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    SubjectRe: [PATCH 2/9] readahead: clean up and simplify the code for filemap page fault readahead
    On Wed, Dec 19, 2007 at 12:54:23AM +0100, Nick Piggin wrote:
    > On Tue, Dec 18, 2007 at 07:50:33PM +0800, Fengguang Wu wrote:
    > > On Tue, Dec 18, 2007 at 09:19:07AM +0100, Nick Piggin wrote:
    > > > On Sun, Dec 16, 2007 at 07:59:30PM +0800, Fengguang Wu wrote:
    > >
    > > > > + read_lock_irq(&mapping->tree_lock);
    > > > > + page = radix_tree_lookup(&mapping->page_tree, offset);
    > > > > + if (likely(page)) {
    > > > > + int got_lock, uptodate;
    > > > > + page_cache_get(page);
    > > > > +
    > > > > + got_lock = !TestSetPageLocked(page);
    > > > > + uptodate = PageUptodate(page);
    > > > > + read_unlock_irq(&mapping->tree_lock);
    > > >
    > > > If we could avoid open coding tree_lock here (and expanding its coverage
    > > > to PageUptodate), that would be nice. I don't think it gains us too much.
    > >
    > > To use find_get_page()? That would be nice to me, too.
    >
    > Exactly.

    Done. Also I think it's better not to split the `found' case into
    the two cases of 'found & lock-ok' and 'found & wait-lock' for now.
    That would reduce some more code:

    -/*
    - * A successful mmap hit is when we didn't need any IO at all,
    - * and got an immediate lock on an up-to-date page. There's not
    - * much to do, except decide on whether we want to trigger read-
    - * ahead.
    - *
    - * We currently do the same thing as we did for a locked page
    - * that we're waiting for.
    - */
    -static void do_mmap_hit(struct vm_area_struct *vma,
    - struct file_ra_state *ra,
    - struct file *file,
    - struct page *page,
    - pgoff_t offset)
    -{
    - do_async_mmap_readahead(vma, ra, file, page, offset);
    -}
    -
    /**
    * filemap_fault - read in file data for page fault handling
    * @vma: vma in which the fault was taken
    @@ -1411,28 +1396,13 @@
    return VM_FAULT_SIGBUS;

    /*
    - * Do we have something in the page cache already that
    - * is unlocked and already up-to-date?
    + * Do we have something in the page cache already?
    */
    page = find_get_page(mapping, offset);
    if (likely(page)) {
    - if (likely(!TestSetPageLocked(page))) {
    - /*
    - * Previous IO error? No read-ahead, but try to
    - * re-do a single read.
    - */
    - if (unlikely(!PageUptodate(page)))
    - goto page_not_uptodate;
    -
    - do_mmap_hit(vma, ra, file, page, offset);
    - goto found_it;
    - }
    -
    /*
    - * We found the page, but it was locked..
    - *
    - * So do async readahead and wait for it to
    - * unlock.
    + * We found the page, so try async readahead before
    + * waiting for the lock.
    */
    do_async_mmap_readahead(vma, ra, file, page, offset);
    lock_page(page);


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