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

From: Mike Kravetz
Date: Tue Nov 10 2015 - 17:42:42 EST

On 11/09/2015 02:55 PM, Mike Kravetz wrote:
> 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.

I have convinced myself that hugetlb_no_page is coded such that page
faults can not race with truncate. hugetlb_no_page handles the case
where there is no PTE for a faulted in address. The general flow in
hugetlb_no_page for the no page found case is:
- check index against i_size, end if beyond
- allocate huge page
- take page table lock for huge page
- check index against i_size again, if beyond free page and return
- add huge page to page table
- unlock page table lock for huge page

The flow for the truncate operation in hugetlb_vmtruncate is:
- set i_size
- take inode/mapping write lock
- hugetlb_vmdelete_list() which removes page table entries. The page
table lock will be taken for each huge page in the range
- release inode/mapping write lock
- remove_inode_hugepages() to actually remove pages

The truncate/page fault race we are concerned with is if a page is faulted
in after hugetlb_vmtruncate sets i_size and unmaps the page, but before
actually removing the page. Obviously, any entry into hugetlb_no_page
after i_size is set will check the value and not allow the fault. In
addition, if the value of i_size is set before the second check in
hugetlb_no_page, it will do the right thing. Therefore, the only place to
race is after the second i_size check in hugetlb_no_page.

Note that the second check for i_size is with the page table lock for
the huge page held. It is not possible for hugetlb_vmtruncate to unmap
the huge page before the page fault completes, as it must acquire the page
table lock. This is the same as a fault happening before the truncate
operation starts and is handled correctly by hugetlb_vmtruncate.

Another way to look at this is by asking the question, Is it possible to
fault on a page in the truncate range after it is unmapped by
hugetlb_vmtruncate/hugetlb_vmdelete_list? To unmap a page,
hugetlb_vmtruncate will:
- set i_size
- take page table lock for huge page
- unmap page
- release page table lock for page

In order to fault in the page, it must take the same page table lock and
check i_size. I do not know of any way for the faulting code to get an
old value for i_size.

Please let me know if my reasoning is incorrect. I will code up a new
(simpler) version of remove_inode_hugepages with the assumption that
truncate can not race with page faults.

Also, I wrote a fairly simple test to have truncate race with page faults.
It was quite easy to hit the second check in hugetlb_no_page where it
notices index is beyond i_size and backs out of the fault. Even after
adding delays in strategic locations of the fault and truncate code, I
could not cause a race as observed by remove_inode_hugepages.
Mike Kravetz
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