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Howlett" To: Suren Baghdasaryan Cc: akpm@linux-foundation.org, michel@lespinasse.org, jglisse@google.com, mhocko@suse.com, vbabka@suse.cz, hannes@cmpxchg.org, mgorman@techsingularity.net, dave@stgolabs.net, willy@infradead.org, peterz@infradead.org, ldufour@linux.ibm.com, paulmck@kernel.org, mingo@redhat.com, will@kernel.org, luto@kernel.org, songliubraving@fb.com, peterx@redhat.com, david@redhat.com, dhowells@redhat.com, hughd@google.com, bigeasy@linutronix.de, kent.overstreet@linux.dev, punit.agrawal@bytedance.com, lstoakes@gmail.com, peterjung1337@gmail.com, rientjes@google.com, chriscli@google.com, axelrasmussen@google.com, joelaf@google.com, minchan@google.com, rppt@kernel.org, jannh@google.com, shakeelb@google.com, tatashin@google.com, edumazet@google.com, gthelen@google.com, gurua@google.com, arjunroy@google.com, soheil@google.com, leewalsh@google.com, posk@google.com, michalechner92@googlemail.com, linux-mm@kvack.org, linux-arm-kernel@lists.infradead.org, linuxppc-dev@lists.ozlabs.org, x86@kernel.org, linux-kernel@vger.kernel.org, kernel-team@android.com Subject: Re: [PATCH v3 21/35] mm/mmap: write-lock adjacent VMAs if they can grow into unmapped area Message-ID: <20230217145052.y526nmjudi6t2ael@revolver> Mail-Followup-To: "Liam R. 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X-MS-Exchange-CrossTenant-AuthSource: SN6PR10MB3022.namprd10.prod.outlook.com X-MS-Exchange-CrossTenant-AuthAs: Internal X-MS-Exchange-CrossTenant-OriginalArrivalTime: 17 Feb 2023 14:50:58.2505 (UTC) X-MS-Exchange-CrossTenant-FromEntityHeader: Hosted X-MS-Exchange-CrossTenant-Id: 4e2c6054-71cb-48f1-bd6c-3a9705aca71b X-MS-Exchange-CrossTenant-MailboxType: HOSTED X-MS-Exchange-CrossTenant-UserPrincipalName: UZBeBtlgbFcz59ZgCMuse2lZoMRm1u3rZtH4QVSumBpvaA84vHXISpEybBhHXJT3X6PIVMCetA+grIMGZb4ocQ== X-MS-Exchange-Transport-CrossTenantHeadersStamped: SA2PR10MB4809 X-Proofpoint-Virus-Version: vendor=baseguard engine=ICAP:2.0.219,Aquarius:18.0.930,Hydra:6.0.562,FMLib:17.11.170.22 definitions=2023-02-17_10,2023-02-17_01,2023-02-09_01 X-Proofpoint-Spam-Details: rule=notspam policy=default score=0 mlxlogscore=737 mlxscore=0 adultscore=0 malwarescore=0 phishscore=0 bulkscore=0 spamscore=0 suspectscore=0 classifier=spam adjust=0 reason=mlx scancount=1 engine=8.12.0-2212070000 definitions=main-2302170135 X-Proofpoint-GUID: RHBuj7a6gvuMBzv70eNE-aFY2GtCQy-y X-Proofpoint-ORIG-GUID: RHBuj7a6gvuMBzv70eNE-aFY2GtCQy-y Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org * Suren Baghdasaryan [230216 14:36]: > On Thu, Feb 16, 2023 at 7:34 AM Liam R. Howlett wrote: > > > > > > First, sorry I didn't see this before v3.. > > Feedback at any time is highly appreciated! > > > > > * Suren Baghdasaryan [230216 00:18]: > > > While unmapping VMAs, adjacent VMAs might be able to grow into the area > > > being unmapped. In such cases write-lock adjacent VMAs to prevent this > > > growth. > > > > > > Signed-off-by: Suren Baghdasaryan > > > --- > > > mm/mmap.c | 8 +++++--- > > > 1 file changed, 5 insertions(+), 3 deletions(-) > > > > > > diff --git a/mm/mmap.c b/mm/mmap.c > > > index 118b2246bba9..00f8c5798936 100644 > > > --- a/mm/mmap.c > > > +++ b/mm/mmap.c > > > @@ -2399,11 +2399,13 @@ do_vmi_align_munmap(struct vma_iterator *vmi, struct vm_area_struct *vma, > > > * down_read(mmap_lock) and collide with the VMA we are about to unmap. > > > */ > > > if (downgrade) { > > > - if (next && (next->vm_flags & VM_GROWSDOWN)) > > > + if (next && (next->vm_flags & VM_GROWSDOWN)) { > > > + vma_start_write(next); > > > downgrade = false; > > > > If the mmap write lock is insufficient to protect us from next/prev > > modifications then we need to move *most* of this block above the maple > > tree write operation, otherwise we have a race here. When I say most, I > > mean everything besides the call to mmap_write_downgrade() needs to be > > moved. > > Which prior maple tree write operation are you referring to? I see > __split_vma() and munmap_sidetree() which both already lock the VMAs > they operate on, so page faults can't happen in those VMAs. The write that removes the VMAs from the maple tree a few lines above.. /* Point of no return */ If the mmap lock is not sufficient, then we need to move the vma_start_write() of prev/next to above the call to vma_iter_clear_gfp() in do_vmi_align_munmap(). But I still think it IS enough. > > > > > If the mmap write lock is sufficient to protect us from next/prev > > modifications then we don't need to write lock the vmas themselves. > > mmap write lock is not sufficient because with per-VMA locks we do not > take mmap lock at all. Understood, but it also does not expand VMAs. > > > > > I believe this is for expand_stack() protection, so I believe it's okay > > to not vma write lock these vmas.. I don't think there are other areas > > where we can modify the vmas without holding the mmap lock, but others > > on the CC list please chime in if I've forgotten something. > > > > So, if I am correct, then you shouldn't lock next/prev and allow the > > vma locking fault method on these vmas. This will work because > > lock_vma_under_rcu() uses mas_walk() on the faulting address. That is, > > your lock_vma_under_rcu() will fail to find anything that needs to be > > grown and go back to mmap lock protection. As it is written today, the > > vma locking fault handler will fail and we will wait for the mmap lock > > to be released even when the vma isn't going to expand. > > So, let's consider a case when the next VMA is not being removed (so > it was neither removed nor locked by munmap_sidetree()) and it is > found by lock_vma_under_rcu() in the page fault handling path. By this point next VMA is either NULL or outside the munmap area, so what you said here is always true. >Page > fault handler can now expand it and push into the area we are > unmapping in unmap_region(). That is the race I'm trying to prevent > here by locking the next/prev VMAs which can be expanded before > unmap_region() unmaps them. Am I missing something? Yes, I think the part you are missing (or I am missing..) is that expand_stack() will never be called without the mmap lock. We don't use the vma locking to expand the stack. ...