free_swap_and_cache() doubt

free_swap_and_cache() doubt

[alad]

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Hi,
  I am reading 2.4.16, let us assume following scenario

swap_map[offset] == 2;
and page->count == 2; (before function execution)
vm_swap is full (nr_swap_pages*2 > total_swap_pages);

first question is the above case possible.

if yes,
  then
    after execution of this function, we would have page->count == 1, i.e.
mapped by some process, with good page->index and what we have is, the
associated
swap entry is already freed.

Am i wrong somewhere ??

-----
Amol


void free_swap_and_cache(swp_entry_t entry)
{
        struct swap_info_struct * p;
        struct page *page = NULL;

        p = swap_info_get(entry);
        if (p) {
                if (swap_entry_free(p, SWP_OFFSET(entry)) == 1)
                        page = find_trylock_page(&swapper_space, entry.val);
                swap_info_put(p);
        }
        if (page) {
                page_cache_get(page);
                /* Only cache user (+us), or swap space full? Free it! */
                if (page_count(page) == 2 || vm_swap_full()) {
                        delete_from_swap_cache(page);
                        SetPageDirty(page);
                }
                UnlockPage(page);
                page_cache_release(page);
        }
}

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?


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Re: free_swap_and_cache() doubt

[Hugh Dickins]

On Sat, 19 Jan 2002 alad@hss.hns.com wrote:
> I am reading 2.4.16, let us assume following scenario
> 
> swap_map[offset] == 2;
> and page->count == 2; (before function execution)
> vm_swap is full (nr_swap_pages*2 > total_swap_pages);
> 
> first question is the above case possible.

Yes.

> if yes, then
> after execution of this function, we would have page->count == 1, i.e.
> mapped by some process, with good page->index and what we have is, the
> associated swap entry is already freed.
> 
> Am i wrong somewhere ??

The only place you are wrong is in using the word "good" of page->index
at the end, and indeed I think your point is that it's not good.  It is
not good, it is stale, but that's okay because page->mapping has been
set to NULL, and page->index has no meaning without page->mapping.

You'll notice that __free_pages_ok() contains many BUG() hurdles
(including check on page->mapping and redundant check on PageSwapCache)
but doesn't mind if page->index is non-zero.

But thanks for making me look at this function, there
is a bug there and I'll post a patch for it right now...

Hugh