Subject: Re: [PATCH v12 23/31] mm: don't do swap readahead during speculative page fault

From: Laurent Dufour <[email-masked]>
Date: Wed, 24 Apr 2019 16:57:50 +0200

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 <[email-masked]>
>> Signed-off-by: Laurent Dufour <[email-masked]>
> 
> Reviewed-by: Jérôme Glisse <[email-masked]>
> 
> 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
>>
>