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Message-ID: <d7e0574d-c74d-4e91-bf60-aa6691df78e3@redhat.com>
Date:   Thu, 30 Nov 2023 14:43:48 +0100
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Subject: Re: [PATCH RFC v2 19/27] mm: mprotect: Introduce PAGE_FAULT_ON_ACCESS
 for mprotect(PROT_MTE)
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To:     Alexandru Elisei <alexandru.elisei@arm.com>
Cc:     Hyesoo Yu <hyesoo.yu@samsung.com>, catalin.marinas@arm.com,
        will@kernel.org, oliver.upton@linux.dev, maz@kernel.org,
        james.morse@arm.com, suzuki.poulose@arm.com, yuzenghui@huawei.com,
        arnd@arndb.de, akpm@linux-foundation.org, mingo@redhat.com,
        peterz@infradead.org, juri.lelli@redhat.com,
        vincent.guittot@linaro.org, dietmar.eggemann@arm.com,
        rostedt@goodmis.org, bsegall@google.com, mgorman@suse.de,
        bristot@redhat.com, vschneid@redhat.com, mhiramat@kernel.org,
        rppt@kernel.org, hughd@google.com, pcc@google.com,
        steven.price@arm.com, anshuman.khandual@arm.com,
        vincenzo.frascino@arm.com, eugenis@google.com, kcc@google.com,
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From:   David Hildenbrand <david@redhat.com>
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In-Reply-To: <ZWiO4PWfK2gKDLGr@raptor>
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On 30.11.23 14:32, Alexandru Elisei wrote:
> Hi,
> 
> On Thu, Nov 30, 2023 at 01:49:34PM +0100, David Hildenbrand wrote:
>>>>> +
>>>>> +out_retry:
>>>>> +	put_page(page);
>>>>> +	if (vmf->flags & FAULT_FLAG_VMA_LOCK)
>>>>> +		vma_end_read(vma);
>>>>> +	if (fault_flag_allow_retry_first(vmf->flags)) {
>>>>> +		err = VM_FAULT_RETRY;
>>>>> +	} else {
>>>>> +		/* Replay the fault. */
>>>>> +		err = 0;
>>>>
>>>> Hello!
>>>>
>>>> Unfortunately, if the page continues to be pinned, it seems like fault will continue to occur.
>>>> I guess it makes system stability issue. (but I'm not familiar with that, so please let me know if I'm mistaken!)
>>>>
>>>> How about migrating the page when migration problem repeats.
>>>
>>> Yes, I had the same though in the previous iteration of the series, the
>>> page was migrated out of the VMA if tag storage couldn't be reserved.
>>>
>>> Only short term pins are allowed on MIGRATE_CMA pages, so I expect that the
>>> pin will be released before the fault is replayed. Because of this, and
>>> because it makes the code simpler, I chose not to migrate the page if tag
>>> storage couldn't be reserved.
>>
>> There are still some cases that are theoretically problematic: vmsplice()
>> can pin pages forever and doesn't use FOLL_LONGTERM yet.
>>
>> All these things also affect other users that rely on movability (e.g., CMA,
>> memory hotunplug).
> 
> I wasn't aware of that, thank you for the information. Then to ensure that the
> process doesn't hang by replying the loop indefinitely, I'll migrate the page if
> tag storage cannot be reserved. Looking over the code again, I think I can reuse
> the same function that migrates tag storage pages out of the MTE VMA (added in
> patch #21), so no major changes needed.

It's going to be interesting if migrating that page fails because it is 
pinned :/

-- 
Cheers,

David / dhildenb