Re: [PATCH 04/31] mm/pgtable: allow pte_offset_map[_lock]() to fail

[Hugh Dickins <hughd@google.com>]

On Mon, 22 May 2023, Qi Zheng wrote:
> On 2023/5/22 12:53, Hugh Dickins wrote:
> 
> [...]
> 
> >   @@ -229,3 +231,57 @@ pmd_t pmdp_collapse_flush(struct vm_area_struct *vma,
> > unsigned long address,
> >   }
> >   #endif
> >   #endif /* CONFIG_TRANSPARENT_HUGEPAGE */
> > +
> > +pte_t *__pte_offset_map(pmd_t *pmd, unsigned long addr, pmd_t *pmdvalp)
> > +{
> > +	pmd_t pmdval;
> > +
> > +	/* rcu_read_lock() to be added later */
> > +	pmdval = pmdp_get_lockless(pmd);
> > +	if (pmdvalp)
> > +		*pmdvalp = pmdval;
> > +	if (unlikely(pmd_none(pmdval) || is_pmd_migration_entry(pmdval)))
> > +		goto nomap;
> > +	if (unlikely(pmd_trans_huge(pmdval) || pmd_devmap(pmdval)))
> > +		goto nomap;
> 
> Will the follow-up patch deal with the above situation specially?

No, the follow-up patch will only insert the rcu_read_lock() and unlock();
and do something (something!) about the PAE mismatched halves case.

> Otherwise, maybe it can be changed to the following check method?
> 
> 	if (unlikely(pmd_none(pmdval) || pmd_leaf(pmdval)))
> 		goto nomap;

Maybe, but I'm not keen.  Partly just because pmd_leaf() is quite a
(good) new invention (I came across a few instances in updating to
the current tree), whereas here I'm just following the old examples,
from zap_pmd_range() etc.  I'd have to spend a while getting to know
pmd_leaf(), and its interaction with strange gotchas like pmd_present().

And partly because I do want as many corrupt cases as possible to
reach the pmd_bad() check below, so generating a warning (and clear).
I might be wrong, I haven't checked through the architectures and how
pmd_leaf() is implemented in each, but my guess is that pmd_leaf()
will tend to miss the pmd_bad() check.

But if you can demonstrate a performance improvement from using
pmd_leaf() there, I expect many people would prefer that improvement
to badness catching: send a patch later to make that change if it's
justified.

Thanks a lot for all your attention to these.

Hugh

> 
> > +	if (unlikely(pmd_bad(pmdval))) {
> > +		pmd_clear_bad(pmd);
> > +		goto nomap;
> > +	}
> > +	return __pte_map(&pmdval, addr);
> > +nomap:
> > +	/* rcu_read_unlock() to be added later */
> > +	return NULL;
> > +}