Date:   Mon, 13 Mar 2023 18:21:30 -0700
From:   Davidlohr Bueso <dave@...olabs.net>
To:     Luis Chamberlain <mcgrof@...nel.org>
Cc:     hughd@...gle.com, akpm@...ux-foundation.org, willy@...radead.org,
        brauner@...nel.org, linux-mm@...ck.org, p.raghav@...sung.com,
        da.gomez@...sung.com, a.manzanares@...sung.com,
        yosryahmed@...gle.com, keescook@...omium.org,
        patches@...ts.linux.dev, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 0/6] tmpfs: add the option to disable swap

On Thu, 09 Mar 2023, Luis Chamberlain wrote:

>Changes on this v2 PATCH series:
>
>  o Added all respective tags for Reviewed-by, Acked-by's
>  o David Hildenbrand suggested on the update-docs patch to mention THP.
>    It turns out tmpfs.rst makes absolutely no mention to THP at all
>    so I added all the relevant options to the docs including the
>    system wide sysfs file. All that should hopefully demistify that
>    and make it clearer.
>  o Yosry Ahmed spell checked my patch "shmem: add support to ignore swap"
>
>Changes since RFCv2 to the first real PATCH series:
>
>  o Added Christian Brauner'd Acked-by for the noswap patch (the only
>    change in that patch is just the new shmem_show_options() change I
>    describe below).
>  o Embraced Yosry Ahmed's recommendation to use mapping_set_unevictable()
>    to at ensure the folios at least appear in the unevictable LRU.
>    Since that is the goal, this accomplishes what we want and the VM
>    takes care of things for us. The shem writepage() still uses a stop-gap
>    to ensure we don't get called for swap when its shmem uses
>    mapping_set_unevictable().
>  o I had evaluated using shmem_lock() instead of calling mapping_set_unevictable()
>    but upon my review this doesn't make much sense, as shmem_lock() was
>    designed to make use of the RLIMIT_MEMLOCK and this was designed for
>    files / IPC / unprivileged perf limits. If we were to use
>    shmem_lock() we'd bump the count on each new inode. Using
>    shmem_lock() would also complicate inode allocation on shmem as
>    we'd to unwind on failure from the user_shm_lock(). It would also
>    beg the question of when to capture a ucount for an inode, should we
>    just share one for the superblock at shmem_fill_super() or do we
>    really need to capture it at every single inode creation? In theory
>    we could end up with different limits. The simple solution is to
>    juse use mapping_set_unevictable() upon inode creation and be done
>    with it, as it cannot fail.
>  o Update the documentation for tmpfs before / after my patch to
>    reflect use cases a bit more clearly between ramfs, tmpfs and brd
>    ramdisks.
>  o I updated the shmem_show_options() to also reveal the noswap option
>    when its used.
>  o Address checkpatch style complaint with spaces before tabs on
>    shmem_fs.h.
>
>Chances since first RFC:
>
>  o Matthew suggested BUG_ON(!folio_test_locked(folio)) is not needed
>    on writepage() callback for shmem so just remove that.
>  o Based on Matthew's feedback the inode is set up early as it is not
>    reset in case we split the folio. So now we move all the variables
>    we can set up really early.
>  o shmem writepage() should only be issued on reclaim, so just move
>    the WARN_ON_ONCE(!wbc->for_reclaim) early so that the code and
>    expectations are easier to read. This also avoid the folio splitting
>    in case of that odd case.
>  o There are a few cases where the shmem writepage() could possibly
>    hit, but in the total_swap_pages we just bail out. We shouldn't be
>    splitting the folio then. Likewise for VM_LOCKED case. But for
>    a writepage() on a VM_LOCKED case is not expected so we want to
>    learn about it so add a WARN_ON_ONCE() on that condition.
>  o Based on Yosry Ahmed's feedback the patch which allows tmpfs to
>    disable swap now just uses mapping_set_unevictable() on inode
>    creation. In that case writepage() should not be called so we
>    augment the WARN_ON_ONCE() for writepage() for that case to ensure
>    that never happens.
>
>To test I've used kdevops [0] 8 vpcu 4 GiB libvirt guest on linux-next.
>
>I'm doing this work as part of future experimentation with tmpfs and the
>page cache, but given a common complaint found about tmpfs is the
>innability to work without the page cache I figured this might be useful
>to others. It turns out it is -- at least Christian Brauner indicates
>systemd uses ramfs for a few use-cases because they don't want to use
>swap and so having this option would let them move over to using tmpfs
>for those small use cases, see systemd-creds(1).
>
>To see if you hit swap:
>
>mkswap /dev/nvme2n1
>swapon /dev/nvme2n1
>free -h
>
>With swap - what we see today
>=============================
>mount -t tmpfs            -o size=5G           tmpfs /data-tmpfs/
>dd if=/dev/urandom of=/data-tmpfs/5g-rand2 bs=1G count=5
>free -h
>               total        used        free      shared  buff/cache   available
>Mem:           3.7Gi       2.6Gi       1.2Gi       2.2Gi       2.2Gi       1.2Gi
>Swap:           99Gi       2.8Gi        97Gi
>
>
>Without swap
>=============
>
>free -h
>               total        used        free      shared  buff/cache   available
>Mem:           3.7Gi       387Mi       3.4Gi       2.1Mi        57Mi       3.3Gi
>Swap:           99Gi          0B        99Gi
>mount -t tmpfs            -o size=5G -o noswap tmpfs /data-tmpfs/
>dd if=/dev/urandom of=/data-tmpfs/5g-rand2 bs=1G count=5
>free -h
>               total        used        free      shared  buff/cache   available
>Mem:           3.7Gi       2.6Gi       1.2Gi       2.3Gi       2.3Gi       1.1Gi
>Swap:           99Gi        21Mi        99Gi
>
>The mix and match remount testing
>=================================
>
># Cannot disable swap after it was first enabled:
>mount -t tmpfs            -o size=5G           tmpfs /data-tmpfs/
>mount -t tmpfs -o remount -o size=5G -o noswap tmpfs /data-tmpfs/
>mount: /data-tmpfs: mount point not mounted or bad option.
>       dmesg(1) may have more information after failed mount system call.
>dmesg -c
>tmpfs: Cannot disable swap on remount
>
># Remount with the same noswap option is OK:
>mount -t tmpfs            -o size=5G -o noswap tmpfs /data-tmpfs/
>mount -t tmpfs -o remount -o size=5G -o noswap tmpfs /data-tmpfs/
>dmesg -c
>
># Trying to enable swap with a remount after it first disabled:
>mount -t tmpfs            -o size=5G -o noswap tmpfs /data-tmpfs/
>mount -t tmpfs -o remount -o size=5G           tmpfs /data-tmpfs/
>mount: /data-tmpfs: mount point not mounted or bad option.
>       dmesg(1) may have more information after failed mount system call.
>dmesg -c
>tmpfs: Cannot enable swap on remount if it was disabled on first mount

Nice! For the whole series:

Reviewed-by: Davidlohr Bueso <dave@...olabs.net>

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