• Linus Torvalds's avatar
    Revert "ext4: make __ext4_get_inode_loc plug" · 72dbcf72
    Linus Torvalds authored
    This reverts commit b03755ad.
    
    This is sad, and done for all the wrong reasons.  Because that commit is
    good, and does exactly what it says: avoids a lot of small disk requests
    for the inode table read-ahead.
    
    However, it turns out that it causes an entirely unrelated problem: the
    getrandom() system call was introduced back in 2014 by commit
    c6e9d6f3
    
     ("random: introduce getrandom(2) system call"), and people
    use it as a convenient source of good random numbers.
    
    But part of the current semantics for getrandom() is that it waits for
    the entropy pool to fill at least partially (unlike /dev/urandom).  And
    at least ArchLinux apparently has a systemd that uses getrandom() at
    boot time, and the improvements in IO patterns means that existing
    installations suddenly start hanging, waiting for entropy that will
    never happen.
    
    It seems to be an unlucky combination of not _quite_ enough entropy,
    together with a particular systemd version and configuration.  Lennart
    says that the systemd-random-seed process (which is what does this early
    access) is supposed to not block any other boot activity, but sadly that
    doesn't actually seem to be the case (possibly due bogus dependencies on
    cryptsetup for encrypted swapspace).
    
    The correct fix is to fix getrandom() to not block when it's not
    appropriate, but that fix is going to take a lot more discussion.  Do we
    just make it act like /dev/urandom by default, and add a new flag for
    "wait for entropy"? Do we add a boot-time option? Or do we just limit
    the amount of time it will wait for entropy?
    
    So in the meantime, we do the revert to give us time to discuss the
    eventual fix for the fundamental problem, at which point we can re-apply
    the ext4 inode table access optimization.
    
    Reported-by: default avatarAhmed S. Darwish <darwish.07@gmail.com>
    Cc: Ted Ts'o <tytso@mit.edu>
    Cc: Willy Tarreau <w@1wt.eu>
    Cc: Alexander E. Patrakov <patrakov@gmail.com>
    Cc: Lennart Poettering <mzxreary@0pointer.de>
    Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
    72dbcf72