Re: [PATCH] mm/hugetlb: Unmap pages if page fault raced with hole punch

[Mike Kravetz]

On 11/08/2015 11:42 PM, Hugh Dickins wrote:
> On Fri, 30 Oct 2015, Mike Kravetz wrote:
>>
>> The 'next = start' code is actually from the original truncate_hugepages
>> routine.  This functionality was combined with that needed for hole punch
>> to create remove_inode_hugepages().
>>
>> The following code was in truncate_hugepages:
>>
>> 	next = start;
>> 	while (1) {
>> 		if (!pagevec_lookup(&pvec, mapping, next, PAGEVEC_SIZE)) {
>> 			if (next == start)
>> 				break;
>> 			next = start;
>> 			continue;
>> 		}
>>
>>
>> So, in the truncate case pages starting at 'start' are deleted until
>> pagevec_lookup fails.  Then, we call pagevec_lookup() again.  If no
>> pages are found we are done.  Else, we repeat the whole process.
>>
>> Does anyone recall the reason for going back and looking for pages at
>> index'es already deleted?  Git doesn't help as that was part of initial
>> commit.  My thought is that truncate can race with page faults.  The
>> truncate code sets inode offset before unmapping and deleting pages.
>> So, faults after the new offset is set should fail.  But, I suppose a
>> fault could race with setting offset and deleting of pages.  Does this
>> sound right?  Or, is there some other reason I am missing?
> 
> I believe your thinking is correct.  But remember that
> truncate_inode_pages_range() is shared by almost all filesystems,
> and different filesystems have different internal locking conventions,
> and different propensities to such a race: it's trying to cover for
> all of them.
> 
> Typically, writing is well serialized (by i_mutex) against truncation,
> but faulting (like reading) sails through without enough of a lock.
> We resort to i_size checks to avoid the worst of it, but there's often
> a corner or two in which those checks are not quite good enough -
> it's easy to check i_size at the beginning, but it needs to be checked
> again at the end too, and what's been done undone - can be awkward.

Well, it looks like the hugetlb_no_page() routine is checking i_size both
before and after.  It appears to be doing the right thing to handle the
race, but I need to stare at the code some more to make sure.

Because of the way the truncate code went back and did an extra lookup
when done with the range, I assumed it was covering some race.  However,
that may not be the case.

> 
> I hope that in the case of hugetlbfs, since you already have the
> additional fault_mutex to handle races between faults and punching,
> it should be possible to get away without that "pincer" restarting.

Yes, it looks like this may work as a straight loop over the range of
pages.  I just need to study the code some more to make sure I am not
missing something.

-- 
Mike Kravetz

> 
> Hugh
> 
>>
>> I would like to continue having remove_inode_hugepages handle both the
>> truncate and hole punch case.  So, what to make sure the code correctly
>> handles both cases.
>>
>> -- 
>> Mike Kravetz
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