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Our VM tests and everything related to our virtualisation infrastructure is currently broken if used with kernel 4.19 or later. The reason for this is that since 4.19, overlayfs uses the O_NOATIME flag when opening files in lowerdir and this doesn't play nice with the way we pass the Nix store to our QEMU guests. On a NixOS system, paths in the Nix store are typically owned by root but the QEMU process is usually run by an ordinary user. Using O_NOATIME on a file where you're not the owner (or superuser) will return with EPERM (Operation not permitted). This is exactly what happens in our VM tests, because we're using overlayfs in the guests to allow writes to the store. Another implication of this is that the default kernel version for NixOS 19.03 has been reverted to Linux 4.14. Work on getting this upstream is still ongoing and the patch I posted previously was incomplete, needs rework and also some more review from upstream maintainers - in summary: This will take a while. So instead of rushing in a kernel patch to nixpkgs, which will affect all users of overlayfs, not just NixOS VM tests, I opted to patch QEMU for now to ignore the O_NOATIME flag in 9p. I think this is also the least impacting change, because even if you care about whether access times are written or not, you get the same behaviour as with Linux 4.19 in conjunction with QEMU. Signed-off-by: aszlig <aszlig@nix.build> Fixes: https://github.com/NixOS/nixpkgs/issues/54509
45 lines
1.7 KiB
Diff
45 lines
1.7 KiB
Diff
commit cdc3e7eeafa9f683214d2c15d52ef384c3de6611
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Author: aszlig <aszlig@nix.build>
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Date: Mon Mar 18 13:21:01 2019 +0100
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9pfs: Ignore O_NOATIME open flag
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Since Linux 4.19, overlayfs uses the O_NOATIME flag on its lowerdir,
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which in turn causes errors when the Nix store is mounted in the guest
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because the file owner of the store paths typically don't match the
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owner of the QEMU process.
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After submitting a patch to the overlayfs mailing list[1], it turns out
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that my patch was incomplete[2] and needs a bit more rework.
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So instead of using an incomplete kernel patch in nixpkgs, which affects
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*all* users of overlayfs, not just NixOS VM tests, I decided that for
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now it's better to patch QEMU instead.
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The change here really only ignores the O_NOATIME flag so that the
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behaviour is similar to what NFS does. From open(2):
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This flag may not be effective on all filesystems. One example is NFS,
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where the server maintains the access time.
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This change is therefore only temporary until the final fix lands in the
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stable kernel releases.
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[1]: https://www.spinics.net/lists/linux-unionfs/msg06755.html
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[2]: https://www.spinics.net/lists/linux-unionfs/msg06756.html
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Signed-off-by: aszlig <aszlig@nix.build>
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diff --git a/hw/9pfs/9p.c b/hw/9pfs/9p.c
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index 55821343e5..0b8425fe18 100644
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--- a/hw/9pfs/9p.c
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+++ b/hw/9pfs/9p.c
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@@ -127,7 +127,6 @@ static int dotl_to_open_flags(int flags)
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{ P9_DOTL_LARGEFILE, O_LARGEFILE },
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{ P9_DOTL_DIRECTORY, O_DIRECTORY },
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{ P9_DOTL_NOFOLLOW, O_NOFOLLOW },
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- { P9_DOTL_NOATIME, O_NOATIME },
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{ P9_DOTL_SYNC, O_SYNC },
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};
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