linux-kernel - Re: [PATCH 1/2] mm: Retry migration earlier upon refcount mismatch

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Message-ID: <367b0403-7477-4857-9e7c-5a749c723432@redhat.com>
Date: Sat, 10 Aug 2024 20:52:44 +0200
From: David Hildenbrand <david@...hat.com>
To: Dev Jain <dev.jain@....com>, akpm@...ux-foundation.org, shuah@...nel.org,
 willy@...radead.org
Cc: ryan.roberts@....com, anshuman.khandual@....com, catalin.marinas@....com,
 cl@...two.org, vbabka@...e.cz, mhocko@...e.com, apopple@...dia.com,
 osalvador@...e.de, baolin.wang@...ux.alibaba.com,
 dave.hansen@...ux.intel.com, will@...nel.org, baohua@...nel.org,
 ioworker0@...il.com, gshan@...hat.com, mark.rutland@....com,
 kirill.shutemov@...ux.intel.com, hughd@...gle.com, aneesh.kumar@...nel.org,
 yang@...amperecomputing.com, peterx@...hat.com, broonie@...nel.org,
 mgorman@...hsingularity.net, linux-arm-kernel@...ts.infradead.org,
 linux-kernel@...r.kernel.org, linux-mm@...ck.org,
 linux-kselftest@...r.kernel.org
Subject: Re: [PATCH 1/2] mm: Retry migration earlier upon refcount mismatch

On 10.08.24 20:42, Dev Jain wrote:
> 
> On 8/9/24 19:17, David Hildenbrand wrote:
>> On 09.08.24 12:31, Dev Jain wrote:
>>> As already being done in __migrate_folio(), wherein we backoff if the
>>> folio refcount is wrong, make this check during the unmapping phase,
>>> upon
>>> the failure of which, the original state of the PTEs will be restored
>>> and
>>> the folio lock will be dropped via migrate_folio_undo_src(), any racing
>>> thread will make progress and migration will be retried.
>>>
>>> Signed-off-by: Dev Jain <dev.jain@....com>
>>> ---
>>>    mm/migrate.c | 9 +++++++++
>>>    1 file changed, 9 insertions(+)
>>>
>>> diff --git a/mm/migrate.c b/mm/migrate.c
>>> index e7296c0fb5d5..477acf996951 100644
>>> --- a/mm/migrate.c
>>> +++ b/mm/migrate.c
>>> @@ -1250,6 +1250,15 @@ static int migrate_folio_unmap(new_folio_t
>>> get_new_folio,
>>>        }
>>>          if (!folio_mapped(src)) {
>>> +        /*
>>> +         * Someone may have changed the refcount and maybe sleeping
>>> +         * on the folio lock. In case of refcount mismatch, bail out,
>>> +         * let the system make progress and retry.
>>> +         */
>>> +        struct address_space *mapping = folio_mapping(src);
>>> +
>>> +        if (folio_ref_count(src) != folio_expected_refs(mapping, src))
>>> +            goto out;
>>
>> This really seems to be the latest point where we can "easily" back
>> off and unlock the source folio -- in this function :)
>>
>> I wonder if we should be smarter in the migrate_pages_batch() loop
>> when we start the actual migrations via migrate_folio_move(): if we
>> detect that a folio has unexpected references *and* it has waiters
>> (PG_waiters), back off then and retry the folio later. If it only has
>> unexpected references, just keep retrying: no waiters -> nobody is
>> waiting for the lock to make progress.
> 
> 
> The patch currently retries migration irrespective of the reason of
> refcount change.
> 
> If you are suggesting that, break the retrying according to two conditions:

That's not what I am suggesting ...

> 
> 
>> This really seems to be the latest point where we can "easily" back
>> off and unlock the source folio -- in this function :)
>> For example, when migrate_folio_move() fails with -EAGAIN, check if
>> there are waiters (PG_waiter?) and undo+unlock to try again later.
> 
> 
> Currently, on -EAGAIN, migrate_folio_move() returns without undoing src
> and dst; even if we were to fall

...

I am wondering if we should detect here if there are waiters and undo 
src+dst.

Does that make sense?

-- 
Cheers,

David / dhildenb