linux-kernel - Re: [PATCH] mm/migrate: fix race between lock page and clear PG

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Date:   Tue, 15 Mar 2022 16:45:13 +0100
From:   David Hildenbrand <david@...hat.com>
To:     Andrew Morton <akpm@...ux-foundation.org>,
        Andrew Yang <andrew.yang@...iatek.com>
Cc:     Matthias Brugger <matthias.bgg@...il.com>,
        Matthew Wilcox <willy@...radead.org>,
        Vlastimil Babka <vbabka@...e.cz>,
        David Howells <dhowells@...hat.com>,
        William Kucharski <william.kucharski@...cle.com>,
        Yang Shi <shy828301@...il.com>, Marc Zyngier <maz@...nel.org>,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        linux-arm-kernel@...ts.infradead.org,
        linux-mediatek@...ts.infradead.org, wsd_upstream@...iatek.com,
        Nicholas Tang <nicholas.tang@...iatek.com>,
        Kuan-Ying Lee <Kuan-Ying.Lee@...iatek.com>
Subject: Re: [PATCH] mm/migrate: fix race between lock page and clear
 PG_Isolated

On 15.03.22 05:21, Andrew Morton wrote:
> On Tue, 15 Mar 2022 11:05:15 +0800 Andrew Yang <andrew.yang@...iatek.com> wrote:
> 
>> When memory is tight, system may start to compact memory for large
>> continuous memory demands. If one process tries to lock a memory page
>> that is being locked and isolated for compaction, it may wait a long time
>> or even forever. This is because compaction will perform non-atomic
>> PG_Isolated clear while holding page lock, this may overwrite PG_waiters
>> set by the process that can't obtain the page lock and add itself to the
>> waiting queue to wait for the lock to be unlocked.
>>
>> CPU1                            CPU2
>> lock_page(page); (successful)
>>                                 lock_page(); (failed)
>> __ClearPageIsolated(page);      SetPageWaiters(page) (may be overwritten)
>> unlock_page(page);
>>
>> The solution is to not perform non-atomic operation on page flags while
>> holding page lock.
> 
> Sure, the non-atomic bitop optimization is really risky and I suspect
> we reach for it too often.  Or at least without really clearly
> demonstrating that it is safe, and documenting our assumptions.

I agree. IIRC, non-atomic variants are mostly only safe while the
refcount is 0. Everything else is just absolutely fragile.

-- 
Thanks,

David / dhildenb