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Subject: Re: [PATCH v12 23/31] mm: don't do swap readahead during speculative
 page fault
To:     Jerome Glisse <jglisse@redhat.com>
Cc:     akpm@linux-foundation.org, mhocko@kernel.org, peterz@infradead.org,
        kirill@shutemov.name, ak@linux.intel.com, dave@stgolabs.net,
        jack@suse.cz, Matthew Wilcox <willy@infradead.org>,
        aneesh.kumar@linux.ibm.com, benh@kernel.crashing.org,
        mpe@ellerman.id.au, paulus@samba.org,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>, hpa@zytor.com,
        Will Deacon <will.deacon@arm.com>,
        Sergey Senozhatsky <sergey.senozhatsky@gmail.com>,
        sergey.senozhatsky.work@gmail.com,
        Andrea Arcangeli <aarcange@redhat.com>,
        Alexei Starovoitov <alexei.starovoitov@gmail.com>,
        kemi.wang@intel.com, Daniel Jordan <daniel.m.jordan@oracle.com>,
        David Rientjes <rientjes@google.com>,
        Ganesh Mahendran <opensource.ganesh@gmail.com>,
        Minchan Kim <minchan@kernel.org>,
        Punit Agrawal <punitagrawal@gmail.com>,
        vinayak menon <vinayakm.list@gmail.com>,
        Yang Shi <yang.shi@linux.alibaba.com>,
        zhong jiang <zhongjiang@huawei.com>,
        Haiyan Song <haiyanx.song@intel.com>,
        Balbir Singh <bsingharora@gmail.com>, sj38.park@gmail.com,
        Michel Lespinasse <walken@google.com>,
        Mike Rapoport <rppt@linux.ibm.com>,
        linux-kernel@vger.kernel.org, linux-mm@kvack.org,
        haren@linux.vnet.ibm.com, npiggin@gmail.com,
        paulmck@linux.vnet.ibm.com, Tim Chen <tim.c.chen@linux.intel.com>,
        linuxppc-dev@lists.ozlabs.org, x86@kernel.org,
        Vinayak Menon <vinmenon@codeaurora.org>
References: <20190416134522.17540-1-ldufour@linux.ibm.com>
 <20190416134522.17540-24-ldufour@linux.ibm.com>
 <20190422213611.GN14666@redhat.com>
From:   Laurent Dufour <ldufour@linux.ibm.com>
Date:   Wed, 24 Apr 2019 16:57:50 +0200
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Le 22/04/2019 à 23:36, Jerome Glisse a écrit :
> On Tue, Apr 16, 2019 at 03:45:14PM +0200, Laurent Dufour wrote:
>> Vinayak Menon faced a panic because one thread was page faulting a page in
>> swap, while another one was mprotecting a part of the VMA leading to a VMA
>> split.
>> This raise a panic in swap_vma_readahead() because the VMA's boundaries
>> were not more matching the faulting address.
>>
>> To avoid this, if the page is not found in the swap, the speculative page
>> fault is aborted to retry a regular page fault.
>>
>> Reported-by: Vinayak Menon <vinmenon@codeaurora.org>
>> Signed-off-by: Laurent Dufour <ldufour@linux.ibm.com>
> 
> Reviewed-by: Jérôme Glisse <jglisse@redhat.com>
> 
> Note that you should also skip non swap entry in do_swap_page() when doing
> speculative page fault at very least you need to is_device_private_entry()
> case.
> 
> But this should either be part of patch 22 or another patch to fix swap
> case.

Thanks Jérôme,

Yes I missed that, I guess the best option would be to abort on non swap 
entry. I'll add that in the patch 22.

>> ---
>>   mm/memory.c | 11 +++++++++++
>>   1 file changed, 11 insertions(+)
>>
>> diff --git a/mm/memory.c b/mm/memory.c
>> index 6e6bf61c0e5c..1991da97e2db 100644
>> --- a/mm/memory.c
>> +++ b/mm/memory.c
>> @@ -2900,6 +2900,17 @@ vm_fault_t do_swap_page(struct vm_fault *vmf)
>>   				lru_cache_add_anon(page);
>>   				swap_readpage(page, true);
>>   			}
>> +		} else if (vmf->flags & FAULT_FLAG_SPECULATIVE) {
>> +			/*
>> +			 * Don't try readahead during a speculative page fault
>> +			 * as the VMA's boundaries may change in our back.
>> +			 * If the page is not in the swap cache and synchronous
>> +			 * read is disabled, fall back to the regular page
>> +			 * fault mechanism.
>> +			 */
>> +			delayacct_clear_flag(DELAYACCT_PF_SWAPIN);
>> +			ret = VM_FAULT_RETRY;
>> +			goto out;
>>   		} else {
>>   			page = swapin_readahead(entry, GFP_HIGHUSER_MOVABLE,
>>   						vmf);
>> -- 
>> 2.21.0
>>
>