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Fewer pages used for mmap descriptors
> Date: Mon, 27 Oct 2025 10:10:40 +0100
> From: Martin Pieuchot <mpi@grenadille.net>
>
> 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?
ok kettenis@
> 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