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Howlett" , Andrew Morton , Vlastimil Babka , Shakeel Butt , David Hildenbrand , Rik van Riel , Harry Yoo , Jann Horn , Mike Rapoport , Michal Hocko , Pedro Falcato , Chris Li , Barry Song , linux-mm@kvack.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH 1/8] mm/rmap: improve anon_vma_clone(), unlink_anon_vmas() comments, add asserts Message-ID: <13c66c95-ca0d-4711-b755-676ec4066811@lucifer.local> References: <3acc90a8613d5e2ea8882d60b5677228e6fe624d.1765970117.git.lorenzo.stoakes@oracle.com> Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: X-ClientProxiedBy: LO4P265CA0143.GBRP265.PROD.OUTLOOK.COM (2603:10a6:600:2c4::19) To DM4PR10MB8218.namprd10.prod.outlook.com (2603:10b6:8:1cc::16) MIME-Version: 1.0 X-MS-PublicTrafficType: Email X-MS-TrafficTypeDiagnostic: DM4PR10MB8218:EE_|LV3PR10MB7724:EE_ X-MS-Office365-Filtering-Correlation-Id: ba44298b-598c-45f3-d2a7-08de4d22b2c6 X-MS-Exchange-SenderADCheck: 1 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On Fri, Dec 19, 2025 at 10:22 AM Liam R. Howlett > wrote: > > > > * Lorenzo Stoakes [251217 07:27]: > > > Add kdoc comments, describe exactly what these functinos are used for in > > > detail, pointing out importantly that the anon_vma_clone() !dst->anon_vma > > > && src->anon_vma dance is ONLY for fork. > > > > > > Both are confusing functions that will be refactored in a subsequent patch > > > but the first stage is establishing documentation and some invariatns. > > > > > > Add some basic CONFIG_DEBUG_VM asserts that help document expected state, > > > specifically: > > > > > > anon_vma_clone() > > > - mmap write lock held. > > > - We do nothing if src VMA is not faulted. > > > - The destination VMA has no anon_vma_chain yet. > > > - We are always operating on the same active VMA (i.e. vma->anon-vma). > > nit: s/vma->anon-vma/vma->anon_vma Thanks will correct. > > > > - If not forking, must operate on the same mm_struct. > > > > > > unlink_anon_vmas() > > > - mmap lock held (read on unmap downgraded). > > Out of curiosity I looked for the place where unlink_anon_vmas() is > called with mmap_lock downgraded to read but could not find it. Could > you please point me to it? In brk() we call: -> do_vmi_align_munmap() -> ... (below) On munmap() we call: -> __vm_munmap() -> do_vmi_munmap() -> do_vmi_align_munmap() -> ... (below) On mremap() when shrinking a VMA in place we call: -> mremap_at() -> shrink_vma() -> do_vmi_munmap() -> do_vmi_align_munmap() -> ... (below) And the ... is: -> vms_complete_munmap_vmas() [ does downgrade since vms->unlock ] -> vms_clear_ptes() -> free_pgtables() I've improved the comment anyway to make it a little clearer. > > > > - That unfaulted VMAs are no-ops. > > > > > > Signed-off-by: Lorenzo Stoakes > > > > Reviewed-by: Liam R. Howlett > > > > > --- > > > mm/rmap.c | 82 +++++++++++++++++++++++++++++++++++++++++++------------ > > > 1 file changed, 64 insertions(+), 18 deletions(-) > > > > > > diff --git a/mm/rmap.c b/mm/rmap.c > > > index d6799afe1114..0e34c0a69fbc 100644 > > > --- a/mm/rmap.c > > > +++ b/mm/rmap.c > > > @@ -257,30 +257,60 @@ static inline void unlock_anon_vma_root(struct anon_vma *root) > > > up_write(&root->rwsem); > > > } > > > > > > -/* > > > - * Attach the anon_vmas from src to dst. > > > - * Returns 0 on success, -ENOMEM on failure. > > > - * > > > - * anon_vma_clone() is called by vma_expand(), vma_merge(), __split_vma(), > > > - * copy_vma() and anon_vma_fork(). The first four want an exact copy of src, > > > - * while the last one, anon_vma_fork(), may try to reuse an existing anon_vma to > > > - * prevent endless growth of anon_vma. Since dst->anon_vma is set to NULL before > > > - * call, we can identify this case by checking (!dst->anon_vma && > > > - * src->anon_vma). > > > - * > > > - * If (!dst->anon_vma && src->anon_vma) is true, this function tries to find > > > - * and reuse existing anon_vma which has no vmas and only one child anon_vma. > > > - * This prevents degradation of anon_vma hierarchy to endless linear chain in > > > - * case of constantly forking task. On the other hand, an anon_vma with more > > > - * than one child isn't reused even if there was no alive vma, thus rmap > > > - * walker has a good chance of avoiding scanning the whole hierarchy when it > > > - * searches where page is mapped. > > > +static void check_anon_vma_clone(struct vm_area_struct *dst, > > > + struct vm_area_struct *src) > > > +{ > > > + /* The write lock must be held. */ > > > + mmap_assert_write_locked(src->vm_mm); > > > + /* If not a fork (implied by dst->anon_vma) then must be on same mm. */ > > > + VM_WARN_ON_ONCE(dst->anon_vma && dst->vm_mm != src->vm_mm); > > > + > > > + /* No source anon_vma is a no-op. */ > > I'm confused about the above comment. Do you mean that if > !src->anon_vma then it's a no-op and therefore this function shouldn't > be called? If so, we could simply have VM_WARN_ON_ONCE(!src->anon_vma) It's a no-op :) so it makes no sense to specify other fields. In a later commit we literally bail out of anon_vma_clone() if it's not specified. In fact the very next patch... > but checks below have more conditions. Can this comment be perhaps > expanded please so that the reader clearly understands what is allowed > and what is not. For example, combination (!src->anon_vma && > !dst->anon_vma) is allowed and we correctly not triggering a warning > here, however that's still a no-op IIUC. Yup it's correct and fine but it's a no-op, hence we have nothing to do, as you say. I thought it was self-documenting, given I literally spell out the expected conditions in the asserts but obviously this isn't entirely clear. I'm trying _not_ to write paragraphs here as that can actually make things _more_ confusing. Will update the comment to say more: /* If we have anything to do src->anon_vma must be provided. */ > > > > + VM_WARN_ON_ONCE(!src->anon_vma && !list_empty(&src->anon_vma_chain)); > > > + VM_WARN_ON_ONCE(!src->anon_vma && dst->anon_vma); > > > + /* We are establishing a new anon_vma_chain. */ > > > + VM_WARN_ON_ONCE(!list_empty(&dst->anon_vma_chain)); > > > + /* > > > + * On fork, dst->anon_vma is set NULL (temporarily). Otherwise, anon_vma > > > + * must be the same across dst and src. > > This is the second time in this small function where we have to remind > that dst->anon_vma==NULL means that we are forking. Maybe it's better > to introduce a `bool forking = dst->anon_vma==NULL;` variable at the > beginning and use it in all these checks? Later we make changes along these lines, so for the purposes of keeping things broken up I'd rather not. And yes, anon_vma is a complicated mess, this is why I'm trying to do things one step at a time, so we document the things you'd have to go research to understand, later we change the code. > > I know, I'm nitpicking but as you said, anon_vma code is very > compicated, so the more clarity we can bring to it the better. Right, sure, but it has to be one thing at a time. > > > > + */ > > > + VM_WARN_ON_ONCE(dst->anon_vma && dst->anon_vma != src->anon_vma); > > > +} > > > + > > > +/** > > > + * anon_vma_clone - Establishes new anon_vma_chain objects in @dst linking to > > > + * all of the anon_vma objects contained within @src anon_vma_chain's. > > > + * @dst: The destination VMA with an empty anon_vma_chain. > > > + * @src: The source VMA we wish to duplicate. > > > + * > > > + * This is the heart of the VMA side of the anon_vma implementation - we invoke > > > + * this function whenever we need to set up a new VMA's anon_vma state. > > > + * > > > + * This is invoked for: > > > + * > > > + * - VMA Merge, but only when @dst is unfaulted and @src is faulted - meaning we > > > + * clone @src into @dst. > > > + * - VMA split. > > > + * - VMA (m)remap. > > > + * - Fork of faulted VMA. > > > + * > > > + * In all cases other than fork this is simply a duplication. Fork additionally > > > + * adds a new active anon_vma. > > > + * > > > + * ONLY in the case of fork do we try to 'reuse' existing anon_vma's in an > > > + * anon_vma hierarchy, reusing anon_vma's which have no VMA associated with them > > > + * but do have a single child. This is to avoid waste of memory when repeatedly > > > + * forking. > > > + * > > > + * Returns: 0 on success, -ENOMEM on failure. > > > */ > > > int anon_vma_clone(struct vm_area_struct *dst, struct vm_area_struct *src) > > > { > > > struct anon_vma_chain *avc, *pavc; > > > struct anon_vma *root = NULL; > > > > > > + check_anon_vma_clone(dst, src); > > > + > > > list_for_each_entry_reverse(pavc, &src->anon_vma_chain, same_vma) { > > > struct anon_vma *anon_vma; > > > > > > @@ -392,11 +422,27 @@ int anon_vma_fork(struct vm_area_struct *vma, struct vm_area_struct *pvma) > > > return -ENOMEM; > > > } > > > > > > +/** > > > + * unlink_anon_vmas() - remove all links between a VMA and anon_vma's, freeing > > > + * anon_vma_chain objects. > > > + * @vma: The VMA whose links to anon_vma objects is to be severed. > > > + * > > > + * As part of the process anon_vma_chain's are freed, > > > + * anon_vma->num_children,num_active_vmas is updated as required and, if the > > > + * relevant anon_vma references no further VMAs, its reference count is > > > + * decremented. > > > + */ > > > void unlink_anon_vmas(struct vm_area_struct *vma) > > > { > > > struct anon_vma_chain *avc, *next; > > > struct anon_vma *root = NULL; > > > > > > + /* Always hold mmap lock, read-lock on unmap possibly. */ > > > + mmap_assert_locked(vma->vm_mm); > > > + > > > + /* Unfaulted is a no-op. */ > > > + VM_WARN_ON_ONCE(!vma->anon_vma && !list_empty(&vma->anon_vma_chain)); > > Hmm. anon_vma_clone() calls unlink_anon_vmas() after setting > dst->anon_vma=NULL in the enomem_failure path. This warning would > imply that in such case dst->anon_vma_chain is always non-empty. But I > don't think we can always expect that... What if the very first call > to anon_vma_chain_alloc() in anon_vma_clone()'s loop failed, I think > this would result in dst->anon_vma_chain being empty, no? OK well that's a good spot, though this is never going to actually happen in reality as an allocation failure here would really be 'too small to fail'. It's a pity we have to give up a completely sensible invariant because of terribly written code for an event that will never happen. But sure will drop this then, that's awful to have to do though :/ Hey maybe we'd have bot reports on this (would require fault injection) if this had been taken to any tree at any point. Ah well. > > > > + > > > /* > > > * Unlink each anon_vma chained to the VMA. This list is ordered > > > * from newest to oldest, ensuring the root anon_vma gets freed last. > > > -- > > > 2.52.0 > > > Thanks, Lorenzo