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SubjectRe: [Patch] shmem_unuse race fix
From
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Marcelo Tosatti <marcelo@conectiva.com.br> writes:

> I think that incrementing the swap entry count will not allow swap from
> removing the swap entry (as the comment says)

I think the culprit is somewhere else. The error occurs in nopage of a
process, not in swapoff.

Looking at the following in try_to_unuse:

found_entry:
entry = SWP_ENTRY(type, i);

/* Get a page for the entry, using the existing swap
cache page if there is one. Otherwise, get a clean
page and read the swap into it. */
page = read_swap_cache(entry);
if (!page) {
swap_free(entry);
return -ENOMEM;
}
if (PageSwapCache(page))
delete_from_swap_cache(page);
read_lock(&tasklist_lock);
for_each_task(p)
unuse_process(p->mm, entry, page);
read_unlock(&tasklist_lock);
shmem_unuse(entry, page);

and in unuse_pte:
if (pte_present(pte)) {
/* If this entry is swap-cached, then page must already
hold the right address for any copies in physical
memory */
if (pte_page(pte) != page)
return;
/* We will be removing the swap cache in a moment, so... */
ptep_mkdirty(dir);
return;
}


I see at least a wrong comment. There is no swap cache any more... And
another thought is: why can we remove the swap cache entry before
finding the process? What prevent reallocation of the swap cache entry
by a page fault?

BTW you can reproduce the problem on a UP system quite easily by
trashing and always disablingænabling two swap partitions round robin.

Greetings
Christoph

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