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Fewer pages used for mmap descriptors
On 26/09/25(Fri) 12:21, Martin Pieuchot wrote:
> The kernel currently allocates a "mmap descriptor" (AKA UVM vnode
> descriptor) for every vnode. This wastes a lot of memory.
>
> Diff below moves the allocation of such descriptors into uvn_attach()
> and greatly reduces the waste. Note that such descriptors are, like
> vnodes, never freed. Nothing is changed in that regard.
>
> On a small VM with 128M or RAM the difference after boot is huge:
>
> before:
> Name Size Requests Fail InUse Pgreq Pgrel Npage Hiwat Minpg Maxpg Idle
> uvmvnodes 80 1533 0 1533 32 0 32 32 0 8 0
>
> after:
> Name Size Requests Fail InUse Pgreq Pgrel Npage Hiwat Minpg Maxpg Idle
> uvmvnodes 80 74 0 74 2 0 2 2 0 8 0
>
> Ok?
Anyone?
Index: kern/exec_subr.c
===================================================================
RCS file: /cvs/src/sys/kern/exec_subr.c,v
diff -u -p -r1.70 exec_subr.c
--- kern/exec_subr.c 20 Sep 2025 13:53:36 -0000 1.70
+++ kern/exec_subr.c 26 Sep 2025 09:12:28 -0000
@@ -40,7 +40,7 @@
#include <sys/mman.h>
#include <sys/resourcevar.h>
-#include <uvm/uvm_extern.h>
+#include <uvm/uvm_vnode.h>
/*
* new_vmcmd():
Index: kern/vfs_subr.c
===================================================================
RCS file: /cvs/src/sys/kern/vfs_subr.c,v
diff -u -p -r1.332 vfs_subr.c
--- kern/vfs_subr.c 25 Sep 2025 09:05:47 -0000 1.332
+++ kern/vfs_subr.c 26 Sep 2025 09:25:53 -0000
@@ -420,7 +420,6 @@ getnewvnode(enum vtagtype tag, struct mo
((TAILQ_FIRST(listhd = &vnode_hold_list) == NULL) || toggle))) {
splx(s);
vp = pool_get(&vnode_pool, PR_WAITOK | PR_ZERO);
- uvm_vnp_obj_alloc(vp);
RBT_INIT(buf_rb_bufs, &vp->v_bufs_tree);
cache_tree_init(&vp->v_nc_tree);
TAILQ_INIT(&vp->v_cache_dst);
Index: uvm/uvm_extern.h
===================================================================
RCS file: /cvs/src/sys/uvm/uvm_extern.h,v
diff -u -p -r1.184 uvm_extern.h
--- uvm/uvm_extern.h 3 Jun 2025 08:38:17 -0000 1.184
+++ uvm/uvm_extern.h 26 Sep 2025 09:12:03 -0000
@@ -443,9 +443,6 @@ void kmeminit_nkmempages(void);
void kmeminit(void);
extern u_int nkmempages;
-struct vnode;
-struct uvm_object *uvn_attach(struct vnode *, vm_prot_t);
-
struct process;
struct kinfo_vmentry;
int fill_vmmap(struct process *, struct kinfo_vmentry *,
Index: uvm/uvm_vnode.c
===================================================================
RCS file: /cvs/src/sys/uvm/uvm_vnode.c,v
diff -u -p -r1.141 uvm_vnode.c
--- uvm/uvm_vnode.c 25 Sep 2025 09:05:47 -0000 1.141
+++ uvm/uvm_vnode.c 26 Sep 2025 10:18:28 -0000
@@ -137,7 +137,7 @@ uvn_init(void)
struct uvm_object *
uvn_attach(struct vnode *vp, vm_prot_t accessprot)
{
- struct uvm_vnode *uvn = vp->v_uvm;
+ struct uvm_vnode *uvn;
struct vattr vattr;
int oldflags, result;
struct partinfo pi;
@@ -148,7 +148,18 @@ uvn_attach(struct vnode *vp, vm_prot_t a
return NULL;
}
- /* first get a lock on the uvn. */
+ if (vp->v_uvm == NULL) {
+ uvn = pool_get(&uvm_vnode_pool, PR_WAITOK | PR_ZERO);
+ KERNEL_ASSERT_LOCKED();
+ if (vp->v_uvm == NULL) {
+ uvm_obj_init(&uvn->u_obj, &uvm_vnodeops, 0);
+ uvn->u_vnode = vp;
+ vp->v_uvm = uvn;
+ } else
+ pool_put(&uvm_vnode_pool, uvn);
+ }
+
+ uvn = vp->v_uvm;
rw_enter(uvn->u_obj.vmobjlock, RW_WRITE);
while (uvn->u_flags & UVM_VNODE_BLOCKED) {
uvn->u_flags |= UVM_VNODE_WANTED;
@@ -423,10 +434,12 @@ void
uvm_vnp_terminate(struct vnode *vp)
{
struct uvm_vnode *uvn = vp->v_uvm;
- struct uvm_object *uobj = &uvn->u_obj;
+ struct uvm_object *uobj;
int oldflags;
- /* check if it is valid */
+ if (uvn == NULL)
+ return;
+ uobj = &uvn->u_obj;
rw_enter(uobj->vmobjlock, RW_WRITE);
if ((uvn->u_flags & UVM_VNODE_VALID) == 0) {
rw_exit(uobj->vmobjlock);
@@ -1357,13 +1370,14 @@ int
uvm_vnp_uncache(struct vnode *vp)
{
struct uvm_vnode *uvn = vp->v_uvm;
- struct uvm_object *uobj = &uvn->u_obj;
-
- /* lock uvn part of the vnode and check if we need to do anything */
+ struct uvm_object *uobj;
+ if (uvn == NULL)
+ return TRUE;
+ uobj = &uvn->u_obj;
rw_enter(uobj->vmobjlock, RW_WRITE);
if ((uvn->u_flags & UVM_VNODE_VALID) == 0 ||
- (uvn->u_flags & UVM_VNODE_BLOCKED) != 0) {
+ (uvn->u_flags & UVM_VNODE_BLOCKED) != 0) {
rw_exit(uobj->vmobjlock);
return TRUE;
}
@@ -1436,13 +1441,13 @@ void
uvm_vnp_setsize(struct vnode *vp, off_t newsize)
{
struct uvm_vnode *uvn = vp->v_uvm;
- struct uvm_object *uobj = &uvn->u_obj;
+ struct uvm_object *uobj;
KERNEL_ASSERT_LOCKED();
-
+ if (uvn == NULL)
+ return;
+ uobj = &uvn->u_obj;
rw_enter(uobj->vmobjlock, RW_WRITE);
-
- /* lock uvn and check for valid object, and if valid: do it! */
if (uvn->u_flags & UVM_VNODE_VALID) {
/*
@@ -1541,17 +1546,4 @@ uvm_vnp_sync(struct mount *mp)
}
rw_exit_write(&uvn_sync_lock);
-}
-
-void
-uvm_vnp_obj_alloc(struct vnode *vp)
-{
- struct uvm_vnode *uvn;
-
- KASSERT(vp->v_uvm == NULL);
-
- uvn = pool_get(&uvm_vnode_pool, PR_WAITOK | PR_ZERO);
- uvm_obj_init(&uvn->u_obj, &uvm_vnodeops, 0);
- uvn->u_vnode = vp;
- vp->v_uvm = uvn;
}
Index: uvm/uvm_vnode.h
===================================================================
RCS file: /cvs/src/sys/uvm/uvm_vnode.h,v
diff -u -p -r1.22 uvm_vnode.h
--- uvm/uvm_vnode.h 25 Sep 2025 09:05:47 -0000 1.22
+++ uvm/uvm_vnode.h 26 Sep 2025 09:25:00 -0000
@@ -94,7 +94,7 @@ struct uvm_vnode {
*/
#define UVM_VNODE_BLOCKED (UVM_VNODE_ALOCK|UVM_VNODE_DYING|UVM_VNODE_RELKILL)
-void uvm_vnp_obj_alloc(struct vnode *);
+struct uvm_object *uvn_attach(struct vnode *, vm_prot_t);
void uvm_vnp_terminate(struct vnode *);
#endif /* _UVM_UVM_VNODE_H_ */
Fewer pages used for mmap descriptors