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SubjectRe: [PATCH v2 0/9] Free user PTE page table pages
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On 01.09.21 18:07, Jason Gunthorpe wrote:
> On Wed, Sep 01, 2021 at 02:32:08PM +0200, David Hildenbrand wrote:
>
>> b) pmd_trans_unstable_or_pte_try_get() and friends are really ugly.
>
> I suspect the good API here is really more like:

That was my exactly my first idea and I tried to rework the code for
roughly 2 days and failed.

Especially in pagefault logic, we temporarily unmap/unlock to map/lock
again later and don't want the page table to just vanish.

I think I met similar cases when allocating a page table and not wanting
it to vanish and not wanting to map/lock it. But I don't recall all the
corner cases: it didn't work for me.

>
> ptep = pte_try_map(pmdp, &pmd_value)
> if (!ptep) {
> // pmd_value is guarenteed to not be a PTE table pointer.
> if (pmd_XXX(pmd_value))
> }
>
> Ie the core code will do whatever stuff, including the THP data race
> avoidance, to either return the next level page table or the value of
> a pmd that is not a enxt level page table. Callers are much clearer in
> this way.
>
> Eg this is a fairly representative sample user:
>
> static int smaps_pte_range(pmd_t *pmd, unsigned long addr, unsigned long end,
> struct mm_walk *walk)
> {
> if (pmd_trans_unstable(pmd))
> goto out;
> pte = pte_offset_map_lock(vma->vm_mm, pmd, addr, &ptl);
>
> And it is obviously pretty easy to integrate any refcount into
> pte_try_map and pte_unmap as in my other email.

It didn't work when I tried.

--
Thanks,

David / dhildenb

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