linux-kernel - Re: [PATCH RFC v2 19/27] mm: mprotect: Introduce PAGE_FAULT_ON_ACCESS for mprotect(PROT

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Message-ID: <4e7a4054-092c-4e34-ae00-0105d7c9343c@redhat.com>
Date:   Thu, 30 Nov 2023 13:49:34 +0100
From:   David Hildenbrand <david@...hat.com>
To:     Alexandru Elisei <alexandru.elisei@....com>,
        Hyesoo Yu <hyesoo.yu@...sung.com>
Cc:     catalin.marinas@....com, will@...nel.org, oliver.upton@...ux.dev,
        maz@...nel.org, james.morse@....com, suzuki.poulose@....com,
        yuzenghui@...wei.com, arnd@...db.de, akpm@...ux-foundation.org,
        mingo@...hat.com, peterz@...radead.org, juri.lelli@...hat.com,
        vincent.guittot@...aro.org, dietmar.eggemann@....com,
        rostedt@...dmis.org, bsegall@...gle.com, mgorman@...e.de,
        bristot@...hat.com, vschneid@...hat.com, mhiramat@...nel.org,
        rppt@...nel.org, hughd@...gle.com, pcc@...gle.com,
        steven.price@....com, anshuman.khandual@....com,
        vincenzo.frascino@....com, eugenis@...gle.com, kcc@...gle.com,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        kvmarm@...ts.linux.dev, linux-fsdevel@...r.kernel.org,
        linux-arch@...r.kernel.org, linux-mm@...ck.org,
        linux-trace-kernel@...r.kernel.org
Subject: Re: [PATCH RFC v2 19/27] mm: mprotect: Introduce PAGE_FAULT_ON_ACCESS
 for mprotect(PROT_MTE)

>>> +
>>> +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).

-- 
Cheers,

David / dhildenb