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    SubjectRe: [PATCH] nfs: remove redundant tests on reading new pages
    On Mon, Dec 01, 2008 at 11:19:22AM +0800, Wu Fengguang wrote:
    > aops->readpages() and its NFS helper readpage_async_filler() will only
    > be called to do readahead I/O for newly allocated pages. So it's not
    > necessary to test for the always 0 dirty/uptodate page flags.
    >
    > The removal of nfs_wb_page() call also fixes a readahead bug: the NFS
    > readahead has been synchronous since 2.6.23, because that call will
    > clear PG_readahead, which is the reminder for asynchronous readahead.
    >
    > More background: the PG_readahead page flag is shared with PG_reclaim,
    > one for read path and the other for write path. clear_page_dirty_for_io()
    > unconditionally clears PG_readahead to prevent possible readahead residuals,
    > assuming itself to be always called in the write path. However, NFS is one
    > and the only exception in that it _always_ calls clear_page_dirty_for_io()
    > in the read path, i.e. for readpages()/readpage().
    >
    > Cc: Trond Myklebust <Trond.Myklebust@netapp.com>
    > Signed-off-by: Wu Fengguang <wfg@linux.intel.com>
    > ---
    > fs/nfs/read.c | 6 ------
    > 1 file changed, 6 deletions(-)
    >
    > This bug was found when playing with my readahead tracing module posted at
    > http://lkml.org/lkml/2008/11/27/373 and confirmed to work.
    >
    > The NFS readahead traces are now
    >
    > [ 59.972526] readahead-initial0(pid=3646(dd), dev=00:0f(0:f), ino=293189(1G), req=0+25, ra=0+60-30, async=0) = 60
    > [ 59.992734] readahead-subsequent(pid=3646(dd), dev=00:0f(0:f), ino=293189(1G), req=30+20, ra=60+30-30, async=1) = 30
    > [ 60.030900] readahead-subsequent(pid=3646(dd), dev=00:0f(0:f), ino=293189(1G), req=60+15, ra=90+30-30, async=1) = 30
    > [ 60.031687] readahead-subsequent(pid=3646(dd), dev=00:0f(0:f), ino=293189(1G), req=90+10, ra=120+30-30, async=1) = 30
    >
    > The async field was always 0 before this patch.

    On the trace messages: they are generated by the command
    'dd if=/tmp/sparse/1G of=/dev/null bs=100k count=9' with readahead_size=120k.

    The printk for generating the messages is:

    printk(KERN_DEBUG "readahead-%s(pid=%d(%s), dev=%02x:%02x(%s), "
    "ino=%lu(%s), req=%lu+%lu, ra=%lu+%d-%d, async=%d) = %d\n",
    ra_pattern_names[pattern],
    current->pid, current->comm,
    MAJOR(mapping->host->i_sb->s_dev),
    MINOR(mapping->host->i_sb->s_dev),
    mapping->host->i_sb->s_id,
    mapping->host->i_ino,
    filp->f_path.dentry->d_name.name,
    offset, req_size,
    ra->start, ra->size, ra->async_size,
    async,
    actual);

    Thanks,
    Fengguang

    > --- linux-2.6.orig/fs/nfs/read.c
    > +++ linux-2.6/fs/nfs/read.c
    > @@ -533,12 +533,6 @@ readpage_async_filler(void *data, struct
    > unsigned int len;
    > int error;
    >
    > - error = nfs_wb_page(inode, page);
    > - if (error)
    > - goto out_unlock;
    > - if (PageUptodate(page))
    > - goto out_unlock;
    > -
    > len = nfs_page_length(page);
    > if (len == 0)
    > return nfs_return_empty_page(page);


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