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Sun, 12 Jan 2025 21:34:54 -0800 (PST) MIME-Version: 1.0 References: <20241230174621.61185-1-ryncsn@gmail.com> <20241230174621.61185-8-ryncsn@gmail.com> In-Reply-To: From: Kairui Song Date: Mon, 13 Jan 2025 13:34:38 +0800 X-Gm-Features: AbW1kvalYfZ6QF02v6ErUsIBUzFgBeGySrLFWZqM8BIJgksFaPrKE-lKE2dFhWI Message-ID: Subject: Re: [PATCH v3 07/13] mm, swap: hold a reference during scan and cleanup flag usage To: Baoquan He Cc: linux-mm@kvack.org, Andrew Morton , Chris Li , Barry Song , Ryan Roberts , Hugh Dickins , Yosry Ahmed , "Huang, Ying" , Nhat Pham , Johannes Weiner , Kalesh Singh , linux-kernel@vger.kernel.org Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: quoted-printable X-Rspam-User: X-Rspamd-Server: rspam03 X-Rspamd-Queue-Id: 82BBE80010 X-Stat-Signature: ayp11qdr4jqgo5ozj5kbcsm5j7j9ioaq X-HE-Tag: 1736746496-445683 X-HE-Meta: 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 T3vgR1BR GTX+sn7OV8BVLUxvWXJNwSFQf42CxagVYYu9utYHqSbHayREFHMDWJPGcnTg9EFx4z+2KIpFBi1TUTMBxQ9c4BD4ma7EBaMJVgYp5v6wk3zkNKbwSBx8GotZD3FZAP7WaHTXCQsrVeFHJ5tw1Z1erVWXL3iEtPEjiHP0BRiIuiEp/7IuAA4X4z7KNQdo3gSJDAC1KkR3Ob2LU1l8/mp2JLZOk2cQnZRcrJJ5cqeaXt29q+PnpnWKwCgi8aXqmCPh5EyKVpriWYjkcdNefqBtZ2S0KtyB4iOGEUw5/rsveNLwsW0GubdAkEqvq+cGAFrg8JxcylnMhPIdmlr1z9jrdIC9SJOyEnz142hZckaFw042KV7e3JQ2qwqlfmQ== X-Bogosity: Ham, tests=bogofilter, spamicity=0.000013, 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 Sat, Jan 4, 2025 at 1:46=E2=80=AFPM Baoquan He wrote: > > On 12/31/24 at 01:46am, Kairui Song wrote: > > From: Kairui Song > > > > The flag SWP_SCANNING was used as an indicator of whether a device > > is being scanned for allocation, and prevents swapoff. Combined with > > SWP_WRITEOK, they work as a set of barriers for a clean swapoff: > > > > 1. Swapoff clears SWP_WRITEOK, allocation requests will see > > ~SWP_WRITEOK and abort as it's serialized by si->lock. > > 2. Swapoff unuses all allocated entries. > > 3. Swapoff waits for SWP_SCANNING flag to be cleared, so ongoing > > allocations will stop, preventing UAF. > > 4. Now swapoff can free everything safely. > > > > This will make the allocation path have a hard dependency on > > si->lock. Allocation always have to acquire si->lock first for > > setting SWP_SCANNING and checking SWP_WRITEOK. > > > > This commit removes this flag, and just uses the existing per-CPU > > refcount instead to prevent UAF in step 3, which serves well for > > such usage without dependency on si->lock, and scales very well too. > > Just hold a reference during the whole scan and allocation process. > > Swapoff will kill and wait for the counter. > > > > And for preventing any allocation from happening after step 1 so the > > unuse in step 2 can ensure all slots are free, swapoff will acquire > > the ci->lock of each cluster one by one to ensure all allocations > > see ~SWP_WRITEOK and abort. > > Changing to use si->users is great, while wondering why we need acquire = =3D > each ci->lock now. After setup 1, we have cleared SWP_WRITEOK, and take > the si off swap_avail_heads list. No matter what, we just need wait for > p->comm's completion and continue, why bothering to loop for the > ci->lock acquiring? > Hi Baoquan, Waiting for p->comm's completion must be done after unuse is called (unuse will need to take the si->users refcound, so it can't be dead yet), but unuse must be called after no one will allocate any new entry. That is guaranteed by the loop ci->lock acquiring.