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    SubjectRe: [RFC][PATCH 1/3] radix priority search tree - objrmap complexity fix
    Andrea Arcangeli <andrea@suse.de> wrote:
    >
    > @@ -151,8 +151,11 @@ int rw_swap_page_sync(int rw, swp_entry_
    > lock_page(page);
    >
    > BUG_ON(page->mapping);
    > - page->mapping = &swapper_space;
    > - page->index = entry.val;
    > + ret = add_to_page_cache(page, &swapper_space, entry.val, GFP_KERNEL);

    Doing a __GFP_FS allocation while holding lock_page() is worrisome. It's
    OK if that page is private, but how do we know that the caller didn't pass
    us some page which is on the LRU?

    The only place where I think we can deadlock is if that GFP_KERNEL
    allocation tries to write out the page we hold a lock on, and we hit an
    error running swap_writepage() and then enter handle_write_error().

    This actually cannot happen because swap_writepage() can only fail if
    bio_alloc() fails, and that uses a mempool. But ick.

    Your patch seems reasonable to run with for now, but to be totally anal
    about it, I'll run with the below monstrosity.



    diff -puN mm/page_io.c~rw_swap_page_sync-fix mm/page_io.c
    --- 25/mm/page_io.c~rw_swap_page_sync-fix Wed Mar 31 16:55:44 2004
    +++ 25-akpm/mm/page_io.c Wed Mar 31 17:15:31 2004
    @@ -19,6 +19,7 @@
    #include <linux/buffer_head.h> /* for block_sync_page() */
    #include <linux/mpage.h>
    #include <linux/writeback.h>
    +#include <linux/radix-tree.h>
    #include <asm/pgtable.h>

    static struct bio *
    @@ -137,9 +138,11 @@ struct address_space_operations swap_aop
    .set_page_dirty = __set_page_dirty_nobuffers,
    };

    +#if defined(CONFIG_SOFTWARE_SUSPEND) || defined(CONFIG_PM_DISK)
    +
    /*
    * A scruffy utility function to read or write an arbitrary swap page
    - * and wait on the I/O.
    + * and wait on the I/O. The caller must have a ref on the page.
    */
    int rw_swap_page_sync(int rw, swp_entry_t entry, struct page *page)
    {
    @@ -148,11 +151,30 @@ int rw_swap_page_sync(int rw, swp_entry_
    .sync_mode = WB_SYNC_ALL,
    };

    - lock_page(page);
    -
    BUG_ON(page->mapping);
    - page->mapping = &swapper_space;
    - page->index = entry.val;
    +
    + /*
    + * We shouldn't perform add_to_page_cache(..., GFP_KERNEL) inside
    + * lock_page(), so here we do bizarre things to arrange for the page
    + * to be locked while ensuring that this CPU has sufficient pooled
    + * radix-tree nodes for a successful add_to_page_cache().
    + */
    + for ( ; ; ) {
    + ret = radix_tree_preload(GFP_KERNEL);
    + if (ret)
    + goto out;
    + if (TestSetPageLocked(page) == 0)
    + break;
    + radix_tree_preload_end();
    + lock_page(page);
    + unlock_page(page);
    + }
    + ret = add_to_page_cache(page, &swapper_space, entry.val, GFP_ATOMIC);
    + radix_tree_preload_end();
    + if (ret) {
    + unlock_page(page);
    + goto out;
    + }

    if (rw == READ) {
    ret = swap_readpage(NULL, page);
    @@ -161,8 +183,15 @@ int rw_swap_page_sync(int rw, swp_entry_
    ret = swap_writepage(page, &swap_wbc);
    wait_on_page_writeback(page);
    }
    - page->mapping = NULL;
    +
    + lock_page(page);
    + remove_from_page_cache(page);
    + unlock_page(page);
    + page_cache_release(page); /* For add_to_page_cache() */
    +
    if (ret == 0 && (!PageUptodate(page) || PageError(page)))
    ret = -EIO;
    +out:
    return ret;
    }
    +#endif
    _

    -
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