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    SubjectRe: [PATCH 6/6] writeback: merge for_kupdate and !for_kupdate cases
    On Tue, Jul 13, 2010 at 06:22:54AM +0800, Andrew Morton wrote:
    > On Mon, 12 Jul 2010 23:52:39 +0800
    > Wu Fengguang <fengguang.wu@intel.com> wrote:
    >
    > > > Also, I'd prefer that the
    > > > comments remain somewhat more descriptive of the circumstances that
    > > > we are operating under. Comments like "retry later to avoid blocking
    > > > writeback of other inodes" is far, far better than "retry later"
    > > > because it has "why" component that explains the reason for the
    > > > logic. You may remember why, but I sure won't in a few months time....
    >
    > me2 (of course). This code is waaaay too complex to be scrimping on comments.
    >
    > > Ah yes the comment is too simple. However the redirty_tail() is not to
    > > avoid blocking writeback of other inodes, but to avoid eating 100% CPU
    > > on busy retrying a dirty inode/page that cannot perform writeback for
    > > a while. (In theory redirty_tail() can still busy retry though, when
    > > there is only one single dirty inode.) So how about
    > >
    > > /*
    > > * somehow blocked: avoid busy retrying
    > > */
    >
    > That's much too short. Expand on the "somehow" - provide an example,
    > describe the common/expected cause. Fully explain what the "busy"
    > retry _is_ and how it can come about.

    It was a long story.. This redirty_tail() was introduced when more_io
    is introduced. The initial patch for more_io does not have the
    redirty_tail(), and when it's merged, several 100% iowait bug reports
    arises:

    reiserfs:
    http://lkml.org/lkml/2007/10/23/93

    jfs:
    commit 29a424f28390752a4ca2349633aaacc6be494db5
    JFS: clear PAGECACHE_TAG_DIRTY for no-write pages

    ext2:
    http://www.spinics.net/linux/lists/linux-ext4/msg04762.html

    They are all old bugs hidden in various filesystems that become
    "obvious" with the more_io patch. At the time, the ext2 bug is thought
    to be "trivial", so you didn't merge that fix. Instead the following
    patch with redirty_tail() is merged:

    http://www.spinics.net/linux/lists/linux-ext4/msg04507.html

    This will in general prevent 100% on ext2 and other possibly unknown FS bugs.

    I'll take David's comments and note the above in changelog.

    Thanks,
    Fengguang


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