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Howlett" , Vlastimil Babka , Mike Rapoport , Suren Baghdasaryan , Michal Hocko , Rik van Riel , Harry Yoo , Jann Horn , Lance Yang , linux-mm@kvack.org, linux-kernel@vger.kernel.org, Pan Deng Subject: Re: [PATCH] mm/vma: don't remove VMA from rmap if pgoff unchanged Message-ID: References: <20260925-speed-up-inplace-rmap-v1-1-babc48ce7c83@kernel.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: X-Rspamd-Server: rspam08 X-Rspamd-Queue-Id: 3ECCB1C000A X-Rspam-User: X-Stat-Signature: rg6b81zpq5kyhct5f8e4enpuwfzcd869 X-HE-Tag: 1790614879-148160 X-HE-Meta: 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 JhZ0y9GX RZ/kk+FLTiPavFgoSwKE6Q/p8GbUcxaB6agUzbxJ2Je9ZLyP0TcDnNOD56H0ohNxvSOUg3U2zF/5QQ+hO28AslGQPIUzoo4+M+daI1SYxZ03ZpXJnUyAwxdYzVyO53WfiORXFKFdLAp8IrUCXc8peGVoeC5QRUR7NSKTR8OkUKl/XE6n9Zj4xXZzp76fGRfKiuyrsLin8MOYPsSIriV8QaX1dsqbh1ZVulc58dyAyYRP7TzxKUFHHe7IqW94dVQbspmENqj9Qm53yk/A3ddtoXmin31mq1TAL2R6FjJ0N5uslboU= Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On Mon, Sep 28, 2026 at 04:37:52PM +0100, Pedro Falcato wrote: > On Fri, Sep 25, 2026 at 07:32:20PM +0100, Lorenzo Stoakes (ARM) wrote: > > When updating a VMA, vma_prepare() unconditionally removes it from its rmap > > interval trees under the rmap lock, and vma_complete() reinserts it before > > releasing the lock. > > > > This is wholly unnecessary if its page offset (file rmap) or anonymous page > > offset (anon rmap) is unchanged. > > > > So, track whether they will change in the newly introduced > > vp->anon_pgoff_unchanged and vp->pgoff_unchanged fields, and use them to > > determine whether to remove the VMA or not. > > > > The rmap lock keeps things safe as no rmap walks can concurrently occur > > during the operation. > > > > Additionally, some architectures (arm, parisc, nios2, csky) have dcache > > flush rmap walkers which take only flush_dcache_mmap_lock(), which is > > likewise held across the operation. > > > > It's also necessary to keep the rb_subtree_last field updated in the > > interval tree so implement anon_rmap_tree_update_inplace() and > > mapping_rmap_tree_update_inplace() to do that. > > > > This is done in vma_complete(), after the VMA's range has been updated, so > > in the interim the field may be invalid. However, given the locks described > > above, this cannot be observed until after the state is corrected. > > > > The anonymous rmap is keyed on anon_vma_chains not VMAs, so in those > > instances anon_rmap_tree_update_vma_inplace() iterates over > > vma->anon_vma_chain, invoking anon_rmap_tree_update_inplace() on each one. > > > > For the anon rmap case, with CONFIG_DEBUG_VM_RB set, avc->cached_vma_last > > is also updated in anon_rmap_tree_update_inplace(). > > > > When performing a VMA shrink or a split where the VMA is the lower one, the > > page offset cannot change, so set the flags unconditionally in these cases. > > > > When merging VMAs the page offset is unchanged only in some cases, so > > update init_multi_vma_prep() to set the flags only if the page offsets > > remain the same. > > > > These changes ultimately result in less rmap lock contention. > > I think this asks for numbers? Well I don't have any :) It logically reduces the contention, and that can only be a good thing. Pan had some numbers, I've asked him to re-run against this one. > > > > > Reported-by: Pan Deng > > Closes: https://lore.kernel.org/linux-mm/20260924054301.2330822-1-pan.deng@intel.com/ > > Signed-off-by: Lorenzo Stoakes > > --- > > Signed-off-by: Lorenzo Stoakes (ARM) > > --- > > include/linux/mm.h | 3 +++ > > mm/interval_tree.c | 33 ++++++++++++++++++++++++++++++++ > > mm/vma.c | 40 ++++++++++++++++++++++++++++++++++----- > > mm/vma.h | 2 ++ > > tools/testing/vma/include/stubs.h | 8 ++++++++ > > 5 files changed, 81 insertions(+), 5 deletions(-) > > > > diff --git a/include/linux/mm.h b/include/linux/mm.h > > index 6e71eaa4af3f..94c2eb055716 100644 > > --- a/include/linux/mm.h > > +++ b/include/linux/mm.h > > @@ -4357,6 +4357,8 @@ void mapping_rmap_tree_insert_after(struct vm_area_struct *vma, > > struct address_space *mapping); > > void mapping_rmap_tree_remove(struct vm_area_struct *vma, > > struct address_space *mapping); > > +void mapping_rmap_tree_update_inplace(struct vm_area_struct *vma); > > + > > struct vm_area_struct * > > mapping_rmap_tree_iter_first(struct address_space *mapping, > > pgoff_t pgoff_start, pgoff_t pgoff_last); > > @@ -4374,6 +4376,7 @@ void anon_rmap_tree_insert(struct anon_vma_chain *avc, > > struct anon_vma *anon_vma); > > void anon_rmap_tree_remove(struct anon_vma_chain *avc, > > struct anon_vma *anon_vma); > > +void anon_rmap_tree_update_inplace(struct anon_vma_chain *avc); > > struct anon_vma_chain * > > anon_rmap_tree_iter_first(struct anon_vma *anon_vma, > > pgoff_t pgoff_start, pgoff_t pgoff_last); > > diff --git a/mm/interval_tree.c b/mm/interval_tree.c > > index 7bbbf15cfbf0..eafde5d12ef5 100644 > > --- a/mm/interval_tree.c > > +++ b/mm/interval_tree.c > > @@ -64,6 +64,21 @@ void mapping_rmap_tree_remove(struct vm_area_struct *vma, > > __mapping_rmap_tree_remove(vma, &mapping->i_mmap); > > } > > > > +/** > > + * mapping_rmap_tree_update_inplace() - Update file rmap tree to reflect an > > + * in-place change in a VMA's size. > > + * @vma: The VMA whose size has changed. > > + * > > + * The file rmap lock must be held. > > + * > > + * Invalid to do so if @vma->vm_pgoff has changed. > > + */ > > +void mapping_rmap_tree_update_inplace(struct vm_area_struct *vma) > > +{ > > + /* Propagate all the way up the tree. */ > > + __mapping_rmap_tree_augment.propagate(&vma->shared.rb, NULL); > > +} > > + > > struct vm_area_struct * > > mapping_rmap_tree_iter_first(struct address_space *mapping, > > pgoff_t pgoff_start, pgoff_t pgoff_last) > > @@ -111,6 +126,24 @@ void anon_rmap_tree_remove(struct anon_vma_chain *avc, > > __anon_rmap_tree_remove(avc, &anon_vma->rb_root); > > } > > > > +/** > > + * anon_rmap_tree_update_inplace() - Update anon rmap tree to reflect an > > + * in-place change in the size of @avc's VMA. > > + * @avc: The anon_vma_chain whose VMA's size has changed. > > + * > > + * The anon rmap root lock must be held. > > + * > > + * Invalid to do so if the VMA's anonymous pgoff has changed. > > + */ > > +void anon_rmap_tree_update_inplace(struct anon_vma_chain *avc) > > +{ > > +#ifdef CONFIG_DEBUG_VM_RB > > + avc->cached_vma_last = avc_last_pgoff(avc); > > +#endif > > + /* Propagate all the way up the tree. */ > > + __anon_rmap_tree_augment.propagate(&avc->rb, NULL); > > +} > > + > > struct anon_vma_chain * > > anon_rmap_tree_iter_first(struct anon_vma *anon_vma, > > pgoff_t pgoff_start, pgoff_t pgoff_last) > > diff --git a/mm/vma.c b/mm/vma.c > > index 077e23694143..8b333ec0c958 100644 > > --- a/mm/vma.c > > +++ b/mm/vma.c > > @@ -201,8 +201,15 @@ static void init_multi_vma_prep(struct vma_prepare *vp, > > if (vp->file) > > vp->mapping = vma->vm_file->f_mapping; > > > > - if (vmg && vmg->skip_vma_uprobe) > > + if (!vmg) > > + return; > > + > > + if (vmg->skip_vma_uprobe) > > vp->skip_vma_uprobe = true; > > + if (vma_start_pgoff(vma) == vmg_start_pgoff(vmg)) > > + vp->pgoff_unchanged = true; > > file_pgoff_unchanged perhaps? since we're distinguishing. Yeah makes sense. > > In any case, I would prefer if we moved this logic to mm/interval_tree.c, or > any rmap-related header. I don't love opencoding rmap tree assumptions this > deep in vma.c code. WDYT? Sure that's fair, will put it over there! > > > Rest obviously looks great to me :) Thanks :) > > > + if (vma_start_anon_pgoff(vma) == vmg_start_anon_pgoff(vmg)) > > + vp->anon_pgoff_unchanged = true; > > } > > > > /* > > @@ -331,6 +338,15 @@ anon_rmap_tree_post_update_vma(struct vm_area_struct *vma) > > anon_rmap_tree_insert(avc, avc->anon_vma); > > } > > > > +static void > > +anon_rmap_tree_update_vma_inplace(struct vm_area_struct *vma) > > +{ > > + struct anon_vma_chain *avc; > > + > > + list_for_each_entry(avc, &vma->anon_vma_chain, same_vma) > > + anon_rmap_tree_update_inplace(avc); > > +} > > + > > /* > > * vma_prepare() - Helper function for handling locking VMAs prior to altering > > * @vp: The initialized vma_prepare struct > > @@ -359,14 +375,16 @@ static void vma_prepare(struct vma_prepare *vp) > > > > if (vp->anon_vma) { > > anon_vma_lock_write(vp->anon_vma); > > - anon_rmap_tree_pre_update_vma(vp->vma); > > + if (!vp->anon_pgoff_unchanged) > > + anon_rmap_tree_pre_update_vma(vp->vma); > > if (vp->adj_next) > > anon_rmap_tree_pre_update_vma(vp->adj_next); > > } > > > > if (vp->file) { > > flush_dcache_mmap_lock(vp->mapping); > > - mapping_rmap_tree_remove(vp->vma, vp->mapping); > > + if (!vp->pgoff_unchanged) > > + mapping_rmap_tree_remove(vp->vma, vp->mapping); > > if (vp->adj_next) > > mapping_rmap_tree_remove(vp->adj_next, vp->mapping); > > } > > @@ -387,7 +405,11 @@ static void vma_complete(struct vma_prepare *vp, struct vma_iterator *vmi, > > if (vp->file) { > > if (vp->adj_next) > > mapping_rmap_tree_insert(vp->adj_next, vp->mapping); > > - mapping_rmap_tree_insert(vp->vma, vp->mapping); > > + /* Need only propagate the change inplace. */ > > + if (vp->pgoff_unchanged) > > + mapping_rmap_tree_update_inplace(vp->vma); > > + else > > + mapping_rmap_tree_insert(vp->vma, vp->mapping); > > And perhaps similar for this, hiding update vs re-insert in interval tree > code (via a helper) sounds cleaner. OK will see how that looks! > > -- > Pedro -- Cheers, Lorenzo