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
