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

[Qi Zheng <qi.zheng@linux.dev>]

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