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SubjectRe: [PATCH] mm,hugetlb: compute page_size_log properly
On Tue, Mar 28, 2017 at 09:55:13AM -0700, Davidlohr Bueso wrote:
> Do we have any consensus here? Keeping SHM_HUGE_* is currently
> winning 2-1. If there are in fact users out there computing the
> value manually, then I am ok with keeping it and properly exporting
> it. Michal?

Well, let's see what it looks like to do that. I went down the rabbit
hole trying to understand why some of the SHM_ flags had the same value
as each other until I realised some of them were internal flags, some
were flags to shmat() and others were flags to shmget(). Hopefully I
disambiguated them nicely in this patch. I also added 8MB and 16GB sizes.
Any more architectures with a pet favourite huge/giant page size we
should add convenience defines for?

diff --git a/include/linux/shm.h b/include/linux/shm.h
index 04e881829625..cd95243efd1a 100644
--- a/include/linux/shm.h
+++ b/include/linux/shm.h
@@ -24,26 +24,13 @@ struct shmid_kernel /* private to the kernel */
struct list_head shm_clist; /* list by creator */
};

-/* shm_mode upper byte flags */
-#define SHM_DEST 01000 /* segment will be destroyed on last detach */
-#define SHM_LOCKED 02000 /* segment will not be swapped */
-#define SHM_HUGETLB 04000 /* segment will use huge TLB pages */
-#define SHM_NORESERVE 010000 /* don't check for reservations */
-
-/* Bits [26:31] are reserved */
-
/*
- * When SHM_HUGETLB is set bits [26:31] encode the log2 of the huge page size.
- * This gives us 6 bits, which is enough until someone invents 128 bit address
- * spaces.
- *
- * Assume these are all power of twos.
- * When 0 use the default page size.
+ * These flags are used internally; they cannot be specified by the user.
+ * They are masked off in newseg(). These values are used by IPC_CREAT
+ * and IPC_EXCL when calling shmget().
*/
-#define SHM_HUGE_SHIFT 26
-#define SHM_HUGE_MASK 0x3f
-#define SHM_HUGE_2MB (21 << SHM_HUGE_SHIFT)
-#define SHM_HUGE_1GB (30 << SHM_HUGE_SHIFT)
+#define SHM_DEST 01000 /* segment will be destroyed on last detach */
+#define SHM_LOCKED 02000 /* segment will not be swapped */

#ifdef CONFIG_SYSVIPC
struct sysv_shm {
diff --git a/include/uapi/linux/shm.h b/include/uapi/linux/shm.h
index 1fbf24ea37fd..44b36cb228d7 100644
--- a/include/uapi/linux/shm.h
+++ b/include/uapi/linux/shm.h
@@ -40,15 +40,34 @@ struct shmid_ds {
/* Include the definition of shmid64_ds and shminfo64 */
#include <asm/shmbuf.h>

-/* permission flag for shmget */
+/* shmget() shmflg values. */
+/* The bottom nine bits are the same as open(2) mode flags */
#define SHM_R 0400 /* or S_IRUGO from <linux/stat.h> */
#define SHM_W 0200 /* or S_IWUGO from <linux/stat.h> */
+/* Bits 9 & 10 are IPC_CREAT and IPC_EXCL */
+#define SHM_HUGETLB (1 << 11) /* segment will use huge TLB pages */
+#define SHM_NORESERVE (1 << 12) /* don't check for reservations */

-/* mode for attach */
-#define SHM_RDONLY 010000 /* read-only access */
-#define SHM_RND 020000 /* round attach address to SHMLBA boundary */
-#define SHM_REMAP 040000 /* take-over region on attach */
-#define SHM_EXEC 0100000 /* execution access */
+/*
+ * When SHM_HUGETLB is set bits [26:31] encode the log2 of the huge page size.
+ * This gives us 6 bits, which is enough until someone invents 128 bit address
+ * spaces. These match MAP_HUGE_SHIFT and MAP_HUGE_MASK.
+ *
+ * Assume these are all powers of two.
+ * When 0 use the default page size.
+ */
+#define SHM_HUGE_SHIFT 26
+#define SHM_HUGE_MASK 0x3f
+#define SHM_HUGE_2MB (21 << SHM_HUGE_SHIFT)
+#define SHM_HUGE_8MB (23 << SHM_HUGE_SHIFT)
+#define SHM_HUGE_1GB (30 << SHM_HUGE_SHIFT)
+#define SHM_HUGE_16GB (34 << SHM_HUGE_SHIFT)
+
+/* shmat() shmflg values */
+#define SHM_RDONLY (1 << 12) /* read-only access */
+#define SHM_RND (1 << 13) /* round attach address to SHMLBA boundary */
+#define SHM_REMAP (1 << 14) /* take-over region on attach */
+#define SHM_EXEC (1 << 15) /* execution access */

/* super user shmctl commands */
#define SHM_LOCK 11
diff --git a/mm/mmap.c b/mm/mmap.c
index 499b988b1639..40b29aca18c1 100644
--- a/mm/mmap.c
+++ b/mm/mmap.c
@@ -1479,7 +1479,7 @@ SYSCALL_DEFINE6(mmap_pgoff, unsigned long, addr, unsigned long, len,
struct user_struct *user = NULL;
struct hstate *hs;

- hs = hstate_sizelog((flags >> MAP_HUGE_SHIFT) & SHM_HUGE_MASK);
+ hs = hstate_sizelog((flags >> MAP_HUGE_SHIFT) & MAP_HUGE_MASK);
if (!hs)
return -EINVAL;

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