linux-kernel - Re: [PATCH v2 28/32] mm/memory: allow pte_offset_map[

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Date:   Mon, 12 Jun 2023 10:10:50 +0100
From:   Ryan Roberts <ryan.roberts@....com>
To:     Hugh Dickins <hughd@...gle.com>,
        Andrew Morton <akpm@...ux-foundation.org>
Cc:     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>,
        Lorenzo Stoakes <lstoakes@...il.com>,
        Huang Ying <ying.huang@...el.com>,
        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 v2 28/32] mm/memory: allow pte_offset_map[_lock]() to fail

On 09/06/2023 21:11, Hugh Dickins wrote:
> On Fri, 9 Jun 2023, Andrew Morton wrote:
>> On Thu, 8 Jun 2023 18:43:38 -0700 (PDT) Hugh Dickins <hughd@...gle.com> wrote:
>>
>>> copy_pte_range(): use pte_offset_map_nolock(), and allow for it to fail;
>>> but with a comment on some further assumptions that are being made there.
>>>
>>> zap_pte_range() and zap_pmd_range(): adjust their interaction so that
>>> a pte_offset_map_lock() failure in zap_pte_range() leads to a retry in
>>> zap_pmd_range(); remove call to pmd_none_or_trans_huge_or_clear_bad().
>>>
>>> Allow pte_offset_map_lock() to fail in many functions.  Update comment
>>> on calling pte_alloc() in do_anonymous_page().  Remove redundant calls
>>> to pmd_trans_unstable(), pmd_devmap_trans_unstable(), pmd_none() and
>>> pmd_bad(); but leave pmd_none_or_clear_bad() calls in free_pmd_range()
>>> and copy_pmd_range(), those do simplify the next level down.
>>>
>>> ...
>>>
>>> @@ -3728,11 +3737,9 @@ vm_fault_t do_swap_page(struct vm_fault *vmf)
>>>  			vmf->page = pfn_swap_entry_to_page(entry);
>>>  			vmf->pte = pte_offset_map_lock(vma->vm_mm, vmf->pmd,
>>>  					vmf->address, &vmf->ptl);
>>> -			if (unlikely(!pte_same(*vmf->pte, vmf->orig_pte))) {
>>> -				spin_unlock(vmf->ptl);
>>> -				goto out;
>>> -			}
>>> -
>>> +			if (unlikely(!vmf->pte ||
>>> +				     !pte_same(*vmf->pte, vmf->orig_pte)))
>>> +				goto unlock;
>>>  			/*
>>>  			 * Get a page reference while we know the page can't be
>>>  			 * freed.
>>
>> This hunk falls afoul of
>> https://lkml.kernel.org/r/20230602092949.545577-5-ryan.roberts@arm.com.
>>
>> I did this:
>>
>> @@ -3729,7 +3738,8 @@ vm_fault_t do_swap_page(struct vm_fault
>>  			vmf->page = pfn_swap_entry_to_page(entry);
>>  			vmf->pte = pte_offset_map_lock(vma->vm_mm, vmf->pmd,
>>  					vmf->address, &vmf->ptl);
>> -			if (unlikely(!pte_same(*vmf->pte, vmf->orig_pte)))
>> +			if (unlikely(!vmf->pte ||
>> +				     !pte_same(*vmf->pte, vmf->orig_pte)))
>>  				goto unlock;
>>  
>>  			/*
> 
> Yes, that's exactly right: thanks, Andrew.

FWIW, I agree.

Thanks,
Ryan


> 
> Hugh