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From: "Yajun Deng" Message-ID: <405f6c44b4214ff466743ed94d16cb2fbea1b7f3@linux.dev> TLS-Required: No Subject: Re: [PATCH] mm/rmap: make num_children and num_active_vmas update in internally To: "Liam R. Howlett" Cc: akpm@linux-foundation.org, david@redhat.com, lorenzo.stoakes@oracle.com, riel@surriel.com, vbabka@suse.cz, harry.yoo@oracle.com, linux-mm@kvack.org, linux-kernel@vger.kernel.org In-Reply-To: <4ifsfk44so7ychuu57mkbhujjl4lh5bxt2ufdseskunxsle366@3p6oo7qulwef> References: <20250905132019.18915-1-yajun.deng@linux.dev> <4ifsfk44so7ychuu57mkbhujjl4lh5bxt2ufdseskunxsle366@3p6oo7qulwef> X-Migadu-Flow: FLOW_OUT September 5, 2025 at 11:16 PM, "Liam R. Howlett" wrote: >=20 >=20* Yajun Deng [250905 09:21]: >=20 >=20>=20 >=20> If the anon_vma_alloc() is called, the num_children of the parent o= f > > the anon_vma will be updated. But this operation occurs outside of > > anon_vma_alloc(). > >=20=20 >=20> The num_active_vmas are also updated outside of anon_vma. > >=20=20 >=20> Pass the parent of anon_vma to the anon_vma_alloc() and update the > > num_children inside it. > >=20=20 >=20> Introduce anon_vma_attach() and anon_vma_detach() to update > > num_active_vmas with the anon_vma. > >=20=20 >=20> Signed-off-by: Yajun Deng > > --- > > mm/rmap.c | 63 ++++++++++++++++++++++++++++-------------------------= -- > > 1 file changed, 32 insertions(+), 31 deletions(-) > >=20=20 >=20> diff --git a/mm/rmap.c b/mm/rmap.c > > index 34333ae3bd80..2a28edfa5734 100644 > > --- a/mm/rmap.c > > +++ b/mm/rmap.c > > @@ -86,15 +86,21 @@ > > static struct kmem_cache *anon_vma_cachep; > > static struct kmem_cache *anon_vma_chain_cachep; > >=20=20 >=20> -static inline struct anon_vma *anon_vma_alloc(void) > > +static inline struct anon_vma *anon_vma_alloc(struct anon_vma *pare= nt) > > { > > struct anon_vma *anon_vma; > >=20=20 >=20> anon_vma =3D kmem_cache_alloc(anon_vma_cachep, GFP_KERNEL); > > - if (anon_vma) { > > - atomic_set(&anon_vma->refcount, 1); > > - anon_vma->num_children =3D 0; > > - anon_vma->num_active_vmas =3D 0; > > + if (!anon_vma) > > + return NULL; > > + > > + atomic_set(&anon_vma->refcount, 1); > > + anon_vma->num_children =3D 0; > > + anon_vma->num_active_vmas =3D 0; > > + if (parent) { > > + anon_vma->parent =3D parent; > > + anon_vma->root =3D parent->root; > > + } else { > > anon_vma->parent =3D anon_vma; > > /* > > * Initialise the anon_vma root to point to itself. If called > > @@ -102,6 +108,7 @@ static inline struct anon_vma *anon_vma_alloc(vo= id) > > */ > > anon_vma->root =3D anon_vma; > > } > > + anon_vma->parent->num_children++; > >=20=20 >=20> return anon_vma; > > } > > @@ -146,6 +153,19 @@ static void anon_vma_chain_free(struct anon_vma= _chain *anon_vma_chain) > > kmem_cache_free(anon_vma_chain_cachep, anon_vma_chain); > > } > >=20=20 >=20> +static inline void anon_vma_attach(struct vm_area_struct *vma, > > + struct anon_vma *anon_vma) > > +{ > > + vma->anon_vma =3D anon_vma; > > + vma->anon_vma->num_active_vmas++; > > +} > > + > > +static inline void anon_vma_detach(struct vm_area_struct *vma) > > +{ > > + vma->anon_vma->num_active_vmas--; > > + vma->anon_vma =3D NULL; > > +} > > + > >=20 >=20It is a bit odd that you are setting a vma value with the prefix of > anon_vma. Surely there is a better name: vma_attach_anon() ? And since > this is editing the vma, should it be in rmap.c or vma.h? >=20 I=20will move them to vma.h. > >=20 >=20> static void anon_vma_chain_link(struct vm_area_struct *vma, > > struct anon_vma_chain *avc, > > struct anon_vma *anon_vma) > > @@ -198,10 +218,9 @@ int __anon_vma_prepare(struct vm_area_struct *v= ma) > > anon_vma =3D find_mergeable_anon_vma(vma); > > allocated =3D NULL; > > if (!anon_vma) { > > - anon_vma =3D anon_vma_alloc(); > > + anon_vma =3D anon_vma_alloc(NULL); > >=20 >=20I don't love passing NULL for parent, it's two if statements to do th= e > same work as before - we already know that parent is NULL by this point= , > but we call a function to check it again. >=20 I=20will add a wapper function. > >=20 >=20> if (unlikely(!anon_vma)) > > goto out_enomem_free_avc; > > - anon_vma->num_children++; /* self-parent link for new root */ > > allocated =3D anon_vma; > > } > >=20=20 >=20> @@ -209,9 +228,8 @@ int __anon_vma_prepare(struct vm_area_struct *= vma) > > /* page_table_lock to protect against threads */ > > spin_lock(&mm->page_table_lock); > > if (likely(!vma->anon_vma)) { > > - vma->anon_vma =3D anon_vma; > > + anon_vma_attach(vma, anon_vma); > > anon_vma_chain_link(vma, avc, anon_vma); > > - anon_vma->num_active_vmas++; > > allocated =3D NULL; > > avc =3D NULL; > > } > > @@ -306,10 +324,8 @@ int anon_vma_clone(struct vm_area_struct *dst, = struct vm_area_struct *src) > > if (!dst->anon_vma && src->anon_vma && > > anon_vma->num_children < 2 && > > anon_vma->num_active_vmas =3D=3D 0) > > - dst->anon_vma =3D anon_vma; > > + anon_vma_attach(dst, anon_vma); > > } > > - if (dst->anon_vma) > > - dst->anon_vma->num_active_vmas++; > > unlock_anon_vma_root(root); > > return 0; > >=20 >=20anon_vma_clone() has a goto label of enomem_failure that needs to be > handled correctly. Looks like you have to avoid zeroing dst before > unlink_anon_vmas(vma) there. >=20 Yes,=20it's an error. > >=20 >=20> @@ -356,31 +372,22 @@ int anon_vma_fork(struct vm_area_struct *vma,= struct vm_area_struct *pvma) > > return 0; > >=20=20 >=20> /* Then add our own anon_vma. */ > > - anon_vma =3D anon_vma_alloc(); > > + anon_vma =3D anon_vma_alloc(pvma->anon_vma); > > if (!anon_vma) > > goto out_error; > > - anon_vma->num_active_vmas++; > > avc =3D anon_vma_chain_alloc(GFP_KERNEL); > > if (!avc) > > goto out_error_free_anon_vma; > >=20 >=20At this point anon_vma has a parent set and the parent->num_children+= +, > but vma->anon_vma !=3D anon_vma yet. If avc fails here, we will put the > anon_vma but leave the parent with num_children incremented, since > unlink_anon_vmas() will not find anything. >=20 Yes,=20it's an error. > >=20 >=20> - /* > > - * The root anon_vma's rwsem is the lock actually used when we > > - * lock any of the anon_vmas in this anon_vma tree. > > - */ > >=20 >=20This information is lost when adding the parent passthrough. >=20 I'll=20add it back. > >=20 >=20> - anon_vma->root =3D pvma->anon_vma->root; > > - anon_vma->parent =3D pvma->anon_vma; > > /* > > * With refcounts, an anon_vma can stay around longer than the > > * process it belongs to. The root anon_vma needs to be pinned until > > * this anon_vma is freed, because the lock lives in the root. > > */ > > get_anon_vma(anon_vma->root); > > - /* Mark this anon_vma as the one where our new (COWed) pages go. *= / > > - vma->anon_vma =3D anon_vma; > > + anon_vma_attach(vma, anon_vma); > >=20 >=20So now we are in the same situation, we know what we need to do with = the > parent, but we have to run through another if statement to get it to > happen instead of assigning it. >=20 Some=20code like it. init_tg_rt_entry() has two callers. One has a parent, the other does not. > >=20 >=20> anon_vma_lock_write(anon_vma); > > anon_vma_chain_link(vma, avc, anon_vma); > > - anon_vma->parent->num_children++; > > anon_vma_unlock_write(anon_vma); > >=20=20 >=20> return 0; > > @@ -419,15 +426,9 @@ void unlink_anon_vmas(struct vm_area_struct *vm= a) > > list_del(&avc->same_vma); > > anon_vma_chain_free(avc); > > } > > - if (vma->anon_vma) { > > - vma->anon_vma->num_active_vmas--; > > + if (vma->anon_vma) > > + anon_vma_detach(vma); > >=20=20 >=20> - /* > > - * vma would still be needed after unlink, and anon_vma will be pre= pared > > - * when handle fault. > > - */ > >=20 >=20It is still worth keeping the comment here too. >=20 Okay. >=20>=20 >=20> - vma->anon_vma =3D NULL; > > - } > > unlock_anon_vma_root(root); > >=20=20 >=20> /* > > --=20 >=20> 2.25.1 > > >