[patch] mm: Fix SHM_HUGETLB to work with users in hugetlb_shm_group

[Ravikiran G Thirumalai <kiran@scalex86.org>]

On Wed, Feb 04, 2009 at 05:11:21PM -0500, wli@movementarian.org wrote:
>On Wed, Feb 04, 2009 at 02:04:28PM -0800, Ravikiran G Thirumalai wrote:
>> ...
>> As I see it we have the following options to fix this inconsistency:
>> 1. Do not depend on RLIMIT_MEMLOCK for hugetlb shm mappings.  If a user
>>    has CAP_IPC_LOCK or if user belongs to /proc/sys/vm/hugetlb_shm_group,
>>    he should be able to use shm memory according to shmmax and shmall OR
>> 2. Update the hugetlbpage documentation to mention the resource limit based
>>    limitation, and remove the useless /proc/sys/vm/hugetlb_shm_group sysctl
>> Which one is better?  I am leaning towards 1. and have a patch ready for 1.
>> but I might be missing some historical reason for using RLIMIT_MEMLOCK with
>> SHM_HUGETLB.
>
>We should do (1) because the hugetlb_shm_group and CAP_IPC_LOCK bits
>should both continue to work as they did prior to RLIMIT_MEMLOCK -based
>management of hugetlb. Please make sure the new RLIMIT_MEMLOCK -based
>management still enables hugetlb shm when hugetlb_shm_group and
>CAP_IPC_LOCK don't apply.
>

OK, here's the patch.

Thanks,
Kiran


Fix hugetlb subsystem so that non root users belonging to hugetlb_shm_group
can actually allocate hugetlb backed shm.

Currently non root users cannot even map one large page using SHM_HUGETLB
when they belong to the gid in /proc/sys/vm/hugetlb_shm_group.
This is because allocation size is verified against RLIMIT_MEMLOCK resource
limit even if the user belongs to hugetlb_shm_group.

This patch
1. Fixes hugetlb subsystem so that users with CAP_IPC_LOCK and users
   belonging to hugetlb_shm_group don't need to be restricted with
   RLIMIT_MEMLOCK resource limits
2. If a user has sufficient memlock limit he can still allocate the hugetlb
   shm segment.

Signed-off-by: Ravikiran Thirumalai <kiran@scalex86.org>

---

 Documentation/vm/hugetlbpage.txt |   11 ++++++-----
 fs/hugetlbfs/inode.c             |   18 ++++++++++++------
 include/linux/mm.h               |    2 ++
 mm/mlock.c                       |   11 ++++++++---
 4 files changed, 28 insertions(+), 14 deletions(-)

Index: linux-2.6-tip/fs/hugetlbfs/inode.c
===================================================================
--- linux-2.6-tip.orig/fs/hugetlbfs/inode.c	2009-02-04 15:21:45.000000000 -0800
+++ linux-2.6-tip/fs/hugetlbfs/inode.c	2009-02-04 15:23:19.000000000 -0800
@@ -943,8 +943,15 @@ static struct vfsmount *hugetlbfs_vfsmou
 static int can_do_hugetlb_shm(void)
 {
 	return likely(capable(CAP_IPC_LOCK) ||
-			in_group_p(sysctl_hugetlb_shm_group) ||
-			can_do_mlock());
+			in_group_p(sysctl_hugetlb_shm_group));
+}
+
+static void acct_huge_shm_lock(size_t size, struct user_struct *user)
+{
+	unsigned long pages = (size + PAGE_SIZE - 1) >> PAGE_SHIFT;
+	spin_lock(&shmlock_user_lock);
+	acct_shm_lock(pages, user);
+	spin_unlock(&shmlock_user_lock);
 }
 
 struct file *hugetlb_file_setup(const char *name, size_t size)
@@ -959,12 +966,11 @@ struct file *hugetlb_file_setup(const ch
 	if (!hugetlbfs_vfsmount)
 		return ERR_PTR(-ENOENT);
 
-	if (!can_do_hugetlb_shm())
+	if (can_do_hugetlb_shm())
+		acct_huge_shm_lock(size, user);
+	else if (!user_shm_lock(size, user))
 		return ERR_PTR(-EPERM);
 
-	if (!user_shm_lock(size, user))
-		return ERR_PTR(-ENOMEM);
-
 	root = hugetlbfs_vfsmount->mnt_root;
 	quick_string.name = name;
 	quick_string.len = strlen(quick_string.name);
Index: linux-2.6-tip/include/linux/mm.h
===================================================================
--- linux-2.6-tip.orig/include/linux/mm.h	2009-02-04 15:21:45.000000000 -0800
+++ linux-2.6-tip/include/linux/mm.h	2009-02-04 15:23:19.000000000 -0800
@@ -737,8 +737,10 @@ extern unsigned long shmem_get_unmapped_
 #endif
 
 extern int can_do_mlock(void);
+extern void acct_shm_lock(unsigned long, struct user_struct *);
 extern int user_shm_lock(size_t, struct user_struct *);
 extern void user_shm_unlock(size_t, struct user_struct *);
+extern spinlock_t shmlock_user_lock;
 
 /*
  * Parameter block passed down to zap_pte_range in exceptional cases.
Index: linux-2.6-tip/mm/mlock.c
===================================================================
--- linux-2.6-tip.orig/mm/mlock.c	2009-02-04 15:21:45.000000000 -0800
+++ linux-2.6-tip/mm/mlock.c	2009-02-04 15:23:19.000000000 -0800
@@ -637,7 +637,13 @@ SYSCALL_DEFINE0(munlockall)
  * Objects with different lifetime than processes (SHM_LOCK and SHM_HUGETLB
  * shm segments) get accounted against the user_struct instead.
  */
-static DEFINE_SPINLOCK(shmlock_user_lock);
+DEFINE_SPINLOCK(shmlock_user_lock);
+
+void acct_shm_lock(unsigned long pages, struct user_struct *user)
+{
+	get_uid(user);
+	user->locked_shm += pages;
+}
 
 int user_shm_lock(size_t size, struct user_struct *user)
 {
@@ -653,8 +659,7 @@ int user_shm_lock(size_t size, struct us
 	if (!allowed &&
 	    locked + user->locked_shm > lock_limit && !capable(CAP_IPC_LOCK))
 		goto out;
-	get_uid(user);
-	user->locked_shm += locked;
+	acct_shm_lock(locked, user);
 	allowed = 1;
 out:
 	spin_unlock(&shmlock_user_lock);
Index: linux-2.6-tip/Documentation/vm/hugetlbpage.txt
===================================================================
--- linux-2.6-tip.orig/Documentation/vm/hugetlbpage.txt	2009-02-04 15:21:45.000000000 -0800
+++ linux-2.6-tip/Documentation/vm/hugetlbpage.txt	2009-02-04 15:23:19.000000000 -0800
@@ -147,11 +147,12 @@ used to change the file attributes on hu
 
 Also, it is important to note that no such mount command is required if the
 applications are going to use only shmat/shmget system calls.  Users who
-wish to use hugetlb page via shared memory segment should be a member of
-a supplementary group and system admin needs to configure that gid into
-/proc/sys/vm/hugetlb_shm_group.  It is possible for same or different
-applications to use any combination of mmaps and shm* calls, though the
-mount of filesystem will be required for using mmap calls.
+wish to use hugetlb page via shared memory segment should either have
+sufficient memlock resource limits or, they need to be a member of
+a supplementary group, and system admin needs to configure that gid into
+/proc/sys/vm/hugetlb_shm_group. It is possible for same or different
+applications to use any combination of mmaps and shm* calls, though
+the mount of filesystem will be required for using mmap calls.
 
 *******************************************************************
 

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