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Message-ID: <87r0kufm15.fsf@yhuang6-desk2.ccr.corp.intel.com>
Date: Mon, 13 Nov 2023 10:02:30 +0800
From: "Huang, Ying" <ying.huang@...el.com>
To: "Yin, Fengwei" <fengwei.yin@...el.com>
Cc: Matthew Wilcox <willy@...radead.org>,
"zhangpeng (AS)" <zhangpeng362@...wei.com>, <linux-mm@...ck.org>,
<linux-kernel@...r.kernel.org>, <akpm@...ux-foundation.org>,
<lstoakes@...il.com>, <hughd@...gle.com>, <david@...hat.com>,
<vbabka@...e.cz>, <peterz@...radead.org>, <mgorman@...e.de>,
<mingo@...hat.com>, <riel@...hat.com>, <hannes@...xchg.org>,
Nanyong Sun <sunnanyong@...wei.com>,
Kefeng Wang <wangkefeng.wang@...wei.com>
Subject: Re: [Question]: major faults are still triggered after mlockall
when numa balancing
"Yin, Fengwei" <fengwei.yin@...el.com> writes:
> On 11/10/2023 1:32 PM, Huang, Ying wrote:
>> Matthew Wilcox <willy@...radead.org> writes:
>>
>>> On Thu, Nov 09, 2023 at 09:47:24PM +0800, zhangpeng (AS) wrote:
>>>> There is a stage in numa fault which will set pte as 0 in do_numa_page() :
>>>> ptep_modify_prot_start() will clear the vmf->pte, until
>>>> ptep_modify_prot_commit() assign a value to the vmf->pte.
>>>
>>> [...]
>>>
>>>> Our problem scenario is as follows:
>>>>
>>>> task 1 task 2
>>>> ------ ------
>>>> /* scan global variables */
>>>> do_numa_page()
>>>> spin_lock(vmf->ptl)
>>>> ptep_modify_prot_start()
>>>> /* set vmf->pte as null */
>>>> /* Access global variables */
>>>> handle_pte_fault()
>>>> /* no pte lock */
>>>> do_pte_missing()
>>>> do_fault()
>>>> do_read_fault()
>>>> ptep_modify_prot_commit()
>>>> /* ptep update done */
>>>> pte_unmap_unlock(vmf->pte, vmf->ptl)
>>>> do_fault_around()
>>>> __do_fault()
>>>> filemap_fault()
>>>> /* page cache is not available
>>>> and a major fault is triggered */
>>>> do_sync_mmap_readahead()
>>>> /* page_not_uptodate and goto
>>>> out_retry. */
>>>>
>>>> Is there any way to avoid such a major fault?
>>>
>>> Yes, this looks like a bug.
>>>
>>> It seems to me that the easiest way to fix this is not to zero the pte
>>> but to make it protnone? That would send task 2 into do_numa_page()
>>> where it would take the ptl, then check pte_same(), see that it's
>>> changed and goto out, which will end up retrying the fault.
>>
>> There are other places in the kernel where the PTE is cleared, for
>> example, move_ptes() in mremap.c. IIUC, we need to audit all them.
>>
>> Another possible solution is to check PTE again with PTL held before
>> reading in file data. This will increase the overhead of major fault
>> path. Is it acceptable?
> What if we check the PTE without page table lock acquired?
The PTE is zeroed temporarily only with PTL held. So, if we acquire the
PTL in filemap_fault() and check the PTE, the PTE which is zeroed in
do_numa_page() will be non-zero now. So we can avoid the major fault.
But, if we don't acquire the PTL, the PTE may still be zero.
--
Best Regards,
Huang, Ying
> Regards
> Yin, Fengwei
>
>>
>>> I'm not particularly expert at page table manipulation, so I'll let
>>> somebody who is propose an actual patch. Or you could try to do it?
>>
>> --
>> Best Regards,
>> Huang, Ying
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