From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-1.web.codeaurora.org [10.30.226.201]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id B5E1927FD49 for ; Fri, 5 Sep 2025 19:44:02 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=10.30.226.201 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1757101443; cv=none; b=KFzRSqEYRD0V4BgWjKhDH+SsTGjiAlww2mQo4oZ3/960bplmh4XCkVIvZR/vf6qnR10+ReLY0xyPWDvieZl2HbFYpQHSmBJup9gspyeRbB6SAA7TOAMnBxZXF9b//XR2yxUoK+fReHz9I1w6cprEKRl/TiSy98hefxQyIEmqfM4= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1757101443; c=relaxed/simple; bh=qj5NWqYmLfQgUNyQbHqhOAioJ4HtjEhlRLgfXr/ggm8=; h=Date:From:To:Cc:Subject:Message-ID:References:MIME-Version: Content-Type:Content-Disposition:In-Reply-To; b=sL9h5jCKvfW7U8sBTSZ3ocwljbDN+/BLk7FG08M2tyrDxRgSjV7XGbORkmR/wWpP0IgYgU2ua9bwlhFJKR60KIcZVNcLVMtkGpiSDlKpkA3CoTnXdx5Pw8pDBJJ2TqHXLkwwnofn9EAmbom6ElDyqFhYkaZnB9Ay3g1eCSoY3qc= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=Gb25GASt; arc=none smtp.client-ip=10.30.226.201 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="Gb25GASt" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 895DBC4CEF1; Fri, 5 Sep 2025 19:44:01 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1757101442; bh=qj5NWqYmLfQgUNyQbHqhOAioJ4HtjEhlRLgfXr/ggm8=; h=Date:From:To:Cc:Subject:References:In-Reply-To:From; b=Gb25GAStY7PK4KWfMQC7rtd4cff2Ft31RumZ/kNKywLAXwUdYfICHHIJhBPjrZ6j7 o7iHpATbrIqVl1AYu1jaVpoUr8IlvHC34mB3TyehId4+9gUdLyyv/gHHn98jalOtC0 MX2S360mJGGFkZmz2ogHqkD/MgCf/035PnB2I9Wq5ZfXBWK5fpwd1RbY1MXhtqJPld H5GqrWgLYTQargRJ06fYCqqcMjdxQlMgkFIz1Gqzc+2aRDBKLVXuNsb4wASbw6IEl6 597PZCICEO+V0BK33bDDozHnZlGhmt7NvXrmXSlYdy89LJVzrbcrFORdWilzQ/GPWA 8B6Fa0HYYbdnA== Date: Fri, 5 Sep 2025 12:43:59 -0700 From: Minchan Kim To: Pedro Falcato Cc: Kalesh Singh , akpm@linux-foundation.org, lorenzo.stoakes@oracle.com, kernel-team@android.com, android-mm@google.com, David Hildenbrand , "Liam R. Howlett" , Vlastimil Babka , Mike Rapoport , Suren Baghdasaryan , Michal Hocko , Jann Horn , linux-mm@kvack.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH] mm: centralize and fix max map count limit checking Message-ID: References: <20250903232437.1454293-1-kaleshsingh@google.com> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: On Thu, Sep 04, 2025 at 12:46:34AM +0100, Pedro Falcato wrote: > On Wed, Sep 03, 2025 at 04:24:35PM -0700, Kalesh Singh wrote: > > The check against the max map count (sysctl_max_map_count) was > > open-coded in several places. This led to inconsistent enforcement > > and subtle bugs where the limit could be exceeded. > > > > For example, some paths would check map_count > sysctl_max_map_count > > before allocating a new VMA and incrementing the count, allowing the > > process to reach sysctl_max_map_count + 1: > > > > int do_brk_flags(...) > > { > > if (mm->map_count > sysctl_max_map_count) > > return -ENOMEM; > > > > /* We can get here with mm->map_count == sysctl_max_map_count */ > > > > vma = vm_area_alloc(mm); > > ... > > mm->map_count++ /* We've now exceeded the threshold. */ > > } > > I think this should be fixed separately, and sent for stable. > > > > > To fix this and unify the logic, introduce a new function, > > exceeds_max_map_count(), to consolidate the check. All open-coded > > checks are replaced with calls to this new function, ensuring the > > limit is applied uniformly and correctly. > > Thanks! In general I like the idea. > > > > > To improve encapsulation, sysctl_max_map_count is now static to > > mm/mmap.c. The new helper also adds a rate-limited warning to make > > debugging applications that exhaust their VMA limit easier. > > > > Cc: Andrew Morton > > Cc: Minchan Kim > > Cc: Lorenzo Stoakes > > Signed-off-by: Kalesh Singh > > --- > > include/linux/mm.h | 11 ++++++++++- > > mm/mmap.c | 15 ++++++++++++++- > > mm/mremap.c | 7 ++++--- > > mm/nommu.c | 2 +- > > mm/util.c | 1 - > > mm/vma.c | 6 +++--- > > 6 files changed, 32 insertions(+), 10 deletions(-) > > > > diff --git a/include/linux/mm.h b/include/linux/mm.h > > index 1ae97a0b8ec7..d4e64e6a9814 100644 > > --- a/include/linux/mm.h > > +++ b/include/linux/mm.h > > @@ -192,7 +192,16 @@ static inline void __mm_zero_struct_page(struct page *page) > > #define MAPCOUNT_ELF_CORE_MARGIN (5) > > #define DEFAULT_MAX_MAP_COUNT (USHRT_MAX - MAPCOUNT_ELF_CORE_MARGIN) > > > > -extern int sysctl_max_map_count; > > +/** > > + * exceeds_max_map_count - check if a VMA operation would exceed max_map_count > > + * @mm: The memory descriptor for the process. > > + * @new_vmas: The number of new VMAs the operation will create. > > + * > > + * Returns true if the operation would cause the number of VMAs to exceed > > + * the sysctl_max_map_count limit, false otherwise. A rate-limited warning > > + * is logged if the limit is exceeded. > > + */ > > +extern bool exceeds_max_map_count(struct mm_struct *mm, unsigned int new_vmas); > > No new "extern" in func declarations please. > > > > > extern unsigned long sysctl_user_reserve_kbytes; > > extern unsigned long sysctl_admin_reserve_kbytes; > > diff --git a/mm/mmap.c b/mm/mmap.c > > index 7306253cc3b5..693a0105e6a5 100644 > > --- a/mm/mmap.c > > +++ b/mm/mmap.c > > @@ -374,7 +374,7 @@ unsigned long do_mmap(struct file *file, unsigned long addr, > > return -EOVERFLOW; > > > > /* Too many mappings? */ > > - if (mm->map_count > sysctl_max_map_count) > > + if (exceeds_max_map_count(mm, 0)) > > return -ENOMEM; > > If the brk example is incorrect, isn't this also wrong? /me is confused > > > > /* > > @@ -1504,6 +1504,19 @@ struct vm_area_struct *_install_special_mapping( > > int sysctl_legacy_va_layout; > > #endif > > > > +static int sysctl_max_map_count __read_mostly = DEFAULT_MAX_MAP_COUNT; > > + > > +bool exceeds_max_map_count(struct mm_struct *mm, unsigned int new_vmas) > > +{ > > + if (unlikely(mm->map_count + new_vmas > sysctl_max_map_count)) { > > + pr_warn_ratelimited("%s (%d): Map count limit %u exceeded\n", > > + current->comm, current->pid, > > + sysctl_max_map_count); > > I'm not entirely sold on the map count warn, even if it's rate limited. It > sounds like something you can hit in nasty edge cases and nevertheless flood > your dmesg (more frustrating if you can't fix the damn program). How about dynamic_debug? a1394bddf9b6, mm: page_alloc: dump migrate-failed pages