Re: [PATCH v3 3/4] mm: FLEXIBLE_THP for improved performance

[Hugh Dickins <hughd@google.com>]

On Mon, 17 Jul 2023, Ryan Roberts wrote:

> >>>> +static int alloc_anon_folio(struct vm_fault *vmf, struct folio **folio)
> >>>> +{
> >>>> +       int i;
> >>>> +       gfp_t gfp;
> >>>> +       pte_t *pte;
> >>>> +       unsigned long addr;
> >>>> +       struct vm_area_struct *vma = vmf->vma;
> >>>> +       int prefer = anon_folio_order(vma);
> >>>> +       int orders[] = {
> >>>> +               prefer,
> >>>> +               prefer > PAGE_ALLOC_COSTLY_ORDER ? PAGE_ALLOC_COSTLY_ORDER : 0,
> >>>> +               0,
> >>>> +       };
> >>>> +
> >>>> +       *folio = NULL;
> >>>> +
> >>>> +       if (vmf_orig_pte_uffd_wp(vmf))
> >>>> +               goto fallback;
> >>>> +
> >>>> +       for (i = 0; orders[i]; i++) {
> >>>> +               addr = ALIGN_DOWN(vmf->address, PAGE_SIZE << orders[i]);
> >>>> +               if (addr >= vma->vm_start &&
> >>>> +                   addr + (PAGE_SIZE << orders[i]) <= vma->vm_end)
> >>>> +                       break;
> >>>> +       }
> >>>> +
> >>>> +       if (!orders[i])
> >>>> +               goto fallback;
> >>>> +
> >>>> +       pte = pte_offset_map(vmf->pmd, vmf->address & PMD_MASK);
> >>>> +       if (!pte)
> >>>> +               return -EAGAIN;
> >>>
> >>> It would be a bug if this happens. So probably -EINVAL?
> >>
> >> Not sure what you mean? Hugh Dickins' series that went into v6.5-rc1 makes it
> >> possible for pte_offset_map() to fail (if I understood correctly) and we have to
> >> handle this. The intent is that we will return from the fault without making any
> >> change, then we will refault and try again.
> > 
> > Thanks for checking that -- it's very relevant. One detail is that
> > that series doesn't affect anon. IOW, collapsing PTEs into a PMD can't
> > happen while we are holding mmap_lock for read here, and therefore,
> > the race that could cause pte_offset_map() on shmem/file PTEs to fail
> > doesn't apply here.
> 
> But Hugh's patches have changed do_anonymous_page() to handle failure from
> pte_offset_map_lock(). So I was just following that pattern. If this really
> can't happen, then I'd rather WARN/BUG on it, and simplify alloc_anon_folio()'s
> prototype to just return a `struct folio *` (and if it's null that means ENOMEM).
> 
> Hugh, perhaps you can comment?

I agree with your use of -EAGAIN there: I find it better to allow for the
possibility, than to go to great effort persuading that it's impossible;
especially because what's possible tomorrow may differ from today.

And notice that, before my changes, there used to be a pmd_trans_unstable()
check above, implying that it is possible for it to fail (for more reasons
than corruption causing pmd_bad()) - one scenario would be that the
pte_alloc() above succeeded *because* someone else had managed to insert
a huge pmd there already (maybe we have MMF_DISABLE_THP but they did not).

But I see from later mail that Yu Zhao now agrees with your -EAGAIN too,
so we are all on the same folio.

Hugh

p.s. while giving opinions, I'm one of those against using "THP" for
large but not pmd-mappable folios; and was glad to see Matthew arguing
the same way when considering THP_SWPOUT in another thread today.