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Howlett" , Vlastimil Babka , Jann Horn , Pedro Falcato , David Hildenbrand , Mike Rapoport , Suren Baghdasaryan , Michal Hocko , Hugh Dickins , Baolin Wang , "Matthew Wilcox (Oracle)" , Jan Kara , linux-kernel@vger.kernel.org, linux-mm@kvack.org, linux-fsdevel@vger.kernel.org, linux-kselftest@vger.kernel.org Subject: Re: [PATCH v3 0/6] mm: make MAP_PRIVATE-/dev/zero mappings truly anonymous Message-Id: <20260928163957.832a27ee9596a25a282c2707@linux-foundation.org> In-Reply-To: <20260926-map-private-dev-zero-v3-0-d4781e84ccfc@kernel.org> References: <20260926-map-private-dev-zero-v3-0-d4781e84ccfc@kernel.org> X-Mailer: Sylpheed 3.8.0beta1 (GTK+ 2.24.33; x86_64-pc-linux-gnu) Mime-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit X-Rspamd-Server: rspam06 X-Stat-Signature: hckudforxihkn5egcm7soh7pr8zxsgka X-Rspam-User: X-Rspamd-Queue-Id: 298D240004 X-HE-Tag: 1790638799-229851 X-HE-Meta: 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 RdDrpQZz qkrlaNPyB2RgtzAcYS6wzvqOCq1vsK3qATBHc29c7t5eCfwjKrSJb1SbqsjPWprqLT6AAJDQN7HUZD4qbg/UAkdFgjuXCaZZ+WIBUcTyWbeT5lc7BY2T7HLpRudqUnWUFl9EhZY5ruxrv5B8nCcONBoIVvAslYMUxJinN2xzb52ftTzQH7yvQNUMdbDd9uTMM0oofxvXnzawg4g10UR8x/P3gFf/G9aTRQhkKQDoqIXObyMLymfxgJburHIr9miUXVijIOtZn7qJyWeZgy3eDb772r5petRnq8Sj+HcXQDrP3Dwit1WMfijNdZrm373ivI4x6 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On Sat, 26 Sep 2026 11:41:05 +0100 "Lorenzo Stoakes (ARM)" wrote: > Historically anonymous memory was obtained in linux by MAP_PRIVATE-mapping > /dev/zero. > > The canonical way of doing these now is mmap() specifying MAP_PRIVATE | > MAP_ANON, but we must continue to support the legacy means of obtaining these > mappings. > > As-is these mappings are an unusual edge-case - they satisfy > vma_is_anonymous() but have non-NULL vma->vm_file, and their page offset is > the offset into the /dev/zero file. > > Commit 93c0c8dc87f6 ("mm/rmap: use anon pgoff to track MAP_PRIVATE > file-backed anon folios") causes all other anonymous folios to be tracked > by their anon index (vma->vm_start >> PAGE_SHIFT at the point of first > fault), leaving MAP_PRIVATE-/dev/zero as the outlier. > > This series remedies the situation by making MAP_PRIVATE-/dev/zero mappings > truly anonymous with !vma->vm_file and correct anonymous page offset. > > It starts by bringing the memory character driver into mm/ - this file > implements /dev/zero, /dev/mem among other things and is already (as > clearly indicated by its name) within the remit of memory management. > > By doing this, the file_is_dev_zero() function can be provided, internal to > mm, which allows for positive identification of these mappings. > > Using this, first prevent any other mappings from mapping memory > anonymously, then make these mappings truly anonymous and eliminate all > code in the kernel that previously had to account for these strange beasts. > > Finally, it adds userland VMA tests to assert the behaviour and selftests > to assert expected merge behaviour. Thanks, I've updated mm-unstable to this version. > v3: > * Added tags (thanks David! :) > * Moved the reworked mmap_prepare vm_ops comment to 3/6 where it is introduced > and fixed a typo in it, as per David. Here's how v3 altered mm.git. Includes a merge fix for the recently-added hotfix "mm/vma: predicate setting mmap_prepare VMA fields on new vma alloc", per yourself. And... really? I already had David's tags so we did a full resend for a single comment typo? Little -fixes had their role... mm/vma.c | 4 ++-- 1 file changed, 2 insertions(+), 2 deletions(-) --- a/mm/vma.c~b +++ a/mm/vma.c @@ -2816,7 +2816,7 @@ static int call_mmap_prepare(struct mmap if (err) return err; - /* It's invalid for mmap_preprare hooks to clear vm_ops. */ + /* It's invalid for mmap_prepare hooks to clear vm_ops. */ if (!desc->vm_ops) return -EINVAL; @@ -2932,7 +2932,7 @@ static unsigned long __mmap_region(struc allocated_new = true; } - if (have_mmap_prepare && allocated_new) + if (have_mmap_prepare && !map_is_anon(&map) && allocated_new) set_vma_user_defined_fields(vma, &map); __mmap_complete(&map, vma); _