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    SubjectRe: pre12 VM doubts and patch
    On Wed, Sep 19, 2001 at 04:16:13PM -0700, Linus Torvalds wrote:
    >
    > On Wed, 19 Sep 2001, Andrea Arcangeli wrote:
    > >
    > > On Wed, Sep 19, 2001 at 08:42:39PM +0100, Hugh Dickins wrote:
    > > > --- 2.4.10-pre12/mm/swap_state.c Wed Sep 19 14:05:54 2001
    > > > +++ linux/mm/swap_state.c Mon Sep 17 06:30:26 2001
    > > > @@ -23,6 +23,17 @@
    > > > */
    > > > static int swap_writepage(struct page *page)
    > > > {
    > > > + /* One for the page cache, one for this user, one for page->buffers */
    > > > + if (page_count(page) > 2 + !!page->buffers)
    > >
    > > this is racy, you have to spin_lock(&pagecache_lock) before you can
    > > expect the page_count() stays constant. then after you checked the page
    > > has count == 1, you must atomically drop it from the pagecache so it's
    > > not visible anymore to the swapin lookups.
    >
    > No.
    >
    > Note how it is a _heuristic_ only. The "safe" answer is always to say "the
    > page is in use", and note that once the page_count has dropped to 2 or
    > less, it won't increase unless somebody else has a swap count..

    the "somebody else has a swap count" is interesting. so we rely on the
    fact any swap_duplicate is always run before the swapcache is unlocked.

    Also the "> 2" should be "> 1", but really I noticed I cannot avoid
    getting a reference so please apply also this patch instead of replacing
    "> 2" with "> 1" (it was a race condition):

    --- 2.4.10pre11aa1/mm/vmscan.c.~1~ Tue Sep 18 21:23:49 2001
    +++ 2.4.10pre11aa1/mm/vmscan.c Thu Sep 20 01:29:58 2001
    @@ -415,7 +415,10 @@
    spin_unlock(&pagemap_lru_lock);

    ClearPageDirty(page);
    +
    + page_cache_get(page);
    writepage(page);
    + page_cache_release(page);

    spin_lock(&pagemap_lru_lock);
    continue;

    > And we check for the "somebody else has a swap count" two lines lower.

    I see.

    > Do you see anything wrong with that logic?

    Looks ok now :), I guess it would be good to write a comment now, so
    maybe other people won't share my worry, the swap_count thing wasn't
    very obvious. thanks,

    Andrea
    -
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