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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: <20230216153405.zo4l2lqpnc2agdzg@revolver> Mail-Followup-To: "Liam R. 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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. 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. 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. > - else if (prev && (prev->vm_flags & VM_GROWSUP)) > + } else if (prev && (prev->vm_flags & VM_GROWSUP)) { > + vma_start_write(prev); > downgrade = false; > - else > + } else > mmap_write_downgrade(mm); > } > > -- > 2.39.1