From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-alma10-1.taild15c8.ts.net [100.103.45.18]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id D024D3D47C5; Mon, 3 Aug 2026 09:19:25 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=100.103.45.18 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1785748768; cv=none; b=ZwKizn6WMeNBLztR8CWVrEVV17gaq7yqWbe0dlOtsbfsREoWvavSP2EGDN2TxU4nG1qRmaw3mKZrkBpr72L0754Ra8jNVuPHpkP4NUB2z5rVrjwaE3rgilNh2tXJMFslYzAxxN7KRW3B1JejDXkBykn8AZfVITvR4ecXbcVmN7E= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1785748768; c=relaxed/simple; bh=LB4FJQsY1IKCjtbKoVcQLFkgQMcPvpd2ST9kc/l/qhk=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=u9fX8rIC2z5xGq0A1umZRcM8fYz+5ignkBAS211RCO+E9GtSBYwhUMfYZBdsGTIe+hVoST0M1QH1TQH92l/hvvF/2WoWXLWGzGaa2inJlBzOsqWCtR7enkVYvoK6YvkmxUanzYWET63QTOOZnt2OtbFUp3mRIWhccKSzE18zLTs= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=JesergGZ; arc=none smtp.client-ip=100.103.45.18 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="JesergGZ" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 0F5741F00A3A; Mon, 3 Aug 2026 09:19:20 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1785748764; bh=cdDQILORMQ8A5KFOekP98a314EWXtxrYKLqfQJeDADU=; h=Date:Subject:To:Cc:References:From:In-Reply-To; b=JesergGZSKI+boCbMxRVMl5UmGIgB/amcI2tVOY6n5Mj9P7k5444Z+OMQYXfZ2ciF 8hWgGu6eVFv4F0q9ArGh8HPaTjsF/rpyH1zTO0xty1nZpRMpY6PXNmFAUb+UCAMHkQ uojekA5uZJqWTx6NPyvLK57S77g+ftn+8/l2OLKJx6aj4zwutuQq5/qcnFCIpoYRLV S5GfoWt5mmue6IQOAGSyha6/Vh8O1zYfPs3e0JaU7QkZyPzuTQt9bcnJnMVJOecgWH M+4xPZ+xRwrVk9vRFg+ByQ3LUQBelz1KzUjBSva4gzuxBnFaxFXRgnKeWifN1XDhf6 RBtwECo/SbacA== Message-ID: <7308cc99-1d1f-4e81-ada0-f0541aca12c7@kernel.org> Date: Mon, 3 Aug 2026 11:19:19 +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: fix always draining LRU caches in collect_longterm_unpinnable_folios() To: Barry Song Cc: Andrew Morton , Jason Gunthorpe , John Hubbard , Peter Xu , Kiryl Shutsemau , Hugh Dickins , Ackerley Tng , linux-mm@kvack.org, linux-kernel@vger.kernel.org, stable@vger.kernel.org References: <20260731-check_and_migrate_movable_folios-v1-1-e0002d7b791e@kernel.org> From: "David Hildenbrand (Arm)" Content-Language: en-US Autocrypt: addr=david@kernel.org; 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charset=UTF-8 Content-Transfer-Encoding: 8bit On 8/3/26 11:17, Barry Song wrote: > On Sat, Aug 1, 2026 at 4:28 AM David Hildenbrand (Arm) wrote: >> >> folio_may_be_lru_cached() is currently only true for small folios, and >> for small folios FOLL_PIN adds GUP_PIN_COUNTING_BIAS references instead >> of 1 in try_grab_folio()/try_grab_folio_fast(). >> >> Consequently, our >> >> folio_ref_count(folio) != folio_expected_ref_count(folio) + 1 >> >> check in collect_longterm_unpinnable_folios() will currently always >> identify "reference mismatch" and first drain the local LRU cache to then >> drain the LRU cache on all CPUs, as collect_longterm_unpinnable_folios() >> is really called after pinning the folios with FOLL_PIN. >> >> Add a comment because the current code is not quite intuitive: we used to >> drain only to make sure the folio_isolate_lru() would succeed. But then we >> also started draining to make later migration more reliable. >> >> We'll refactor that code soon a bit, to also make it usable in other >> context where we really want to remove any references from LRU caches. >> >> Let's add CC stable, because having an easy way for excessive LRU cache >> draining on all CPUs does not sound right. In common scenarios we >> don't expect to every have to drain. >> >> Fixes: 98c6d259319e ("mm/gup: check ref_count instead of lru before migration") >> Fixes: a09a8a1fbb37 ("mm/gup: local lru_add_drain() to avoid lru_add_drain_all()") >> Cc: stable@vger.kernel.org >> Signed-off-by: David Hildenbrand (Arm) >> --- >> Found by code inspection. If someone has a testcase that can easily >> trigger this and result in migration problems, please test! But this >> change seems to be "obvious the right thing to do". >> --- >> mm/gup.c | 10 ++++++++-- >> 1 file changed, 8 insertions(+), 2 deletions(-) >> >> diff --git a/mm/gup.c b/mm/gup.c >> index 99902c15703b0..41c3317e0f0f4 100644 >> --- a/mm/gup.c >> +++ b/mm/gup.c >> @@ -2273,6 +2273,7 @@ static unsigned long collect_longterm_unpinnable_folios( >> >> for (folio = pofs_get_folio(pofs, i); folio; >> folio = pofs_next_folio(folio, pofs, &i)) { >> + const int pin_refs = folio_has_pincount(folio) ? 1 : GUP_PIN_COUNTING_BIAS; >> >> if (folio_is_longterm_pinnable(folio)) >> continue; >> @@ -2287,15 +2288,20 @@ static unsigned long collect_longterm_unpinnable_folios( >> continue; >> } >> >> + /* >> + * We drain not only to make the folio_isolate_lru() succeed, >> + * but also to remove any other folio references from LRU >> + * caches. >> + */ >> if (drained == 0 && folio_may_be_lru_cached(folio) && >> folio_ref_count(folio) != >> - folio_expected_ref_count(folio) + 1) { >> + folio_expected_ref_count(folio) + pin_refs) { >> lru_add_drain(); >> drained = 1; >> } >> if (drained == 1 && folio_may_be_lru_cached(folio) && >> folio_ref_count(folio) != >> - folio_expected_ref_count(folio) + 1) { >> + folio_expected_ref_count(folio) + pin_refs) { >> lru_add_drain_all(); >> drained = 2; >> } > > Not regarding the fix itself, I agree that the fix is correct. I am > just a bit curious why we cannot simply do: > > if (folio_may_be_lru_cached(folio) && !folio_test_lru(folio)) { > lru_add_drain(); > if (!folio_test_lru(folio)) > lru_add_drain_all(); > } > > Or at least the following diff? Best to read 98c6d259319e, which documents why this is required. -- Cheers, David