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

[Qi Zheng]

On 2023/5/24 10:22, Hugh Dickins wrote:
> 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.

IIUC, pmd_leaf() is just for checking a leaf mapped PMD, and will
not cover pmd_bad() case. Can see the examples in vmalloc_to_page()
and apply_to_pmd_range().

> 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.

Probably not a lot of performance gain, just makes the check more
concise.

Thanks,
Qi

> 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;
> > > +}

--
Thanks,
Qi