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Thu, 19 Oct 2023 08:20:03 -0700 (PDT) MIME-Version: 1.0 References: <20231009064230.2952396-1-surenb@google.com> <20231009064230.2952396-2-surenb@google.com> <7495754c-9267-74af-b943-9b0f86619b5d@redhat.com> In-Reply-To: <7495754c-9267-74af-b943-9b0f86619b5d@redhat.com> From: Suren Baghdasaryan Date: Thu, 19 Oct 2023 08:19:50 -0700 Message-ID: Subject: Re: [PATCH v3 1/3] mm/rmap: support move to different root anon_vma in folio_move_anon_rmap() To: David Hildenbrand Cc: Peter Xu , akpm@linux-foundation.org, viro@zeniv.linux.org.uk, brauner@kernel.org, shuah@kernel.org, aarcange@redhat.com, lokeshgidra@google.com, hughd@google.com, mhocko@suse.com, axelrasmussen@google.com, rppt@kernel.org, willy@infradead.org, Liam.Howlett@oracle.com, jannh@google.com, zhangpeng362@huawei.com, bgeffon@google.com, kaleshsingh@google.com, ngeoffray@google.com, jdduke@google.com, linux-mm@kvack.org, linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org, linux-kselftest@vger.kernel.org, kernel-team@android.com Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: quoted-printable X-Rspamd-Queue-Id: 0CB90A0035 X-Rspam-User: X-Stat-Signature: 4453muq8mjhgogd4hb8rozhtyhaq5fb1 X-Rspamd-Server: rspam01 X-HE-Tag: 1697728804-602534 X-HE-Meta: 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 A1R3NZYJ 0EWCNOtwb1y+jrmERx/uWkqq54IXXMONpRyy/r4ZS/0jQ17cPZOcKeuMDurPI3EIaGWUAPfT3X8zs8yKdWXlZmewiEMArDRfDMEV4+hgLfsgzxzTY4iDXNZOIdWr60HQpRXenhbMqrP4m7bUAMKQtMmTvlXIfFuqJWffiK+L2mECBH+/QBJ6JZ+JksFSC4k4P/IZOnw1WItPCo/PM7y6+5MJ0maTBFg9uTXsebxtV1m5tCiPkQEUTf7OtUxWHBBSWw+pDUN3i7N10IqYL3y+ACn0Grjp8vmU3Wf5CQn821QPQCZMm9Y+ZDc7oT2sRUnTdBElg+xvk3Uo+sGU= 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 Fri, Oct 13, 2023 at 1:04=E2=80=AFAM David Hildenbrand wrote: > > On 13.10.23 00:01, Peter Xu wrote: > > On Sun, Oct 08, 2023 at 11:42:26PM -0700, Suren Baghdasaryan wrote: > >> From: Andrea Arcangeli > >> > >> For now, folio_move_anon_rmap() was only used to move a folio to a > >> different anon_vma after fork(), whereby the root anon_vma stayed > >> unchanged. For that, it was sufficient to hold the folio lock when > >> calling folio_move_anon_rmap(). > >> > >> However, we want to make use of folio_move_anon_rmap() to move folios > >> between VMAs that have a different root anon_vma. As folio_referenced(= ) > >> performs an RMAP walk without holding the folio lock but only holding = the > >> anon_vma in read mode, holding the folio lock is insufficient. > >> > >> When moving to an anon_vma with a different root anon_vma, we'll have = to > >> hold both, the folio lock and the anon_vma lock in write mode. > >> Consequently, whenever we succeeded in folio_lock_anon_vma_read() to > >> read-lock the anon_vma, we have to re-check if the mapping was changed > >> in the meantime. If that was the case, we have to retry. > >> > >> Note that folio_move_anon_rmap() must only be called if the anon page = is > >> exclusive to a process, and must not be called on KSM folios. > >> > >> This is a preparation for UFFDIO_MOVE, which will hold the folio lock, > >> the anon_vma lock in write mode, and the mmap_lock in read mode. > >> > >> Signed-off-by: Andrea Arcangeli > >> Signed-off-by: Suren Baghdasaryan > >> --- > >> mm/rmap.c | 24 ++++++++++++++++++++++++ > >> 1 file changed, 24 insertions(+) > >> > >> diff --git a/mm/rmap.c b/mm/rmap.c > >> index c1f11c9dbe61..f9ddc50269d2 100644 > >> --- a/mm/rmap.c > >> +++ b/mm/rmap.c > >> @@ -542,7 +542,9 @@ struct anon_vma *folio_lock_anon_vma_read(struct f= olio *folio, > >> struct anon_vma *root_anon_vma; > >> unsigned long anon_mapping; > >> > >> +retry: > >> rcu_read_lock(); > >> +retry_under_rcu: > >> anon_mapping =3D (unsigned long)READ_ONCE(folio->mapping); > >> if ((anon_mapping & PAGE_MAPPING_FLAGS) !=3D PAGE_MAPPING_ANON) > >> goto out; > >> @@ -552,6 +554,16 @@ struct anon_vma *folio_lock_anon_vma_read(struct = folio *folio, > >> anon_vma =3D (struct anon_vma *) (anon_mapping - PAGE_MAPPING_ANO= N); > >> root_anon_vma =3D READ_ONCE(anon_vma->root); > >> if (down_read_trylock(&root_anon_vma->rwsem)) { > >> + /* > >> + * folio_move_anon_rmap() might have changed the anon_vma= as we > >> + * might not hold the folio lock here. > >> + */ > >> + if (unlikely((unsigned long)READ_ONCE(folio->mapping) != =3D > >> + anon_mapping)) { > >> + up_read(&root_anon_vma->rwsem); > >> + goto retry_under_rcu; > > > > Is adding this specific label worthwhile? How about rcu unlock and got= o > > retry (then it'll also be clear that we won't hold rcu read lock for > > unpredictable time)? > > +1, sounds good to me Sorry for the delay, I was travelling for a week. I was hesitant about RCU unlocking and then immediately re-locking but your point about holding it for unpredictable time makes sense. Will change. Thanks! > > -- > Cheers, > > David / dhildenb >