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t=1681803497; bh=aywSGl7+sE6dTjQmK39DnKLM7QTCjnhC/MpmTJAZGeo=; h=Date:From:To:Cc:Subject:References:In-Reply-To:From; b=qPWwyDYJVFjNXr0MQOogfVNnADpvKkZYjxdvzqivGrwTCvkQibNVJJXBcc5gho//z z96gMpaI2SvUA5mPKw0t1mRmdJS9uRVI+OxgwkHM4/PUNOfNqvs955fP4Np2UiDY9v xutEPwMwPhAyX9c74/oHI2plz6BjafMXsZflMj6wgojaKmtiivzS1N3oYLS3C1s5ZT ehZy1uYfIfjGqj7tuxcZk0HTQlSHChBVI2EXZpbKbUorVHI8/MECayY+SakWjCwtPu gQABjByOkhR6Lt2Ws3uR69rPDranKEZx+FK6NeKB68XjDANGPvb59cku3i9Utm/xw7 Y4lgqLWQ8cFyg== Date: Tue, 18 Apr 2023 09:38:10 +0200 From: Christian Brauner To: Hugh Dickins Cc: Luis Chamberlain , akpm@linux-foundation.org, willy@infradead.org, linux-mm@kvack.org, p.raghav@samsung.com, da.gomez@samsung.com, a.manzanares@samsung.com, dave@stgolabs.net, yosryahmed@google.com, keescook@chromium.org, patches@lists.linux.dev, linux-kernel@vger.kernel.org Subject: Re: [PATCH v2 6/6] shmem: add support to ignore swap Message-ID: <20230418-zynisch-satzglied-55821361f70a@brauner> References: <20230309230545.2930737-1-mcgrof@kernel.org> <20230309230545.2930737-7-mcgrof@kernel.org> <79eae9fe-7818-a65c-89c6-138b55d609a@google.com> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline In-Reply-To: <79eae9fe-7818-a65c-89c6-138b55d609a@google.com> X-Rspam-User: X-Rspamd-Server: rspam03 X-Stat-Signature: n7imicdfh37nadbwpwfe7cy9db3esmxz X-Rspamd-Queue-Id: 47559140003 X-HE-Tag: 1681803499-784880 X-HE-Meta: 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 GEXTiM+t a67zAjmIIPmg/s6JUiYHpRgvfQE+sRdNVyqbVpSC41P7LrjMiUQAOH2RqMgON3Q1v6vVWUysgde53vXQNRfi1BYSrHduDAgtf51e/gyv6LestKL+BQwS+Gs6Sf6o5ZIZKazQl8pcOA+QSHq0/oL9fAtIirWHMtF7Bs23cZNPGl+HfY6Ig8zJzHf2S7z7uEEpDkL9Dygp13lQAlJ4wlNr+VXfB4l3kH9+atM2Hsj6/j/WYVNET1KlDrZahOQKln+O/NXVec4JgiMnqqDz83fwYKramS9HMbOU8H0ul9zbe6iP5TyKQ5Zrqo0FDQQ== 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 Mon, Apr 17, 2023 at 10:50:59PM -0700, Hugh Dickins wrote: > On Thu, 9 Mar 2023, Luis Chamberlain wrote: > > > In doing experimentations with shmem having the option to avoid swap > > becomes a useful mechanism. One of the *raves* about brd over shmem is > > you can avoid swap, but that's not really a good reason to use brd if > > we can instead use shmem. Using brd has its own good reasons to exist, > > but just because "tmpfs" doesn't let you do that is not a great reason > > to avoid it if we can easily add support for it. > > > > I don't add support for reconfiguring incompatible options, but if > > we really wanted to we can add support for that. > > > > To avoid swap we use mapping_set_unevictable() upon inode creation, > > and put a WARN_ON_ONCE() stop-gap on writepages() for reclaim. > > I have one big question here, which betrays my ignorance: > I hope that you or Christian can reassure me on this. > > tmpfs has fs_flags FS_USERNS_MOUNT. I know nothing about namespaces, > nothing; but from overhearings, wonder if an ordinary user in a namespace > might be able to mount their own tmpfs with "noswap", and thereby evade > all accounting of the locked memory. > > That would be an absolute no-no for this patch; but I assume that even > if so, it can be easily remedied by inserting an appropriate (unknown > to me!) privilege check where the "noswap" option is validated. Oh, good catch. Thanks! So you would just need sm like: diff --git a/mm/shmem.c b/mm/shmem.c index 787e83791eb5..21ce9b26bb4d 100644 --- a/mm/shmem.c +++ b/mm/shmem.c @@ -3571,6 +3571,10 @@ static int shmem_parse_one(struct fs_context *fc, struct fs_parameter *param) ctx->seen |= SHMEM_SEEN_INUMS; break; case Opt_noswap: + if ((fc->user_ns != &init_user_ns) || !capable(CAP_SYS_ADMIN)) { + return invalfc(fc, + "Turning off swap in unprivileged tmpfs mounts unsupported"); + } ctx->noswap = true; ctx->seen |= SHMEM_SEEN_NOSWAP; break; The fc->user_ns is the userns that the tmpfs mount will be mounted in, i.e., fc->user_ns will become sb->s_user_ns if FS_USERNS_MOUNT is raised. So with the check above we require that the tmpfs instance must ultimately belong to the initial userns and that the caller has CAP_SYS_ADMIN in the initial userns (CAP_SYS_ADMIN guards swapon and swapoff) according to capabilities(7).