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[2003:d8:2f23:9c00:d1f6:f7fe:8f14:7e34]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-45b9a6ecfafsm339135615e9.21.2025.09.09.04.50.51 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Tue, 09 Sep 2025 04:50:52 -0700 (PDT) Message-ID: <655d2d29-8fe6-4684-aba4-4803bed0d4d0@redhat.com> Date: Tue, 9 Sep 2025 13:50:51 +0200 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH] mm/gup: Drain batched mlock folio processing before attempting migration To: Will Deacon , Hugh Dickins Cc: linux-mm@kvack.org, linux-kernel@vger.kernel.org, Keir Fraser , Jason Gunthorpe , John Hubbard , Frederick Mayle , Andrew Morton , Peter Xu , Rik van Riel , Vlastimil Babka , Ge Yang References: <20250815101858.24352-1-will@kernel.org> <9e7d31b9-1eaf-4599-ce42-b80c0c4bb25d@google.com> <8376d8a3-cc36-ae70-0fa8-427e9ca17b9b@google.com> <7ce169c2-09b7-39e3-d00b-ba1db6dd258c@google.com> From: David Hildenbrand Content-Language: en-US Autocrypt: addr=david@redhat.com; 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charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit On 09.09.25 13:39, Will Deacon wrote: > On Fri, Aug 29, 2025 at 08:46:52AM -0700, Hugh Dickins wrote: >> On Fri, 29 Aug 2025, Will Deacon wrote: >>> On Thu, Aug 28, 2025 at 01:47:14AM -0700, Hugh Dickins wrote: >>>> diff --git a/mm/gup.c b/mm/gup.c >>>> index adffe663594d..9f7c87f504a9 100644 >>>> --- a/mm/gup.c >>>> +++ b/mm/gup.c >>>> @@ -2291,6 +2291,8 @@ static unsigned long collect_longterm_unpinnable_folios( >>>> struct folio *folio; >>>> long i = 0; >>>> >>>> + lru_add_drain(); >>>> + >>>> for (folio = pofs_get_folio(pofs, i); folio; >>>> folio = pofs_next_folio(folio, pofs, &i)) { >>>> >>>> @@ -2307,7 +2309,8 @@ static unsigned long collect_longterm_unpinnable_folios( >>>> continue; >>>> } >>>> >>>> - if (!folio_test_lru(folio) && drain_allow) { >>>> + if (drain_allow && folio_ref_count(folio) != >>>> + folio_expected_ref_count(folio) + 1) { >>>> lru_add_drain_all(); >>> >>> How does this synchronise with the folio being added to the mlock batch >>> on another CPU? >>> >>> need_mlock_drain(), which is what I think lru_add_drain_all() ends up >>> using to figure out which CPU batches to process, just looks at the >>> 'nr' field in the batch and I can't see anything in mlock_folio() to >>> ensure any ordering between adding the folio to the batch and >>> incrementing its refcount. >>> >>> Then again, my hack to use folio_test_mlocked() would have a similar >>> issue because the flag is set (albeit with barrier semantics) before >>> adding the folio to the batch, meaning the drain could miss the folio. >>> >>> I guess there's some higher-level synchronisation making this all work, >>> but it would be good to understand that as I can't see that >>> collect_longterm_unpinnable_folios() can rely on much other than the pin. >> >> No such strict synchronization: you've been misled if people have told >> you that this pinning migration stuff is deterministically successful: >> it's best effort - or will others on the Cc disagree? >> >> Just as there's no synchronization between the calculation inside >> folio_expected_ref_count() and the reading of folio's refcount. >> >> It wouldn't make sense for this unpinnable collection to anguish over >> such synchronization, when a moment later the migration is liable to >> fail (on occasion) for other transient reasons. All ending up reported >> as -ENOMEM apparently? that looks unhelpful. > > I see this was tangentially discussed with David on the patches you sent > and I agree that it's a distinct issue from what we're solving here, > however, -ENOMEM is a particularly problematic way to report transient > errors with migration due to a race. For KVM, the -ENOMEM will bubble > back up to userspace and the VMM is likely to destroy the VM altogether > whereas -EAGAIN would return back to the guest and retry the faulting > instruction. Migration code itself will retry multiple times, which usually takes care of most races. Not all of course. Now, I recall John was working on that at some point (I recall an RFC patch, but I might be daydreaming), and I recall discussions at LSF/MM around improving handling when we are mixing a flood of short-therm gup with a single long-term gup that wants to migrate these (short-term pinned) pages. Essentially, we would have to temporarily prevent new short-term GUP pins in order to make the long-term GUP-pin succeed in migrating the folio. -- Cheers David / dhildenb