Stop playing silly games with the VM_ACCOUNT flag

The mmap_region() code would temporarily set the VM_ACCOUNT flag for
anonymous shared mappings just to inform shmem_zero_setup() that it
should enable accounting for the resulting shm object.  It would then
clear the flag after calling ->mmap (for the /dev/zero case) or doing
shmem_zero_setup() (for the MAP_ANON case).

This just resulted in vma merge issues, but also made for just
unnecessary confusion.  Use the already-existing VM_NORESERVE flag for
this instead, and let shmem_{zero|file}_setup() just figure it out from
that.

This also happens to make it obvious that the new DRI2 GEM layer uses a
non-reserving backing store for its object allocation - which is quite
possibly not intentional.  But since I didn't want to change semantics
in this patch, I left it alone, and just updated the caller to use the
new flag semantics.

Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

4 files changed, 29 insertions, 27 deletions

diff --git a/drivers/gpu/drm/drm_gem.c b/drivers/gpu/drm/drm_gem.c
index 9da581452874..6915fb82d0b0 100644
--- a/drivers/gpu/drm/drm_gem.c
+++ b/drivers/gpu/drm/drm_gem.c
@@ -136,7 +136,7 @@ drm_gem_object_alloc(struct drm_device *dev, size_t size)
         obj = kcalloc(1, sizeof(*obj), GFP_KERNEL);
 
         obj->dev = dev;
-        obj->filp = shmem_file_setup("drm mm object", size, 0);
+        obj->filp = shmem_file_setup("drm mm object", size, VM_NORESERVE);
         if (IS_ERR(obj->filp)) {
                 kfree(obj);
                 return NULL;
diff --git a/ipc/shm.c b/ipc/shm.c
index a9e09ad2263e..c0a021f7f41a 100644
--- a/ipc/shm.c
+++ b/ipc/shm.c
@@ -368,14 +368,14 @@ static int newseg(struct ipc_namespace *ns, struct ipc_params *params)
                 file = hugetlb_file_setup(name, size);
                 shp->mlock_user = current_user();
         } else {
-                int acctflag = VM_ACCOUNT;
+                int acctflag = 0;
                 /*
                  * Do not allow no accounting for OVERCOMMIT_NEVER, even
                  * if it's asked for.
                  */
                 if  ((shmflg & SHM_NORESERVE) &&
                                 sysctl_overcommit_memory != OVERCOMMIT_NEVER)
-                        acctflag = 0;
+                        acctflag = VM_NORESERVE;
                 file = shmem_file_setup(name, size, acctflag);
         }
         error = PTR_ERR(file);
diff --git a/mm/mmap.c b/mm/mmap.c
index c581df14d0de..214b6a258eeb 100644
--- a/mm/mmap.c
+++ b/mm/mmap.c
@@ -1090,6 +1090,15 @@ int vma_wants_writenotify(struct vm_area_struct *vma)
                 mapping_cap_account_dirty(vma->vm_file->f_mapping);
 }
 
+/*
+ * We account for memory if it's a private writeable mapping,
+ * and VM_NORESERVE wasn't set.
+ */
+static inline int accountable_mapping(unsigned int vm_flags)
+{
+        return (vm_flags & (VM_NORESERVE | VM_SHARED | VM_WRITE)) == VM_WRITE;
+}
+
 unsigned long mmap_region(struct file *file, unsigned long addr,
                           unsigned long len, unsigned long flags,
                           unsigned int vm_flags, unsigned long pgoff,
@@ -1117,23 +1126,24 @@ munmap_back:
         if (!may_expand_vm(mm, len >> PAGE_SHIFT))
                 return -ENOMEM;
 
-        if (flags & MAP_NORESERVE)
+        /*
+         * Set 'VM_NORESERVE' if we should not account for the
+         * memory use of this mapping. We only honor MAP_NORESERVE
+         * if we're allowed to overcommit memory.
+         */
+        if ((flags & MAP_NORESERVE) && sysctl_overcommit_memory != OVERCOMMIT_NEVER)
+                vm_flags |= VM_NORESERVE;
+        if (!accountable)
                 vm_flags |= VM_NORESERVE;
 
-        if (accountable && (!(flags & MAP_NORESERVE) ||
-                            sysctl_overcommit_memory == OVERCOMMIT_NEVER)) {
-                if (vm_flags & VM_SHARED) {
-                        /* Check memory availability in shmem_file_setup? */
-                        vm_flags |= VM_ACCOUNT;
-                } else if (vm_flags & VM_WRITE) {
-                        /*
-                         * Private writable mapping: check memory availability
-                         */
-                        charged = len >> PAGE_SHIFT;
-                        if (security_vm_enough_memory(charged))
-                                return -ENOMEM;
-                        vm_flags |= VM_ACCOUNT;
-                }
+        /*
+         * Private writable mapping: check memory availability
+         */
+        if (accountable_mapping(vm_flags)) {
+                charged = len >> PAGE_SHIFT;
+                if (security_vm_enough_memory(charged))
+                        return -ENOMEM;
+                vm_flags |= VM_ACCOUNT;
         }
 
         /*
@@ -1184,14 +1194,6 @@ munmap_back:
                         goto free_vma;
         }
 
-        /* We set VM_ACCOUNT in a shared mapping's vm_flags, to inform
-         * shmem_zero_setup (perhaps called through /dev/zero's ->mmap)
-         * that memory reservation must be checked; but that reservation
-         * belongs to shared memory object, not to vma: so now clear it.
-         */
-        if ((vm_flags & (VM_SHARED|VM_ACCOUNT)) == (VM_SHARED|VM_ACCOUNT))
-                vma->vm_flags &= ~VM_ACCOUNT;
-
         /* Can addr have changed??
          *
          * Answer: Yes, several device drivers can do it in their
diff --git a/mm/shmem.c b/mm/shmem.c
index 5d0de96c9789..19d566ccdeea 100644
--- a/mm/shmem.c
+++ b/mm/shmem.c
@@ -2628,7 +2628,7 @@ struct file *shmem_file_setup(char *name, loff_t size, unsigned long flags)
                 goto close_file;
 
 #ifdef CONFIG_SHMEM
-        SHMEM_I(inode)->flags = flags & VM_ACCOUNT;
+        SHMEM_I(inode)->flags = (flags & VM_NORESERVE) ? 0 : VM_ACCOUNT;
 #endif
         d_instantiate(dentry, inode);
         inode->i_size = size;