From mboxrd@z Thu Jan 1 00:00:00 1970 Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.remote-ip=100.103.45.18 ARC-Seal: i=1; d=subspace.kernel.org; s=arc20261015; a=rsa-sha256; cv=none; t=1793093013; b=EAjcppYe/RoP4s9XdpJEwGsqlRlUWKmawTl+ZKGhNhQMaThs2cHuBnPTWdl70l5AryLE SWHjf6Lv2Oj7YT6i2TB9sr+we+vsc3Y3Gc5xkiPqVROlgBNGmiCt2zAp+J+Sitsif8WDq 3N/rXRzMxTBbyXtpXcPnimabYiVJ2ssmOimyCrgLtjKAsw49BFlQMdrwLFrU5YWuRVh5f vYBZcxLulB5lo/T3fe7yCWsyddtgSpZdSnir+uL0AE5IPCWZLfS47ThAXujT5hvUNJys8 UJmcZARM8qLMyAFN0pcNesoBKWzP8O/SVYzDyD9e1S8tzxTYAvEMF4dLpYmhd1AECzQ== ARC-Message-Signature: i=1; d=subspace.kernel.org; s=arc20261015; a=rsa-sha256; c=relaxed/relaxed; t=1793093013; h=Received:DKIM-Signature:Date:From:To:Cc:Subject:Message-ID: References:MIME-Version:Content-Type:Content-Disposition:In-Reply-To; bh=gKyZ8Btgv2m5r1QzOsn9naTI7JOU43dREIO8vvPdksk=; b=uaOFWQX9maxwUkbgV90mzT36M+9DzrSG2YsBLgtiZ8dJXF5sGYKk//jBEciKgPdBsKV2 qQ4bHIh41TVx4439w4+L3pBfYnWA5DPnaRpZYJjUUyr8J9oA+v1pUDogOCAJ6pANUEcxz LAJIWjaN92wUfa5Y/tHDtnWsQE8+kd7hUfLQ+Q4ixQcNnk47qmeZFDyIP9R99BKbJ4zWc MCLu7TRvnsun+lVzaffJy4cy4teS8KvS9iSAPvZO9NBHRii9obpi7rLPlyo6AOyebUTB7 fi0X97+srhrG/qHTxwaF3SHtKCSVH9uhAbniYnR73lV+pfrh+WZQLYSqf4yLTobMITw== ARC-Authentication-Results: i=1; smtp.subspace.kernel.org; dkim=pass header.d=kernel.org header.i=@kernel.org header.a=rsa-sha256 header.s=k20260515 header.b=RRIblIbn; arc=none smtp.remote-ip=100.103.45.18 Received: from smtp.kernel.org (quasi.space.kernel.org [100.103.45.18]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature ECDSA (prime256v1)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id A8916606A31A; Tue, 27 Oct 2026 09:23:32 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key, unprotected) header.d=kernel.org header.i=@kernel.org header.a=rsa-sha256 header.s=k20260515 header.b=RRIblIbn Received: by smtp.kernel.org (Postfix) with ESMTPSA id C891B1F01825; Tue, 27 Oct 2026 09:23:17 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1793093012; bh=gKyZ8Btgv2m5r1QzOsn9naTI7JOU43dREIO8vvPdksk=; h=Date:From:To:Cc:Subject:References:In-Reply-To; b=RRIblIbn2zLnU8Sr7/XSlAPM/1uYHmreLkJHjINQaRJOZvC9Sgpx42DrJf6dcQTQC ya74iDcLD6vrow+ocCz0IReZYx0P1Q9NsVCfnovJb8AJLEC/y7eFeLPDmrSWxdwf/o TksWubM3qOV6hJ2NxKiMMuWprf9uAh0CfC93xkb8BJ1+c+RRuIAFhPkXhr4omthO8q f7xxqtEGJZKjUL6xCIL7yGA94ZBM9y8WqbzHraQifdUrB3+WWEq4dkNyC5l1Ja6A+i a65v0Umr9leKkV34R9WoWIlBEKeGporP8FyFgIucxi1ALew3as/VXqSDuvgEofOLI8 FlmLdC2A7uqbA== Date: Tue, 27 Oct 2026 09:23:15 +0000 From: "Lorenzo Stoakes (ARM)" To: "David Hildenbrand (Arm)" Cc: Andrew Morton , Johannes Weiner , Michal Hocko , Qi Zheng , Shakeel Butt , Kairui Song , Barry Song , Axel Rasmussen , Yuanchu Xie , Wei Xu , Baoquan He , Baolin Wang , "Liam R. Howlett" , Vlastimil Babka , Mike Rapoport , Suren Baghdasaryan , Michal Hocko , Kees Cook , Peter Zijlstra , Jann Horn , Pedro Falcato , Hugh Dickins , Pasha Tatashin , Pratyush Yadav , "Matthew Wilcox (Oracle)" , Jan Kara , Zi Yan , Lance Yang , Rik van Riel , Harry Yoo , Alistair Popple , Nico Pache , Ryan Roberts , Dev Jain , Usama Arif , Kiryl Shutsemau , Matthew Brost , John Hubbard , Peter Xu , Will Deacon , "Aneesh Kumar K.V" , Nick Piggin , Oscar Salvador , Minchan Kim , Muchun Song , Xu Xin , Chengming Zhou , Chris Li , Kemeng Shi , Nhat Pham , Youngjun Park , Gregory Price , Joshua Hahn , Rakie Kim , Ying Huang , Jason Gunthorpe , linux-mm@kvack.org, linux-kernel@vger.kernel.org, kexec@lists.infradead.org, linux-fsdevel@vger.kernel.org, linux-arch@vger.kernel.org, linux-cxl@vger.kernel.org, Chris Mason , stable@vger.kernel.org Subject: Re: [PATCH 37/46] mm: fix deadlock of mremap() against UFFDIO_MOVE Message-ID: References: <20261027-fixes_core-v1-0-6f74bec9a5bb@kernel.org> <20261027-fixes_core-v1-37-6f74bec9a5bb@kernel.org> X-Mailing-List: linux-fsdevel@vger.kernel.org List-Id: List-Post: List-Help: List-Subscribe: List-Unsubscribe: List-Owner: Precedence: list MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20261027-fixes_core-v1-37-6f74bec9a5bb@kernel.org> On Tue, Oct 27, 2026 at 12:17:57AM +0100, David Hildenbrand (Arm) wrote: > From: Chris Mason > > With per-VMA locks, UFFDIO_MOVE can run concurrently with mremap() > on different VMAs in the same mm. Those VMAs can share page tables, so > both operations can acquire the same pair of page-table locks. > > UFFDIO_MOVE orders the locks by address, while mremap() takes the source > lock first. These conflicting orders can deadlock; mremap() holding > mmap_lock for writing no longer helps. > > Move double_pt_lock() and double_pt_unlock() to mm/internal.h and use > them for all page-table moves in mremap(). Preserve the source entry > checks after acquiring both locks. Remove the comments claiming that > exclusive mmap_lock makes lock ordering irrelevant. > > In move_ptes(), make sure to just recheck source and destination PMD > after taking the locks and drop the overly-long comment. > > Fix generated by an LLM and sent by Chris Mason in private. > > Fixes: 867a43a34ff8 ("userfaultfd: use per-vma locks in userfaultfd operations") > Cc: stable@vger.kernel.org > Signed-off-by: Chris Mason > [ david: use new helpers consistently; simplify description and comments; > move_ptes() fixup ] > Signed-off-by: David Hildenbrand (Arm) OK nice, reviewed off-list and this revision looks correct + covers all cases, so: Reviewed-by: Lorenzo Stoakes (ARM) > --- > include/linux/userfaultfd_k.h | 2 - > mm/huge_memory.c | 19 ++++------ > mm/hugetlb.c | 14 ++----- > mm/internal.h | 28 ++++++++++++++ > mm/mremap.c | 87 +++++++++++++++++-------------------------- > mm/userfaultfd.c | 27 -------------- > 6 files changed, 72 insertions(+), 105 deletions(-) > > diff --git a/include/linux/userfaultfd_k.h b/include/linux/userfaultfd_k.h > index a14b8a9ffb7b1..565e98f342dc8 100644 > --- a/include/linux/userfaultfd_k.h > +++ b/include/linux/userfaultfd_k.h > @@ -153,8 +153,6 @@ extern int mrwprotect_range(struct userfaultfd_ctx *ctx, unsigned long start, > unsigned long len, bool enable_rwp); > > /* move_pages */ > -void double_pt_lock(spinlock_t *ptl1, spinlock_t *ptl2); > -void double_pt_unlock(spinlock_t *ptl1, spinlock_t *ptl2); > int move_pages_huge_pmd(struct mm_struct *mm, pmd_t *dst_pmd, pmd_t *src_pmd, pmd_t dst_pmdval, > struct vm_area_struct *dst_vma, > struct vm_area_struct *src_vma, > diff --git a/mm/huge_memory.c b/mm/huge_memory.c > index 1b50cc2b35351..08e58cda7d37d 100644 > --- a/mm/huge_memory.c > +++ b/mm/huge_memory.c > @@ -2645,15 +2645,11 @@ bool move_huge_pmd(struct vm_area_struct *vma, unsigned long old_addr, > return false; > } > > - /* > - * We don't have to worry about the ordering of src and dst > - * ptlocks because exclusive mmap_lock prevents deadlock. > - */ > - old_ptl = __pmd_trans_huge_lock(old_pmd, vma); > - if (old_ptl) { > - new_ptl = pmd_lockptr(mm, new_pmd); > - if (new_ptl != old_ptl) > - spin_lock_nested(new_ptl, SINGLE_DEPTH_NESTING); > + old_ptl = pmd_lockptr(mm, old_pmd); > + new_ptl = pmd_lockptr(mm, new_pmd); > + > + double_pt_lock(old_ptl, new_ptl); > + if (likely(pmd_is_huge(*old_pmd))) { > pmd = pmdp_huge_get_and_clear(mm, old_addr, old_pmd); > if (pmd_present(pmd)) > force_flush = true; > @@ -2682,11 +2678,10 @@ bool move_huge_pmd(struct vm_area_struct *vma, unsigned long old_addr, > set_pmd_at(mm, new_addr, new_pmd, pmd); > if (force_flush) > flush_pmd_tlb_range(vma, old_addr, old_addr + PMD_SIZE); > - if (new_ptl != old_ptl) > - spin_unlock(new_ptl); > - spin_unlock(old_ptl); > + double_pt_unlock(old_ptl, new_ptl); > return true; > } > + double_pt_unlock(old_ptl, new_ptl); > return false; > } > > diff --git a/mm/hugetlb.c b/mm/hugetlb.c > index da5d04edf7c10..84141527fe73d 100644 > --- a/mm/hugetlb.c > +++ b/mm/hugetlb.c > @@ -5153,15 +5153,9 @@ static void move_huge_pte(struct vm_area_struct *vma, unsigned long old_addr, > spinlock_t *src_ptl, *dst_ptl; > pte_t pte; > > - dst_ptl = huge_pte_lock(h, mm, dst_pte); > + dst_ptl = huge_pte_lockptr(h, mm, dst_pte); > src_ptl = huge_pte_lockptr(h, mm, src_pte); > - > - /* > - * We don't have to worry about the ordering of src and dst ptlocks > - * because exclusive mmap_lock (or the i_mmap_lock) prevents deadlock. > - */ > - if (src_ptl != dst_ptl) > - spin_lock_nested(src_ptl, SINGLE_DEPTH_NESTING); > + double_pt_lock(dst_ptl, src_ptl); > > pte = huge_ptep_get_and_clear(mm, old_addr, src_pte, sz); > > @@ -5190,9 +5184,7 @@ static void move_huge_pte(struct vm_area_struct *vma, unsigned long old_addr, > set_huge_pte_at(mm, new_addr, dst_pte, pte, sz); > } > > - if (src_ptl != dst_ptl) > - spin_unlock(src_ptl); > - spin_unlock(dst_ptl); > + double_pt_unlock(dst_ptl, src_ptl); > } > > int move_hugetlb_page_tables(struct vm_area_struct *vma, > diff --git a/mm/internal.h b/mm/internal.h > index b1b0f29d2216e..7e4d88b2fb02c 100644 > --- a/mm/internal.h > +++ b/mm/internal.h > @@ -619,6 +619,34 @@ static inline void force_page_cache_readahead(struct address_space *mapping, > force_page_cache_ra(&ractl, nr_to_read); > } > > +/* > + * Take two page table locks at the same level in address order such that > + * we cannot deadlock. > + */ > +static inline void double_pt_lock(spinlock_t *ptl1, spinlock_t *ptl2) > + __acquires(ptl1) > + __acquires(ptl2) > +{ > + if (ptl1 > ptl2) > + swap(ptl1, ptl2); > + spin_lock(ptl1); > + if (ptl1 != ptl2) > + spin_lock_nested(ptl2, SINGLE_DEPTH_NESTING); > + else > + __acquire(ptl2); > +} > + > +static inline void double_pt_unlock(spinlock_t *ptl1, spinlock_t *ptl2) > + __releases(ptl1) > + __releases(ptl2) > +{ > + spin_unlock(ptl1); > + if (ptl1 != ptl2) > + spin_unlock(ptl2); > + else > + __release(ptl2); > +} > + > unsigned find_lock_entries(struct address_space *mapping, pgoff_t *start, > pgoff_t end, struct folio_batch *fbatch, pgoff_t *indices); > unsigned find_get_entries(struct address_space *mapping, pgoff_t *start, > diff --git a/mm/mremap.c b/mm/mremap.c > index 1237718848f5c..70b7264aa6bf5 100644 > --- a/mm/mremap.c > +++ b/mm/mremap.c > @@ -202,7 +202,7 @@ static int move_ptes(struct pagetable_move_control *pmc, > struct mm_struct *mm = vma->vm_mm; > pte_t *old_ptep, *new_ptep; > pte_t old_pte, pte; > - pmd_t dummy_pmdval; > + pmd_t old_pmdval, new_pmdval; > spinlock_t *old_ptl, *new_ptl; > bool force_flush = false; > unsigned long old_addr = pmc->old_addr; > @@ -234,31 +234,30 @@ static int move_ptes(struct pagetable_move_control *pmc, > if (pmc->need_rmap_locks) > take_rmap_locks(vma); > > - /* > - * We don't have to worry about the ordering of src and dst > - * pte locks because exclusive mmap_lock prevents deadlock. > - */ > - old_ptep = pte_offset_map_lock(mm, old_pmd, old_addr, &old_ptl); > + old_ptep = pte_offset_map_rw_nolock(mm, old_pmd, old_addr, &old_pmdval, > + &old_ptl); > if (!old_ptep) { > err = -EAGAIN; > goto out; > } > - /* > - * Now new_pte is none, so collapse_scan_file() path can not find > - * this by traversing file->f_mapping, so there is no concurrency with > - * retract_page_tables(). In addition, we already hold the exclusive > - * mmap_lock, so this new_pte page is stable, so there is no need to get > - * pmdval and do pmd_same() check. > - */ > - new_ptep = pte_offset_map_rw_nolock(mm, new_pmd, new_addr, &dummy_pmdval, > - &new_ptl); > + new_ptep = pte_offset_map_rw_nolock(mm, new_pmd, new_addr, &new_pmdval, > + &new_ptl); > if (!new_ptep) { > - pte_unmap_unlock(old_ptep, old_ptl); > + pte_unmap(old_ptep); > + err = -EAGAIN; > + goto out; > + } > + > + double_pt_lock(old_ptl, new_ptl); > + if (unlikely(!pmd_same(old_pmdval, pmdp_get_lockless(old_pmd)) || > + !pmd_same(new_pmdval, pmdp_get_lockless(new_pmd)))) { > + double_pt_unlock(old_ptl, new_ptl); > + pte_unmap(new_ptep); > + pte_unmap(old_ptep); > err = -EAGAIN; > goto out; > } > - if (new_ptl != old_ptl) > - spin_lock_nested(new_ptl, SINGLE_DEPTH_NESTING); > + > flush_tlb_batched_pending(vma->vm_mm); > lazy_mmu_mode_enable(); > > @@ -317,10 +316,9 @@ static int move_ptes(struct pagetable_move_control *pmc, > lazy_mmu_mode_disable(); > if (force_flush) > flush_tlb_range(vma, old_end - len, old_end); > - if (new_ptl != old_ptl) > - spin_unlock(new_ptl); > + double_pt_unlock(old_ptl, new_ptl); > pte_unmap(new_ptep - 1); > - pte_unmap_unlock(old_ptep - 1, old_ptl); > + pte_unmap(old_ptep - 1); > out: > if (pmc->need_rmap_locks) > drop_rmap_locks(vma); > @@ -395,14 +393,9 @@ static bool move_normal_pmd(struct pagetable_move_control *pmc, > if (WARN_ON_ONCE(!pmd_none(*new_pmd))) > return false; > > - /* > - * We don't have to worry about the ordering of src and dst > - * ptlocks because exclusive mmap_lock prevents deadlock. > - */ > - old_ptl = pmd_lock(mm, old_pmd); > + old_ptl = pmd_lockptr(mm, old_pmd); > new_ptl = pmd_lockptr(mm, new_pmd); > - if (new_ptl != old_ptl) > - spin_lock_nested(new_ptl, SINGLE_DEPTH_NESTING); > + double_pt_lock(old_ptl, new_ptl); > > pmd = *old_pmd; > > @@ -418,9 +411,7 @@ static bool move_normal_pmd(struct pagetable_move_control *pmc, > pmd_populate(mm, new_pmd, pmd_pgtable(pmd)); > flush_tlb_range(vma, pmc->old_addr, pmc->old_addr + PMD_SIZE); > out_unlock: > - if (new_ptl != old_ptl) > - spin_unlock(new_ptl); > - spin_unlock(old_ptl); > + double_pt_unlock(old_ptl, new_ptl); > > return res; > } > @@ -452,14 +443,10 @@ static bool move_normal_pud(struct pagetable_move_control *pmc, > if (WARN_ON_ONCE(!pud_none(*new_pud))) > return false; > > - /* > - * We don't have to worry about the ordering of src and dst > - * ptlocks because exclusive mmap_lock prevents deadlock. > - */ > - old_ptl = pud_lock(mm, old_pud); > + old_ptl = pud_lockptr(mm, old_pud); > new_ptl = pud_lockptr(mm, new_pud); > - if (new_ptl != old_ptl) > - spin_lock_nested(new_ptl, SINGLE_DEPTH_NESTING); > + > + double_pt_lock(old_ptl, new_ptl); > > pud = *old_pud; > if (unlikely(pud_none(pud))) > @@ -473,9 +460,7 @@ static bool move_normal_pud(struct pagetable_move_control *pmc, > pud_populate(mm, new_pud, pud_pgtable(pud)); > flush_tlb_range(vma, pmc->old_addr, pmc->old_addr + PUD_SIZE); > out_unlock: > - if (new_ptl != old_ptl) > - spin_unlock(new_ptl); > - spin_unlock(old_ptl); > + double_pt_unlock(old_ptl, new_ptl); > > return true; > } > @@ -503,16 +488,14 @@ static bool move_huge_pud(struct pagetable_move_control *pmc, > if (WARN_ON_ONCE(!pud_none(*new_pud))) > return false; > > - /* > - * We don't have to worry about the ordering of src and dst > - * ptlocks because exclusive mmap_lock prevents deadlock. > - */ > - old_ptl = __pud_trans_huge_lock(old_pud, vma); > - if (unlikely(!old_ptl)) > - return false; > + old_ptl = pud_lockptr(mm, old_pud); > new_ptl = pud_lockptr(mm, new_pud); > - if (new_ptl != old_ptl) > - spin_lock_nested(new_ptl, SINGLE_DEPTH_NESTING); > + > + double_pt_lock(old_ptl, new_ptl); > + if (unlikely(!pud_trans_huge(*old_pud))) { > + double_pt_unlock(old_ptl, new_ptl); > + return false; > + } > > /* Clear the pud */ > pud = pudp_huge_get_and_clear(mm, pmc->old_addr, old_pud); > @@ -523,9 +506,7 @@ static bool move_huge_pud(struct pagetable_move_control *pmc, > /* mark soft_ditry when we add pud level soft dirty support */ > set_pud_at(mm, pmc->new_addr, new_pud, pud); > flush_pud_tlb_range(vma, pmc->old_addr, pmc->old_addr + HPAGE_PUD_SIZE); > - if (new_ptl != old_ptl) > - spin_unlock(new_ptl); > - spin_unlock(old_ptl); > + double_pt_unlock(old_ptl, new_ptl); > > return true; > } > diff --git a/mm/userfaultfd.c b/mm/userfaultfd.c > index 710e913c73bfa..76de84510340c 100644 > --- a/mm/userfaultfd.c > +++ b/mm/userfaultfd.c > @@ -1206,33 +1206,6 @@ int mrwprotect_range(struct userfaultfd_ctx *ctx, unsigned long start, > return 0; > } > > -void double_pt_lock(spinlock_t *ptl1, > - spinlock_t *ptl2) > - __acquires(ptl1) > - __acquires(ptl2) > -{ > - if (ptl1 > ptl2) > - swap(ptl1, ptl2); > - /* lock in virtual address order to avoid lock inversion */ > - spin_lock(ptl1); > - if (ptl1 != ptl2) > - spin_lock_nested(ptl2, SINGLE_DEPTH_NESTING); > - else > - __acquire(ptl2); > -} > - > -void double_pt_unlock(spinlock_t *ptl1, > - spinlock_t *ptl2) > - __releases(ptl1) > - __releases(ptl2) > -{ > - spin_unlock(ptl1); > - if (ptl1 != ptl2) > - spin_unlock(ptl2); > - else > - __release(ptl2); > -} > - > static inline bool is_pte_pages_stable(pte_t *dst_pte, pte_t *src_pte, > pte_t orig_dst_pte, pte_t orig_src_pte, > pmd_t *dst_pmd, pmd_t dst_pmdval) > > -- > 2.43.0 > -- Cheers, Lorenzo