lkml.org 
[lkml]   [2023]   [Mar]   [31]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
Date
SubjectRe: [PATCH v2] mm: khugepaged: Fix kernel BUG in hpage_collapse_scan_file
From
On 3/31/23 05:33, Zach O'Keefe wrote:

> Thanks, Ivan.
>
> In the process of reviewing this, I starting thinking if the !shmem case was
> also susceptible to a similar race, and I *think* it might be. Unfortunately, my
> time has ran out, and I haven't been able to validate ; I'm less familiar with
> the file-side of things.
>
> The underlying problem is race with truncation/hole-punch under OOM condition.
> The nice do-while loop near the top of collapse_file() attempts to avoid this
> scenario by making sure enough slots are available. However, when we drop xarray
> lock, we open ourselves up to concurrent removal + slot deletion. Those slots
> then need to be allocated again -- which under OOM condition is failable.
>
> The syzbot reproducer picks on shmem, but I think this can occur for file as
> well. If we find a hole, we unlock the xarray and call
> page_cache_sync_readahead(), which if it succeeds, IIUC, will have allocated a
> new slot in our mapping pointing to the new page. We *then* locks the page. Only
> after the page is locked are we protected from concurrent removal (Note: this is
> what provides us protection in many of the xas_store() cases ; we've held the
> slot's contained page-lock since verifying the slot exists, protecting us from
> removal / reallocation races).
>
> Maybe I'm just low on caffeine at the end of the day, and am missing something,
> but if I had more time, I'd be looking into the file-side some more to verify.
> Apologies that hasn't occurred to me until now ; I was looking at one of your
> comments and double-checked why I *thought* we were safe.
>
> Anyways, irrespective of that looming issues, some more notes to follow:
>
>> The 'xas_store' call during page cache scanning can potentially
>> translate 'xas' into the error state (with the reproducer provided
>> by the syzkaller the error code is -ENOMEM). However, there are no
>> further checks after the 'xas_store', and the next call of 'xas_next'
>> at the start of the scanning cycle doesn't increase the xa_index,
>> and the issue occurs.
>>
>> This patch will add the xarray state error checking after the
>> 'xas_store' and the corresponding result error code. It will
>> also add xarray state error checking via WARN_ON_ONCE macros,
>> to be sure that ENOMEM or other possible errors don't occur
>> at the places they shouldn't.
>
> Thanks for the additions here. I think it's worthwhile providing even more
> details about the specifics of the race we are fixing and/or guarding against to
> help ppl understand how that -ENOMEM comes about if the do-while loop has
> "Ensured" we have slots available (additionally, I think that comment can be
> augmented).
>
>> Tested via syzbot.
>>
>> Reported-by: syzbot+9578faa5475acb35fa50@syzkaller.appspotmail.com
>> Link: https://syzkaller.appspot.com/bug?id=7d6bb3760e026ece7524500fe44fb024a0e959fc
>> Signed-off-by: Ivan Orlov <ivan.orlov0322@gmail.com>
>> ---
>> V1 -> V2: Add WARN_ON_ONCE error checking and comments
>>
>> mm/khugepaged.c | 20 ++++++++++++++++++++
>> 1 file changed, 20 insertions(+)
>>
>> diff --git a/mm/khugepaged.c b/mm/khugepaged.c
>> index 92e6f56a932d..8b6580b13339 100644
>> --- a/mm/khugepaged.c
>> +++ b/mm/khugepaged.c
>> @@ -55,6 +55,7 @@ enum scan_result {
>> SCAN_CGROUP_CHARGE_FAIL,
>> SCAN_TRUNCATED,
>> SCAN_PAGE_HAS_PRIVATE,
>> + SCAN_STORE_FAILED,
>> };
>
> I'm still reluctant to add a new error code for this as this seems like quite a
> rare race that requires OOM to trigger. I'd be happier just reusing SCAN_FAIL,
> or, something we might get some millage out of later: SCAN_OOM.
>
> Also, a reminder to update include/trace/events/huge_memory.h, if you go that
> route.
>
>>
>> #define CREATE_TRACE_POINTS
>> @@ -1840,6 +1841,15 @@ static int collapse_file(struct mm_struct *mm, unsigned long addr,
>> goto xa_locked;
>> }
>> xas_store(&xas, hpage);
>> + if (xas_error(&xas)) {
>> + /* revert shmem_charge performed
>> + * in the previous condition
>> + */
>
> Nit: Here, and following, I think standard convention for multiline comment is
> to have an empty first and last line, eg:
>
> + /*
> + * revert shmem_charge performed
> + * in the previous condition
> + */
>
> Though, checkpatch.pl --strict didn't seem to care.
>
>> + mapping->nrpages--;
>> + shmem_uncharge(mapping->host, 1);
>> + result = SCAN_STORE_FAILED;
>> + goto xa_locked;
>> + }
>> nr_none++;
>> continue;
>> }
>> @@ -1992,6 +2002,11 @@ static int collapse_file(struct mm_struct *mm, unsigned long addr,
>>
>> /* Finally, replace with the new page. */
>> xas_store(&xas, hpage);
>> + /* We can't get an ENOMEM here (because the allocation happened before)
>> + * but let's check for errors (XArray implementation can be
>> + * changed in the future)
>> + */
>> + WARN_ON_ONCE(xas_error(&xas));
>
> Nit: it's not just that allocation happened before -- need some guarantee we've
> been protected from concurrent removal. This is what made me look at the file
> side.
>
>> continue;
>> out_unlock:
>> unlock_page(page);
>> @@ -2029,6 +2044,11 @@ static int collapse_file(struct mm_struct *mm, unsigned long addr,
>> /* Join all the small entries into a single multi-index entry */
>> xas_set_order(&xas, start, HPAGE_PMD_ORDER);
>> xas_store(&xas, hpage);
>> + /* Here we can't get an ENOMEM (because entries were
>> + * previously allocated) But let's check for errors
>> + * (XArray implementation can be changed in the future)
>> + */
>> + WARN_ON_ONCE(xas_error(&xas));
>
> Ditto.
>
> Apologies I won't be around to see this change through -- I'm just out of time,
> and will be shutting my computer down tomorrow for 3 months. Sorry for the poor
> timing, for raising issues, then disappearing. Hopefully I'm wrong and the
> file-side isn't a concern.
>
> Best,
> Zach
>
>> xa_locked:
>> xas_unlock_irq(&xas);
>> xa_unlocked:
>> --
>> 2.34.1
>>

Hello, Zach! Thank you very much for the detailed review! I thought that
locking is our guarantee to not get an -ENOMEM, but I didn't have the
deep understanding of the problem's cause, due to the fact I'm just
starting my memory management journey :) In any case, I will do a
research about this problem, and hopefully after you get out of the
vacation you will see a new patch fully fixes this problem. Have a nice
vacation! Thanks again!

\
 
 \ /
  Last update: 2023-03-31 08:47    [W:0.096 / U:0.068 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site