linux-kernel - Re: [PATCH 04/31] mm/pgtable: allow pte_offset_map[

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Message-ID: <1efc993b-5b41-4895-a4d-20d38eb95de5@google.com>
Date:   Tue, 23 May 2023 19:22:28 -0700 (PDT)
From:   Hugh Dickins <hughd@...gle.com>
To:     Qi Zheng <qi.zheng@...ux.dev>
cc:     Hugh Dickins <hughd@...gle.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Mike Kravetz <mike.kravetz@...cle.com>,
        Mike Rapoport <rppt@...nel.org>,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        Matthew Wilcox <willy@...radead.org>,
        David Hildenbrand <david@...hat.com>,
        Suren Baghdasaryan <surenb@...gle.com>,
        Qi Zheng <zhengqi.arch@...edance.com>,
        Yang Shi <shy828301@...il.com>,
        Mel Gorman <mgorman@...hsingularity.net>,
        Peter Xu <peterx@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Will Deacon <will@...nel.org>, Yu Zhao <yuzhao@...gle.com>,
        Alistair Popple <apopple@...dia.com>,
        Ralph Campbell <rcampbell@...dia.com>,
        Ira Weiny <ira.weiny@...el.com>,
        Steven Price <steven.price@....com>,
        SeongJae Park <sj@...nel.org>,
        Naoya Horiguchi <naoya.horiguchi@....com>,
        Christophe Leroy <christophe.leroy@...roup.eu>,
        Zack Rusin <zackr@...are.com>, Jason Gunthorpe <jgg@...pe.ca>,
        Axel Rasmussen <axelrasmussen@...gle.com>,
        Anshuman Khandual <anshuman.khandual@....com>,
        Pasha Tatashin <pasha.tatashin@...een.com>,
        Miaohe Lin <linmiaohe@...wei.com>,
        Minchan Kim <minchan@...nel.org>,
        Christoph Hellwig <hch@...radead.org>,
        Song Liu <song@...nel.org>,
        Thomas Hellstrom <thomas.hellstrom@...ux.intel.com>,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [PATCH 04/31] mm/pgtable: allow pte_offset_map[_lock]() to
 fail

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;
> > +}