Benjamin Herrenschmidt wrote:
>> * objects have rwsem to protect migration.
>> * no_page() does:
>> - takes that object read sem
>> - if object is in vram or other non-memory location then do
>> io_remap_pfn_range() and get a dummy page struct pointer
>> - else get the struct page of the object page in memory
>> - release the object read sem and return whatever struct page we got
>> * migration does:
>> - take that object write sem
>> - copy the data to the new location
>> - call unmap_mapping_ranges() for that object
>> - release the object write sem
>
> Ok, there is one fault in my reasoning: io_remap_pfn_range() isn't
> designed to be used in that context (it's really made to be called with
> the mmap_sem held for writing) and thus doesn't check if the PTE is
> still empty after locking it which is necessary if you are only holding
> the read semaphore.
>
> That means that it's still all possible, but not using
> io_remap_pfn_range(). Best is to provide a specific new function, called
> something like map_one_io_page() or something like that, which does
> something along the lines of
>
> pgd = pgd_offset(mm, address);
> pud = pud_alloc(mm, pgd, address);
> if (!pud)
> return VM_FAULT_OOM;
> pmd = pmd_alloc(mm, pud, address);
> if (!pmd)
> return VM_FAULT_OOM;
> pte = pte_alloc_map(mm, pmd, address);
> if (!pte)
> return VM_FAULT_OOM;
> pte = pte_offset_map_lock(mm, pmd, address, &ptl);
> if (pte_none(*page_table)) {
> flush_icache_page(vma, new_page);
> entry = mk_pte(new_page, vma->vm_page_prot);
> if (write_access)
> entry = maybe_mkwrite(pte_mkdirty(entry), vma);
> set_pte_at(mm, address, page_table, entry);
> } else {
> page_cache_release(new_page);
> goto unlock;
> }
> update_mmu_cache(vma, address, entry);
> lazy_mmu_prot_update(entry);
> unlock:
> pte_unmap_unlock(pte, ptl);
>
> Note that it's clear that this is to be used exclusively for mapping on
> non real pages and it doesn't handle racing with truncate (concurrent
> unmap_mapping_ranges(), which is fine in our case as we have the object
> semaphore).
>
> We're looking into doing something like that for Cell to not require
> sparsemem anymore and thus not create struct page's for SPE local stores
> and registers which is a real pain...
>
> We should probably move that discussion to linux-mm and/or lkml tho :)
>
I'm finding this an interesting discussion. If it shifts to lkml, for
instance, is there a way to follow *and post* on the thread without
either subscribing to lkml or requiring myself to be on the CC list?
Keith
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