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Wed, 01 Mar 2023 11:07:32 -0800 (PST) MIME-Version: 1.0 References: <20230227173632.3292573-1-surenb@google.com> <20230227173632.3292573-19-surenb@google.com> In-Reply-To: From: Suren Baghdasaryan Date: Wed, 1 Mar 2023 11:07:21 -0800 Message-ID: Subject: Re: [PATCH v4 18/33] mm: write-lock VMAs before removing them from VMA tree To: Hyeonggon Yoo <42.hyeyoo@gmail.com> 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, liam.howlett@oracle.com, 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 Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: quoted-printable X-Stat-Signature: appaft1h5bk5a9exeo4p7fx7cqxoihdx X-Rspam-User: X-Rspamd-Queue-Id: AD1CD20027 X-Rspamd-Server: rspam06 X-HE-Tag: 1677697653-298497 X-HE-Meta: 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 cH7Zi0BB 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 X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: On Wed, Mar 1, 2023 at 10:34=E2=80=AFAM Suren Baghdasaryan wrote: > > On Tue, Feb 28, 2023 at 11:57=E2=80=AFPM Hyeonggon Yoo <42.hyeyoo@gmail.c= om> wrote: > > > > On Wed, Mar 01, 2023 at 07:43:33AM +0000, Hyeonggon Yoo wrote: > > > On Mon, Feb 27, 2023 at 09:36:17AM -0800, Suren Baghdasaryan wrote: > > > > Write-locking VMAs before isolating them ensures that page fault > > > > handlers don't operate on isolated VMAs. > > > > > > > > Signed-off-by: Suren Baghdasaryan > > > > --- > > > > mm/mmap.c | 1 + > > > > mm/nommu.c | 5 +++++ > > > > 2 files changed, 6 insertions(+) > > > > > > > > diff --git a/mm/mmap.c b/mm/mmap.c > > > > index 1f42b9a52b9b..f7ed357056c4 100644 > > > > --- a/mm/mmap.c > > > > +++ b/mm/mmap.c > > > > @@ -2255,6 +2255,7 @@ int split_vma(struct vma_iterator *vmi, struc= t vm_area_struct *vma, > > > > static inline int munmap_sidetree(struct vm_area_struct *vma, > > > > struct ma_state *mas_detach) > > > > { > > > > + vma_start_write(vma); > > > > mas_set_range(mas_detach, vma->vm_start, vma->vm_end - 1); > > > > > > I may be missing something, but have few questions: > > > > > > 1) Why does a writer need to both write-lock a VMA and mark the= VMA detached > > > when unmapping it, isn't it enough to just only write-lock a= VMA? > > We need to mark the VMA detached to avoid handling page fault in a > detached VMA. The possible scenario is: > > lock_vma_under_rcu > vma =3D mas_walk(&mas) > munmap_sidetree > vma_start_write= (vma) > > mas_store_gfp() // remove VMA from the tree > vma_end_write_a= ll() > vma_start_read(vma) > // we locked the VMA but it is not part of the tree anymore. > > So, marking the VMA locked before vma_end_write_all() and checking > vma->detached after vma_start_read() helps us avoid handling faults in > the detached VMA. > > > > > > > > 2) as VMAs that are going to be removed are already locked in v= ma_prepare(), > > > so I think this hunk could be dropped? > > > > After sending this just realized that I did not consider simple munmap = case :) > > But I still think 1) and 3) are valid question. > > > > > > > > > if (mas_store_gfp(mas_detach, vma, GFP_KERNEL)) > > > > return -ENOMEM; > > > > diff --git a/mm/nommu.c b/mm/nommu.c > > > > index 57ba243c6a37..2ab162d773e2 100644 > > > > --- a/mm/nommu.c > > > > +++ b/mm/nommu.c > > > > @@ -588,6 +588,7 @@ static int delete_vma_from_mm(struct vm_area_st= ruct *vma) > > > > current->pid); > > > > return -ENOMEM; > > > > } > > > > + vma_start_write(vma); > > > > cleanup_vma_from_mm(vma); > > > > > > 3) I think this hunk could be dropped as Per-VMA lock depends o= n MMU anyway. > > Ah, yes, you are right. We can safely remove the changes in nommu.c > Andrew, should I post a fixup or you can make the removal directly in > mm-unstable? I went ahead and posted the fixup for this at: https://lore.kernel.org/all/20230301190457.1498985-1-surenb@google.com/ > Thanks, > Suren. > > > > > > > Thanks, > > > Hyeonggon > > > > > > > > > > > /* remove from the MM's tree and list */ > > > > @@ -1519,6 +1520,10 @@ void exit_mmap(struct mm_struct *mm) > > > > */ > > > > mmap_write_lock(mm); > > > > for_each_vma(vmi, vma) { > > > > + /* > > > > + * No need to lock VMA because this is the only mm user= and no > > > > + * page fault handled can race with it. > > > > + */ > > > > cleanup_vma_from_mm(vma); > > > > delete_vma(mm, vma); > > > > cond_resched(); > > > > -- > > > > 2.39.2.722.g9855ee24e9-goog > > > > > > > > > > > > > > > -- > > To unsubscribe from this group and stop receiving emails from it, send = an email to kernel-team+unsubscribe@android.com. > >