From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-1.web.codeaurora.org [10.30.226.201]) (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 185D522B5AC for ; Mon, 18 Aug 2025 13:38:51 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=10.30.226.201 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1755524332; cv=none; b=CXrxmil2ERIJeLcO5gpcqUuas6b8tX8JvYIQhTScB53GQKjP9FZMQROT8h2gjq617yOzfmsaLud68cYTdjWwTLQf6/W6FgyzXU7ZQypbu9zV3S0DY7f/NYLrnlMMM+vdZ+Nhb3LqGbPhvQf6Xko1zNJF4XW0s7xB3hEE7kNVLdw= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1755524332; c=relaxed/simple; bh=+aNZzQBZ0JlhnJvSIA3wkd/Gq74n4JLtTqgPhcnHKK4=; h=Date:From:To:Cc:Subject:Message-ID:References:MIME-Version: Content-Type:Content-Disposition:In-Reply-To; b=dFxGfGUszKQElep9ngofmIh7PZ0bIpbKiYovsqV9I3eXO+AWoNH+cO22IobhXSsadj6BCkCJq3hWTuFjdLnu7iBJSvC78WFaidDAYCfY5q3phJMleV0jDQmOH535wNdsaq8bueGqTz3T/ZVQMIvWlbe0OQNnNuIeWTTw99Z7UrE= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=fW4xOacR; arc=none smtp.client-ip=10.30.226.201 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="fW4xOacR" Received: by smtp.kernel.org (Postfix) with ESMTPSA id EB254C113D0; Mon, 18 Aug 2025 13:38:49 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1755524331; bh=+aNZzQBZ0JlhnJvSIA3wkd/Gq74n4JLtTqgPhcnHKK4=; h=Date:From:To:Cc:Subject:References:In-Reply-To:From; b=fW4xOacRzXH3IrnYOIVgVa+T2AJK1ao9BMPxptTUbco/VPJngPBp8knPwsg/thDRz 1liCwAs8eIyPe1EIAig3FbsP2aWicQDUP8wwCS5h1EWflfgN9L7x2o+HjRT6AewyWM HJB8U5rj9jzIP/0eMXAqWR8pZy94NMWnahPxrANIruc48NhZCxarpFIrsyvru+xbYj wnoIMJeA4T0xDjD2UI/6zJNgdSW6KI1yKXbEzxTlS+pgmWurHvJ47YLQNcWxyhD+mE /UiEN0R65KIOmvMG5Jy4lWlmt6XFIBqM78q7KF2bbWi/9wFohm6EVbiRk1s14u33AL rwoSA5VVT512g== Date: Mon, 18 Aug 2025 14:38:46 +0100 From: Will Deacon To: John Hubbard Cc: linux-mm@kvack.org, linux-kernel@vger.kernel.org, Hugh Dickins , Keir Fraser , Jason Gunthorpe , David Hildenbrand , Frederick Mayle , Andrew Morton , Peter Xu Subject: Re: [PATCH] mm/gup: Drain batched mlock folio processing before attempting migration Message-ID: References: <20250815101858.24352-1-will@kernel.org> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: On Fri, Aug 15, 2025 at 06:03:17PM -0700, John Hubbard wrote: > On 8/15/25 3:18 AM, Will Deacon wrote: > > When taking a longterm GUP pin via pin_user_pages(), > > __gup_longterm_locked() tries to migrate target folios that should not > > be longterm pinned, for example because they reside in a CMA region or > > movable zone. This is done by first pinning all of the target folios > > anyway, collecting all of the longterm-unpinnable target folios into a > > list, dropping the pins that were just taken and finally handing the > > list off to migrate_pages() for the actual migration. > > > > It is critically important that no unexpected references are held on the > > folios being migrated, otherwise the migration will fail and > > pin_user_pages() will return -ENOMEM to its caller. Unfortunately, it is > > relatively easy to observe migration failures when running pKVM (which > > uses pin_user_pages() on crosvm's virtual address space to resolve > > stage-2 page faults from the guest) on a 6.15-based Pixel 6 device and > > this results in the VM terminating prematurely. > > > > In the failure case, 'crosvm' has called mlock(MLOCK_ONFAULT) on its > > mapping of guest memory prior to the pinning. Subsequently, when > > pin_user_pages() walks the page-table, the relevant 'pte' is not > > present and so the faulting logic allocates a new folio, mlocks it > > with mlock_folio() and maps it in the page-table. > > > > Since commit 2fbb0c10d1e8 ("mm/munlock: mlock_page() munlock_page() > > batch by pagevec"), mlock/munlock operations on a folio (formerly page), > > are deferred. For example, mlock_folio() takes an additional reference > > on the target folio before placing it into a per-cpu 'folio_batch' for > > later processing by mlock_folio_batch(), which drops the refcount once > > the operation is complete. Processing of the batches is coupled with > > the LRU batch logic and can be forcefully drained with > > lru_add_drain_all() but as long as a folio remains unprocessed on the > > batch, its refcount will be elevated. > > > > This deferred batching therefore interacts poorly with the pKVM pinning > > I would go even a little broader (more general), and claim that this > deferred batching interacts poorly with gup FOLL_LONGTERM when trying > to pin folios in CMA or ZONE_MOVABLE, in fact. That's much better, thanks. > > diff --git a/mm/gup.c b/mm/gup.c > > index adffe663594d..656835890f05 100644 > > --- a/mm/gup.c > > +++ b/mm/gup.c > > @@ -2307,7 +2307,8 @@ static unsigned long collect_longterm_unpinnable_folios( > > continue; > > } > > > > - if (!folio_test_lru(folio) && drain_allow) { > > + if (drain_allow && > > + (!folio_test_lru(folio) || folio_test_mlocked(folio))) { > > That should work, yes. > > Alternatively, after thinking about this a bit today, it seems to me that the > mlock batching is a little too bold, given the presence of gup/pup. And so I'm > tempted to fix the problem closer to the root cause, like this (below). > > But maybe this is actually *less* wise than what you have proposed... > > I'd like to hear other mm folks' opinion on this approach: > > diff --git a/mm/mlock.c b/mm/mlock.c > index a1d93ad33c6d..edecdd32996e 100644 > --- a/mm/mlock.c > +++ b/mm/mlock.c > @@ -278,7 +278,15 @@ void mlock_new_folio(struct folio *folio) > > folio_get(folio); > if (!folio_batch_add(fbatch, mlock_new(folio)) || > - folio_test_large(folio) || lru_cache_disabled()) > + folio_test_large(folio) || lru_cache_disabled() || > + /* > + * If this is being called as part of a gup FOLL_LONGTERM operation in > + * CMA/MOVABLE zones with MLOCK_ONFAULT active, then the newly faulted > + * in folio will need to immediately migrate to a pinnable zone. > + * Allowing the mlock operation to batch would break the ability to > + * migrate the folio. Instead, force immediate processing. > + */ > + (current->flags & PF_MEMALLOC_PIN)) > mlock_folio_batch(fbatch); > local_unlock(&mlock_fbatch.lock); > } So after Hugh's eagle eyes spotted mlock_folio() in my description, it turns out that the mlock happens on the user page fault path rather than during the pin itself. I think that means that checking for PF_MEMALLOC_PIN isn't going to work, as the pinning comes later. Hrm. I posted some stacktraces in my reply to Hugh that might help (and boy do I have plenty more of those). Will