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AJvYcCV2ArxTALQgJdJUVGMwTZH4e+h3RfbGXM1HWlEFdOHfEfGew8ZIHry8FUBNUuWr24cML/Es4EfbmPkE60eNwD3rLTs= X-Gm-Message-State: AOJu0YzHto0APSZhctDHCkjhIcWo4XMYJKb6lCJ3AUmBcYLgDLx4Z3Ak WPT2nSy/BXfJ+veVxcm+hhod9qEsgwBXKMLd8foGNYbM6A/Afn7qT9fp36uHtkIF7jmxZhD69UR hOh+rjRBoT7HbK5LexYeOXREJulE= X-Google-Smtp-Source: AGHT+IHMeY5jM/wD/wv97U9VYE3QYen31/sqybOI0/uXha2NbNByjUWShGK4eJgrwM7PUPoyfc0VBUbdKDa6O/Bs/8I= X-Received: by 2002:a17:907:8e93:b0:a6e:f62d:bd02 with SMTP id a640c23a62f3a-a7245c84f2emr1146127066b.7.1719529359712; Thu, 27 Jun 2024 16:02:39 -0700 (PDT) MIME-Version: 1.0 References: <20240627221413.671680-1-yang@os.amperecomputing.com> <20240627155425.a31792e7c4709facfcbd417c@linux-foundation.org> In-Reply-To: <20240627155425.a31792e7c4709facfcbd417c@linux-foundation.org> From: Yang Shi Date: Thu, 27 Jun 2024 16:02:27 -0700 Message-ID: Subject: Re: [v2 PATCH] mm: gup: do not call try_grab_folio() in slow path To: Andrew Morton Cc: Yang Shi , peterx@redhat.com, yangge1116@126.com, david@redhat.com, linux-mm@kvack.org, linux-kernel@vger.kernel.org, stable@vger.kernel.org, Vivek Kasireddy Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: quoted-printable X-Rspamd-Server: rspam07 X-Rspamd-Queue-Id: 81089C0015 X-Stat-Signature: jjrqk178ecyp5ugjfbd3op4isf9eu6os X-Rspam-User: X-HE-Tag: 1719529361-325715 X-HE-Meta: U2FsdGVkX19ZbJFQhGQTnvqGp1OPs2Vs9bCmuQsR8EAZGSK+kEmlgHUSOzpPTdl0YIGJoxyQLn2h1jZhRu86cp7Xkwl/z7LYz5PrOekyYsBZMxm+6FzpXPMxkZN7caxIqdVEBp/bRjNBj5MX+dRxu+k7nNnNQFI1/LQmET1RmluAttaI/K2s19YF9kuOX9Elo9KqAsL5d7Q2/WLDufAAmdRcNPkL7YniSpnGISQVK07nyLqMCbemZkOjYjuQmf70NvLcljzQjrd07EcRfkCZnqqUcjhYIJTnHCf/KTWs88LklFu7Gk38DhUTpxvl9RfJZCG3Ge3M6+q5c+ebmKbM8qPOvg+P+3wBJPoOEK35TrnKGImHS+Hj3Um7aoo675jXT1J397DVGAYeL1Ct0+EL3R8bmfVzoLf55Cn0uCVOeAvSzlahTqHUOJjVQlbYqtr6rNYFbZ/KldWUoGHb1zZEVoSqw/GzCiuuIDPhCsCjaB1g9+5Jf2l0F8jQyKzLH/ASkdn6T/mq+1mrmQ62Otb3DnXJ0BMXRxRvhvXfLoqOTTysZruAw9Tje3/fI6U/ffZJ/w/8CG6Lr4UCk/MdI2DeRx3ShTyxyNyfUfbQ94SQFwqlxisTDoWZL0Wm9sBQVzFLE3+WdbCbcNInJjVVK2taxjXcHskGcmoIJHClWJD76VtbN52nXHBL+Ucf1OWiCHU5EqSMMUkZvpY/ikVnAg92YXkU5QoW+AJN8i5okL7OU4y9aOAeHXGr86djCKYkuBzypu7AXOnjKMhn7DoM85ENeXRNCyMvgJ8ZhVFATA/zcBUbRfBLq1P40sFEwQnHXgX3dWoF6L6x864kt5zjL0uh77+HNmEuSiYEpZvYIVi/r+9ad277FC9k59igtGTr3VCe2pYxJdW5Qcqjj9H1Ps/e9di/bwzl1/5YLIwc5h1FYJTD2pPFx4lcC4yW/oIi5KLBQlcbLcTrr85ZrwRKK/9 O3sQ/riY 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 X-Bogosity: Ham, tests=bogofilter, spamicity=0.001038, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On Thu, Jun 27, 2024 at 3:54=E2=80=AFPM Andrew Morton wrote: > > On Thu, 27 Jun 2024 15:14:13 -0700 Yang Shi = wrote: > > > The try_grab_folio() is supposed to be used in fast path and it elevate= s > > folio refcount by using add ref unless zero. We are guaranteed to have > > at least one stable reference in slow path, so the simple atomic add > > could be used. The performance difference should be trivial, but the > > misuse may be confusing and misleading. > > > > In another thread [1] a kernel warning was reported when pinning folio > > in CMA memory when launching SEV virtual machine. The splat looks like= : > > > > [ 464.325306] WARNING: CPU: 13 PID: 6734 at mm/gup.c:1313 __get_user_p= ages+0x423/0x520 > > [ 464.325464] CPU: 13 PID: 6734 Comm: qemu-kvm Kdump: loaded Not taint= ed 6.6.33+ #6 > > [ 464.325477] RIP: 0010:__get_user_pages+0x423/0x520 > > [ 464.325515] Call Trace: > > [ 464.325520] > > [ 464.325523] ? __get_user_pages+0x423/0x520 > > [ 464.325528] ? __warn+0x81/0x130 > > [ 464.325536] ? __get_user_pages+0x423/0x520 > > [ 464.325541] ? report_bug+0x171/0x1a0 > > [ 464.325549] ? handle_bug+0x3c/0x70 > > [ 464.325554] ? exc_invalid_op+0x17/0x70 > > [ 464.325558] ? asm_exc_invalid_op+0x1a/0x20 > > [ 464.325567] ? __get_user_pages+0x423/0x520 > > [ 464.325575] __gup_longterm_locked+0x212/0x7a0 > > [ 464.325583] internal_get_user_pages_fast+0xfb/0x190 > > [ 464.325590] pin_user_pages_fast+0x47/0x60 > > [ 464.325598] sev_pin_memory+0xca/0x170 [kvm_amd] > > [ 464.325616] sev_mem_enc_register_region+0x81/0x130 [kvm_amd] > > > > ... > > > > Fixes: 57edfcfd3419 ("mm/gup: accelerate thp gup even for "pages !=3D N= ULL"") > > Cc: [6.6+] > > So we want something against Linus mainline for backporting ease. > > > 3. Rebased onto the latest mm-unstable > > mm-unstable is quite different - memfd_pin_folios() doesn't exist in > mainline! > > So can you please prepare the fix against current -linus? I'll hang > onto this patch to guide myself when I redo Vivek's "mm/gup: Introduce > memfd_pin_folios() for pinning memfd folios" series on top. Sure, I'm going to come up with another patch on top of Linus's tree. > > >