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    SubjectRe: [PATCH 4/6] writeback: dont redirty tail an inode with dirty pages
    On Tue, Jul 13, 2010 at 06:13:17AM +0800, Andrew Morton wrote:
    > On Mon, 12 Jul 2010 23:31:27 +0800
    > Wu Fengguang <fengguang.wu@intel.com> wrote:
    >
    > > > > + } else if (inode->i_state & I_DIRTY) {
    > > > > + /*
    > > > > + * At least XFS will redirty the inode during the
    > > > > + * writeback (delalloc) and on io completion (isize).
    > > > > + */
    > > > > + redirty_tail(inode);
    > > >
    > > > I'd drop the mention of XFS here - any filesystem that does delayed
    > > > allocation or unwritten extent conversion after Io completion will
    > > > cause this. Perhaps make the comment:
    > > >
    > > > /*
    > > > * Filesystems can dirty the inode during writeback
    > > > * operations, such as delayed allocation during submission
    > > > * or metadata updates after data IO completion.
    > > > */
    > >
    > > Thanks, comments updated accordingly.
    > >
    > > ---
    > > writeback: don't redirty tail an inode with dirty pages
    > >
    > > This avoids delaying writeback for an expired (XFS) inode with lots of
    > > dirty pages, but no active dirtier at the moment. Previously we only do
    > > that for the kupdate case.
    > >
    >
    > You didn't actually explain the _reason_ for making this change.
    > Please always do that.

    OK. It's actually extending commit b3af9468ae from the kupdate-only case to
    both kupdate and !kupdate cases.

    The commit documented the reason:

    Debug traces show that in per-bdi writeback, the inode under writeback
    almost always get redirtied by a busy dirtier. We used to call
    redirty_tail() in this case, which could delay inode for up to 30s.

    This is unacceptable because it now happens so frequently for plain cp/dd,
    that the accumulated delays could make writeback of big files very slow.

    So let's distinguish between data redirty and metadata only redirty.
    The first one is caused by a busy dirtier, while the latter one could
    happen in XFS, NFS, etc. when they are doing delalloc or updating isize.

    Commit b3af9468ae only does that for kupdate case because requeue_io() was
    only called in the kupdate case. Now we are merging the kupdate and !kupdate
    cases in patch 6/6 (why not?), so is this patch.

    > The patch is... surprisingly complicated, although the end result
    > looks OK. This is not aided by the partial duplication between
    > mapping_tagged(PAGECACHE_TAG_DIRTY) and I_DIRTY_PAGES. I don't think
    > we can easily remove I_DIRTY_PAGES because it's used for the
    > did-someone-just-dirty-a-page test here.

    I double checked I_DIRTY_PAGES. The main difference to PAGECACHE_TAG_DIRTY is:
    I_DIRTY_PAGES (at the line removed by this patch) means there are _new_ pages
    get dirtied during writeback, while PAGECACHE_TAG_DIRTY means there are dirty
    pages. In this sense, if the I_DIRTY_PAGES handling is the same as
    PAGECACHE_TAG_DIRTY, the code can be merged into PAGECACHE_TAG_DIRTY, as this
    patch does.

    The other minor differences are

    - in *_set_page_dirty*(), PAGECACHE_TAG_DIRTY is set racelessly, while
    I_DIRTY_PAGES might be set on the inode for a page just truncated.
    The difference has no real impact on this patch (it's actually
    slightly better now).

    - afs_fsync() always set I_DIRTY_PAGES after calling afs_writepages().
    The call was there in the first day (introduce by David Howells).
    What was the intention, hmm..?

    > This code is way too complex and fragile and I fear that anything we do
    > to it will break something :(

    Agreed. Let's try to simplify it :)

    Thanks,
    Fengguang


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