /proc/version
Linux version 3.7.5 (alex at alex-pc) (gcc version 4.6.3 20120201
(prerelease) (crosstool-NG linaro-1.13.1-2012.02-20120222 - Linaro GCC
2012.02) ) #1 SMP Thu Jan 31 19:01:37 MSK 2013
I'm running the kernel in the QEMU model of the board RM Versatile
Express for Cortex-A9.
I use the config vexpress_defconfig to compile the kernel.
The test usually outputs something like:
root at crtest:~/ipc-fail# ./ipc-fail
Attached to 0x76e22000
Failed to re-attach the SHM: Invalid argument
The SHM segment was formely attached to 0x76e22000
root at crtest:~/ipc-fail# ./ipc-fail
Attached to 0x76df6000
Failed to re-attach the SHM: Invalid argument
The SHM segment was formely attached to 0x76df6000
However it sometimes succeeds:
root@crtest:~/ipc-fail# ./ipc-fail
Attached to 0x76e94000
As you can see the test fails when the address returned by
the function shmat() isn't SHMLBA-aligned.
I think that this is caused by the fact that the kernel function do_shmat()
requires the argument shmaddr to be SHMLBA-aligned:
[ipc/shm.c]
[...]
958 long do_shmat(int shmid, char __user *shmaddr, int shmflg, ulong
*raddr,
959 unsigned long shmlba)
960 {
[...]
974
975 err = -EINVAL;
976 if (shmid < 0)
977 goto out;
978 else if ((addr = (ulong)shmaddr)) {
979 if (addr & (shmlba - 1)) {
980 if (shmflg & SHM_RND)
981 addr &= ~(shmlba - 1); /* round down */
982 else
983 #ifndef __ARCH_FORCE_SHMLBA
984 if (addr & ~PAGE_MASK)
985 #endif
986 goto out;
However, it can't guarantee that its returned address is SHMLBA-aligned
because the function arch_get_unmapped_area():
[arch/arm/mm/mmap.c]
[...]
66 unsigned long
67 arch_get_unmapped_area(struct file *filp, unsigned long addr,
68 unsigned long len, unsigned long pgoff, unsigned long flags)
69 {
70 struct mm_struct *mm = current->mm;
71 struct vm_area_struct *vma;
72 unsigned long start_addr;
73 int do_align = 0;
74 int aliasing = cache_is_vipt_aliasing();
75
76 /*
77 * We only need to do colour alignment if either the I or D
78 * caches alias.
79 */
80 if (aliasing)
81 do_align = filp || (flags & MAP_SHARED);
[...]
114 full_search:
115 if (do_align)
116 addr = COLOUR_ALIGN(addr, pgoff);
117 else
118 addr = PAGE_ALIGN(addr);
119
120 for (vma = find_vma(mm, addr); ; vma = vma->vm_next) {
121 /* At this point: (!vma || addr < vma->vm_end). */
122 if (TASK_SIZE - len < addr) {
123 /*
124 * Start a new search - just in case we missed
125 * some holes.
126 */
127 if (start_addr != TASK_UNMAPPED_BASE) {
128 start_addr = addr = TASK_UNMAPPED_BASE;
129 mm->cached_hole_size = 0;
130 goto full_search;
131 }
132 return -ENOMEM;
133 }
134 if (!vma || addr + len <= vma->vm_start) {
135 /*
136 * Remember the place where we stopped the search:
137 */
138 mm->free_area_cache = addr + len;
139 return addr;
140 }
141 if (addr + mm->cached_hole_size < vma->vm_start)
142 mm->cached_hole_size = vma->vm_start - addr;
143 addr = vma->vm_end;
144 if (do_align)
145 addr = COLOUR_ALIGN(addr, pgoff);
146 }
147 }
aligns the returned address on a SHMLBA-boundary only if I or D caches alias
as the comment reads:
76 /*
77 * We only need to do colour alignment if either the I or D
78 * caches alias.
79 */
that isn't true for ARMv7.
So the question is whether it's possible to align a SHM segement
on a SHMLBA boundary unconditionally.
--
Sincerely yours,
Alexander Kartashov
Intern
Core team
www.parallels.com
Skype: aleksandr.kartashov
Email: alekskartashov at parallels.com
^ permalink raw reply [flat|nested] 4+ messages in thread