Messages in this thread | | | Date | Wed, 2 Apr 2003 15:09:03 -0800 | From | Andrew Morton <> | Subject | Re: [PATCH 2.5.66-mm2] Fix page_convert_anon locking issues |
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Dave McCracken <dmccr@us.ibm.com> wrote: > > The sequence is the following:
Boy you owe me a big fat comment on top of this one.
> 1. take a copy of the reference to the page (the pgd or pmd entry) > 2. validate the copy > 3. establish a pointer into the page > 4. pull the data from the page (pmd or pte entry) > 5. validate the original reference again > 6. use the data > > This guarantees that the data is from a page that's still valid, since the > pgd or pmd entry are cleared when the page is released. We're helped by > the fact that for an invalid page we can simply return failure.
+ if (page_to_pfn(page) != pte_pfn(*pte)) + goto out_unmap; + + if (addr) + *addr = address; +
==>munmap here
+ return pte;
i_shared_sem won't stop that. The pte points into thin air, and may now point at a value which looks like our page.
> > But then again, why is it not possible to just do: > > > > list_for_each_entry(vma, &mapping->i_mmap, shared) { > > if (!pte_chain) > > pte_chain = pte_chain_alloc(GFP_KERNEL); > > spin_lock(&mm->page_table_lock); > > pte = find_pte(vma, page, NULL); > > if (pte) > > pte_chain = page_add_rmap(page, pte, pte_chain); > > spin_unlock(&mm->page_table_lock); > > } > > > > pte_chain_free(pte_chain); > > up(&mapping->i_shared_sem); > > > > ? > > Because the page is in transition from !PageAnon to PageAnon.
These are file-backed pages. So what does PageAnon really mean?
> We have to > hold the pte_chain lock during the entire transition in case someone else > tries to do something like page_remove_rmap, which would break.
How about setting PageAnon at the _start_ of the operation? page_remove_rmap() will cope with that OK.
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